Version 2, June 1991
Copyright (C) 1989, 1991 Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
NO WARRANTY
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and an idea of what it does. Copyright (C) 19yy name of author This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) 19yy name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. signature of Ty Coon, 1 April 1989 Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. You can write new code in Emacs Lisp and install it as an extension to the editor. However, Emacs Lisp is more than a mere "extension language"; it is a full computer programming language in its own right. You can use it as you would any other programming language.
Because Emacs Lisp is designed for use in an editor, it has special features for scanning and parsing text as well as features for handling files, buffers, displays, subprocesses, and so on. Emacs Lisp is closely integrated with the editing facilities; thus, editing commands are functions that can also conveniently be called from Lisp programs, and parameters for customization are ordinary Lisp variables.
This manual attempts to be a full description of Emacs Lisp. For a beginner's introduction to Emacs Lisp, see An Introduction to Emacs Lisp Programming, by Bob Chassell, also published by the Free Software Foundation. This manual presumes considerable familiarity with the use of Emacs for editing; see The GNU Emacs Manual for this basic information.
Generally speaking, the earlier chapters describe features of Emacs Lisp that have counterparts in many programming languages, and later chapters describe features that are peculiar to Emacs Lisp or relate specifically to editing.
This is edition 2.5.
This manual has gone through numerous drafts. It is nearly complete but not flawless. There are a few topics that are not covered, either because we consider them secondary (such as most of the individual modes) or because they are yet to be written. Because we are not able to deal with them completely, we have left out several parts intentionally. This includes most information about usage on VMS.
The manual should be fully correct in what it does cover, and it is therefore open to criticism on anything it says--from specific examples and descriptive text, to the ordering of chapters and sections. If something is confusing, or you find that you have to look at the sources or experiment to learn something not covered in the manual, then perhaps the manual should be fixed. Please let us know.
As you use the manual, we ask that you mark pages with corrections so you can later look them up and send them in. If you think of a simple, real-life example for a function or group of functions, please make an effort to write it up and send it in. Please reference any comments to the chapter name, section name, and function name, as appropriate, since page numbers and chapter and section numbers will change and we may have trouble finding the text you are talking about. Also state the number of the edition you are criticizing.
Please mail comments and corrections to
bug-lisp-manual@gnu.org
We let mail to this list accumulate unread until someone decides to
apply the corrections. Months, and sometimes years, go by between
updates. So please attach no significance to the lack of a reply--your
mail will be acted on in due time. If you want to contact the
Emacs maintainers more quickly, send mail to
bug-gnu-emacs@gnu.org
.
Lisp (LISt Processing language) was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it ideal for other purposes as well, such as writing editing commands.
Dozens of Lisp implementations have been built over the years, each with its own idiosyncrasies. Many of them were inspired by Maclisp, which was written in the 1960s at MIT's Project MAC. Eventually the implementors of the descendants of Maclisp came together and developed a standard for Lisp systems, called Common Lisp. In the meantime, Gerry Sussman and Guy Steele at MIT developed a simplified but very powerful dialect of Lisp, called Scheme.
GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common Lisp. If you know Common Lisp, you will notice many similarities. However, many features of Common Lisp have been omitted or simplified in order to reduce the memory requirements of GNU Emacs. Sometimes the simplifications are so drastic that a Common Lisp user might be very confused. We will occasionally point out how GNU Emacs Lisp differs from Common Lisp. If you don't know Common Lisp, don't worry about it; this manual is self-contained.
A certain amount of Common Lisp emulation is available via the `cl' library See section `Common Lisp Extension' in Common Lisp Extensions.
Emacs Lisp is not at all influenced by Scheme; but the GNU project has an implementation of Scheme, called Guile. We use Guile in all new GNU software that calls for extensibility.
This section explains the notational conventions that are used in this manual. You may want to skip this section and refer back to it later.
Throughout this manual, the phrases "the Lisp reader" and "the Lisp printer" refer to those routines in Lisp that convert textual representations of Lisp objects into actual Lisp objects, and vice versa. See section Printed Representation and Read Syntax, for more details. You, the person reading this manual, are thought of as "the programmer" and are addressed as "you". "The user" is the person who uses Lisp programs, including those you write.
Examples of Lisp code appear in this font or form: (list 1 2
3)
. Names that represent metasyntactic variables, or arguments to a
function being described, appear in this font or form:
first-number.
nil
and t
In Lisp, the symbol nil
has three separate meanings: it
is a symbol with the name `nil'; it is the logical truth value
false; and it is the empty list--the list of zero elements.
When used as a variable, nil
always has the value nil
.
As far as the Lisp reader is concerned, `()' and `nil' are
identical: they stand for the same object, the symbol nil
. The
different ways of writing the symbol are intended entirely for human
readers. After the Lisp reader has read either `()' or `nil',
there is no way to determine which representation was actually written
by the programmer.
In this manual, we use ()
when we wish to emphasize that it
means the empty list, and we use nil
when we wish to emphasize
that it means the truth value false. That is a good convention to use
in Lisp programs also.
(cons 'foo ()) ; Emphasize the empty list (not nil) ; Emphasize the truth value false
In contexts where a truth value is expected, any non-nil
value
is considered to be true. However, t
is the preferred way
to represent the truth value true. When you need to choose a
value which represents true, and there is no other basis for
choosing, use t
. The symbol t
always has the value
t
.
In Emacs Lisp, nil
and t
are special symbols that always
evaluate to themselves. This is so that you do not need to quote them
to use them as constants in a program. An attempt to change their
values results in a setting-constant
error. The same is true of
any symbol whose name starts with a colon (`:'). See section Variables That Never Change.
A Lisp expression that you can evaluate is called a form. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with `=>':
(car '(1 2)) => 1
You can read this as "(car '(1 2))
evaluates to 1".
When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with `==>'. We may or may not show the result of the evaluation of the expanded form.
(third '(a b c)) ==> (car (cdr (cdr '(a b c)))) => c
Sometimes to help describe one form we show another form that produces identical results. The exact equivalence of two forms is indicated with `=='.
(make-sparse-keymap) == (list 'keymap)
Many of the examples in this manual print text when they are
evaluated. If you execute example code in a Lisp Interaction buffer
(such as the buffer `*scratch*'), the printed text is inserted into
the buffer. If you execute the example by other means (such as by
evaluating the function eval-region
), the printed text is
displayed in the echo area. You should be aware that text displayed in
the echo area is truncated to a single line.
Examples in this manual indicate printed text with `-|',
irrespective of where that text goes. The value returned by evaluating
the form (here bar
) follows on a separate line.
(progn (print 'foo) (print 'bar)) -| foo -| bar => bar
Some examples signal errors. This normally displays an error message in the echo area. We show the error message on a line starting with `error-->'. Note that `error-->' itself does not appear in the echo area.
(+ 23 'x) error--> Wrong type argument: number-or-marker-p, x
Some examples show modifications to text in a buffer, with "before" and "after" versions of the text. These examples show the contents of the buffer in question between two lines of dashes containing the buffer name. In addition, `-!-' indicates the location of point. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place between two characters where point is currently located.)
---------- Buffer: foo ---------- This is the -!-contents of foo. ---------- Buffer: foo ---------- (insert "changed ") => nil ---------- Buffer: foo ---------- This is the changed -!-contents of foo. ---------- Buffer: foo ----------
Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category--function, variable, or whatever--is printed next to the right margin. The description follows on succeeding lines, sometimes with examples.
In a function description, the name of the function being described appears first. It is followed on the same line by a list of argument names. These names are also used in the body of the description, to stand for the values of the arguments.
The appearance of the keyword &optional
in the argument list
indicates that the subsequent arguments may be omitted (omitted
arguments default to nil
). Do not write &optional
when
you call the function.
The keyword &rest
(which must be followed by a single argument
name) indicates that any number of arguments can follow. The single
following argument name will have a value, as a variable, which is a
list of all these remaining arguments. Do not write &rest
when
you call the function.
Here is a description of an imaginary function foo
:
foo
subtracts integer1 from integer2,
then adds all the rest of the arguments to the result. If integer2
is not supplied, then the number 19 is used by default.
(foo 1 5 3 9) => 16 (foo 5) => 14
More generally,
(foo w x y...) == (+ (- x w) y...)
Any argument whose name contains the name of a type (e.g., integer, integer1 or buffer) is expected to be of that type. A plural of a type (such as buffers) often means a list of objects of that type. Arguments named object may be of any type. (See section Lisp Data Types, for a list of Emacs object types.) Arguments with other sorts of names (e.g., new-file) are discussed specifically in the description of the function. In some sections, features common to the arguments of several functions are described at the beginning.
See section Lambda Expressions, for a more complete description of optional and rest arguments.
Command, macro, and special form descriptions have the same format, but the word `Function' is replaced by `Command', `Macro', or `Special Form', respectively. Commands are simply functions that may be called interactively; macros process their arguments differently from functions (the arguments are not evaluated), but are presented the same way.
Special form descriptions use a more complex notation to specify optional and repeated arguments because they can break the argument list down into separate arguments in more complicated ways. `[optional-arg]' means that optional-arg is optional and `repeated-args...' stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure. Here is an example:
(count-loop (i 0 10) (prin1 i) (princ " ") (prin1 (aref vector i)) (terpri))
If from and to are omitted, var is bound to
nil
before the loop begins, and the loop exits if var is
non-nil
at the beginning of an iteration. Here is an example:
(count-loop (done) (if (pending) (fixit) (setq done t)))
In this special form, the arguments from and to are optional, but must both be present or both absent. If they are present, inc may optionally be specified as well. These arguments are grouped with the argument var into a list, to distinguish them from body, which includes all remaining elements of the form.
A variable is a name that can hold a value. Although any variable can be set by the user, certain variables that exist specifically so that users can change them are called user options. Ordinary variables and user options are described using a format like that for functions except that there are no arguments.
Here is a description of the imaginary electric-future-map
variable.
User option descriptions have the same format, but `Variable' is replaced by `User Option'.
These facilities provide information about which version of Emacs is in use.
(emacs-version) => "GNU Emacs 20.3.5 (i486-pc-linux-gnulibc1, X toolkit) of Sat Feb 14 1998 on psilocin.gnu.org"
Called interactively, the function prints the same information in the echo area.
current-time
(see section Time of Day).
emacs-build-time => (13623 62065 344633)
"20.3.1"
. The last number in this string is not
really part of the Emacs release version number; it is incremented each
time you build Emacs in any given directory.
The following two variables have existed since Emacs version 19.23:
This manual was written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert J. Chassell helped to review and edit the manual, with the support of the Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by Warren A. Hunt, Jr. of Computational Logic, Inc.
Corrections were supplied by Karl Berry, Jim Blandy, Bard Bloom, Stephane Boucher, David Boyes, Alan Carroll, Richard Davis, Lawrence R. Dodd, Peter Doornbosch, David A. Duff, Chris Eich, Beverly Erlebacher, David Eckelkamp, Ralf Fassel, Eirik Fuller, Stephen Gildea, Bob Glickstein, Eric Hanchrow, George Hartzell, Nathan Hess, Masayuki Ida, Dan Jacobson, Jak Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, Glenn M. Lewis, K. Richard Magill, Brian Marick, Roland McGrath, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potorti, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Per Starback, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn.
A Lisp object is a piece of data used and manipulated by Lisp programs. For our purposes, a type or data type is a set of possible objects.
Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for "the" type of an object.
A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called primitive types. Each object belongs to one and only one primitive type. These types include integer, float, cons, symbol, string, vector, subr, byte-code function, plus several special types, such as buffer, that are related to editing. (See section Editing Types.)
Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type.
Note that Lisp is unlike many other languages in that Lisp objects are self-typing: the primitive type of the object is implicit in the object itself. For example, if an object is a vector, nothing can treat it as a number; Lisp knows it is a vector, not a number.
In most languages, the programmer must declare the data type of each variable, and the type is known by the compiler but not represented in the data. Such type declarations do not exist in Emacs Lisp. A Lisp variable can have any type of value, and it remembers whatever value you store in it, type and all.
This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters.
The printed representation of an object is the format of the
output generated by the Lisp printer (the function prin1
) for
that object. The read syntax of an object is the format of the
input accepted by the Lisp reader (the function read
) for that
object. See section Reading and Printing Lisp Objects.
Most objects have more than one possible read syntax. Some types of object have no read syntax, since it may not make sense to enter objects of these types directly in a Lisp program. Except for these cases, the printed representation of an object is also a read syntax for it.
In other languages, an expression is text; it has no other form. In Lisp, an expression is primarily a Lisp object and only secondarily the text that is the object's read syntax. Often there is no need to emphasize this distinction, but you must keep it in the back of your mind, or you will occasionally be very confused.
Every type has a printed representation. Some types have no read
syntax--for example, the buffer type has none. Objects of these types
are printed in hash notation: the characters `#<' followed by
a descriptive string (typically the type name followed by the name of
the object), and closed with a matching `>'. Hash notation cannot
be read at all, so the Lisp reader signals the error
invalid-read-syntax
whenever it encounters `#<'.
(current-buffer) => #<buffer objects.texi>
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (see section Evaluation). However,
evaluation and reading are separate activities. Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later. See section Input Functions, for a description of
read
, the basic function for reading objects.
A comment is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a semicolon (`;') starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system.
The `#@count' construct, which skips the next count characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (see section Byte Compilation). It isn't meant for source files, however.
See section Tips on Writing Comments, for conventions for formatting comments.
There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former exist in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp.
The range of values for integers in Emacs Lisp is -134217728 to
134217727 (28 bits; i.e.,
to
on most machines. (Some machines may provide a wider range.) It is
important to note that the Emacs Lisp arithmetic functions do not check
for overflow. Thus (1+ 134217727)
is -134217728 on most
machines.
The read syntax for integers is a sequence of (base ten) digits with an optional sign at the beginning and an optional period at the end. The printed representation produced by the Lisp interpreter never has a leading `+' or a final `.'.
-1 ; The integer -1. 1 ; The integer 1. 1. ; Also The integer 1. +1 ; Also the integer 1. 268435457 ; Also the integer 1 on a 28-bit implementation.
See section Numbers, for more information.
Emacs supports floating point numbers (though there is a compilation option to disable them). The precise range of floating point numbers is machine-specific.
The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, `1500.0', `15e2', `15.0e2', `1.5e3', and `.15e4' are five ways of writing a floating point number whose value is 1500. They are all equivalent.
See section Numbers, for more information.
A character in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character A is represented as the integer 65.
Individual characters are not often used in programs. It is far more common to work with strings, which are sequences composed of characters. See section String Type.
Characters in strings, buffers, and files are currently limited to the range of 0 to 524287--nineteen bits. But not all values in that range are valid character codes. Codes 0 through 127 are ASCII codes; the rest are non-ASCII (see section Non-ASCII Characters). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift.
Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is a very bad idea. You should always use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark.
The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, `?A' for the character A, `?B' for the character B, and `?a' for the character a.
For example:
?Q => 81 ?q => 113
You can use the same syntax for punctuation characters, but it is often a good idea to add a `\' so that the Emacs commands for editing Lisp code don't get confused. For example, `?\ ' is the way to write the space character. If the character is `\', you must use a second `\' to quote it: `?\\'.
You can express the characters Control-g, backspace, tab, newline, vertical tab, formfeed, return, and escape as `?\a', `?\b', `?\t', `?\n', `?\v', `?\f', `?\r', `?\e', respectively. Thus,
?\a => 7 ; C-g ?\b => 8 ; backspace, BS, C-h ?\t => 9 ; tab, TAB, C-i ?\n => 10 ; newline, C-j ?\v => 11 ; vertical tab, C-k ?\f => 12 ; formfeed character, C-l ?\r => 13 ; carriage return, RET, C-m ?\e => 27 ; escape character, ESC, C-[ ?\\ => 92 ; backslash character, \
These sequences which start with backslash are also known as escape sequences, because backslash plays the role of an escape character; this usage has nothing to do with the character ESC.
Control characters may be represented using yet another read syntax. This consists of a question mark followed by a backslash, caret, and the corresponding non-control character, in either upper or lower case. For example, both `?\^I' and `?\^i' are valid read syntax for the character C-i, the character whose value is 9.
Instead of the `^', you can use `C-'; thus, `?\C-i' is equivalent to `?\^I' and to `?\^i':
?\^I => 9 ?\C-I => 9
In strings and buffers, the only control characters allowed are those that exist in ASCII; but for keyboard input purposes, you can turn any character into a control character with `C-'. The character codes for these non-ASCII control characters include the bit as well as the code for the corresponding non-control character. Ordinary terminals have no way of generating non-ASCII control characters, but you can generate them straightforwardly using X and other window systems.
For historical reasons, Emacs treats the DEL character as the control equivalent of ?:
?\^? => 127 ?\C-? => 127
As a result, it is currently not possible to represent the character Control-?, which is a meaningful input character under X, using `\C-'. It is not easy to change this, as various Lisp files refer to DEL in this way.
For representing control characters to be found in files or strings, we recommend the `^' syntax; for control characters in keyboard input, we prefer the `C-' syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it.
A meta character is a character typed with the META modifier key. The integer that represents such a character has the bit set (which on most machines makes it a negative number). We use high bits for this and other modifiers to make possible a wide range of basic character codes.
In a string, the bit attached to an ASCII character indicates a meta character; thus, the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters. (In Emacs versions 18 and older, this convention was used for characters outside of strings as well.)
The read syntax for meta characters uses `\M-'. For example, `?\M-A' stands for M-A. You can use `\M-' together with octal character codes (see below), with `\C-', or with any other syntax for a character. Thus, you can write M-A as `?\M-A', or as `?\M-\101'. Likewise, you can write C-M-b as `?\M-\C-b', `?\C-\M-b', or `?\M-\002'.
The case of a graphic character is indicated by its character code; for example, ASCII distinguishes between the characters `a' and `A'. But ASCII has no way to represent whether a control character is upper case or lower case. Emacs uses the bit to indicate that the shift key was used in typing a control character. This distinction is possible only when you use X terminals or other special terminals; ordinary terminals do not report the distinction to the computer in any way.
The X Window System defines three other modifier bits that can be set in a character: hyper, super and alt. The syntaxes for these bits are `\H-', `\s-' and `\A-'. (Case is significant in these prefixes.) Thus, `?\H-\M-\A-x' represents Alt-Hyper-Meta-x.
Finally, the most general read syntax for a character represents the
character code in either octal or hex. To use octal, write a question
mark followed by a backslash and the octal character code (up to three
octal digits); thus, `?\101' for the character A,
`?\001' for the character C-a, and ?\002
for the
character C-b. Although this syntax can represent any ASCII
character, it is preferred only when the precise octal value is more
important than the ASCII representation.
?\012 => 10 ?\n => 10 ?\C-j => 10 ?\101 => 65 ?A => 65
To use hex, write a question mark followed by a backslash, `x',
and the hexadecimal character code. You can use any number of hex
digits, so you can represent any character code in this way.
Thus, `?\x41' for the character A, `?\x1' for the
character C-a, and ?\x8e0
for the character
``a'.
A backslash is allowed, and harmless, preceding any character without a special escape meaning; thus, `?\+' is equivalent to `?+'. There is no reason to add a backslash before most characters. However, you should add a backslash before any of the characters `()\|;'`"#.,' to avoid confusing the Emacs commands for editing Lisp code. Also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of the easily readable escape sequences, such as `\t', instead of an actual whitespace character such as a tab.
A symbol in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary use, the name is unique--no two symbols have the same name.
A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended. But you can use one symbol in all of these ways, independently.
A symbol name can contain any characters whatever. Most symbol names are written with letters, digits, and the punctuation characters `-+=*/'. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a `\' at the beginning of the name to force interpretation as a symbol.) The characters `_~!@$%^&:<>{}' are less often used but also require no special punctuation. Any other characters may be included in a symbol's name by escaping them with a backslash. In contrast to its use in strings, however, a backslash in the name of a symbol simply quotes the single character that follows the backslash. For example, in a string, `\t' represents a tab character; in the name of a symbol, however, `\t' merely quotes the letter `t'. To have a symbol with a tab character in its name, you must actually use a tab (preceded with a backslash). But it's rare to do such a thing.
Common Lisp note: In Common Lisp, lower case letters are always "folded" to upper case, unless they are explicitly escaped. In Emacs Lisp, upper case and lower case letters are distinct.
Here are several examples of symbol names. Note that the `+' in the fifth example is escaped to prevent it from being read as a number. This is not necessary in the sixth example because the rest of the name makes it invalid as a number.
foo ; A symbol named `foo'. FOO ; A symbol named `FOO', different from `foo'. char-to-string ; A symbol named `char-to-string'. 1+ ; A symbol named `1+' ; (not `+1', which is an integer). \+1 ; A symbol named `+1' ; (not a very readable name). \(*\ 1\ 2\) ; A symbol named `(* 1 2)' (a worse name). +-*/_~!@$%^&=:<>{} ; A symbol named `+-*/_~!@$%^&=:<>{}'. ; These characters need not be escaped.
A sequence is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp, lists and arrays. Thus, an object of type list or of type array is also considered a sequence.
Arrays are further subdivided into strings, vectors, char-tables and
bool-vectors. Vectors can hold elements of any type, but string
elements must be characters, and bool-vector elements must be t
or nil
. The characters in a string can have text properties like
characters in a buffer (see section Text Properties); vectors and
bool-vectors do not support text properties even when their elements
happen to be characters. Char-tables are like vectors except that they
are indexed by any valid character code.
Lists, strings and the other array types are different, but they have
important similarities. For example, all have a length l, and all
have elements which can be indexed from zero to l minus one.
Several functions, called sequence functions, accept any kind of
sequence. For example, the function elt
can be used to extract
an element of a sequence, given its index. See section Sequences, Arrays, and Vectors.
It is generally impossible to read the same sequence twice, since
sequences are always created anew upon reading. If you read the read
syntax for a sequence twice, you get two sequences with equal contents.
There is one exception: the empty list ()
always stands for the
same object, nil
.
A cons cell is an object that consists of two pointers or slots, called the CAR slot and the CDR slot. Each slot can point to or hold to any Lisp object. We also say that the "the CAR of this cons cell is" whatever object its CAR slot currently points to, and likewise for the CDR.
A list is a series of cons cells, linked together so that the CDR slot of each cons cell holds either the next cons cell or the empty list. See section Lists, for functions that work on lists. Because most cons cells are used as part of lists, the phrase list structure has come to refer to any structure made out of cons cells.
The names CAR and CDR derive from the history of Lisp. The
original Lisp implementation ran on an IBM 704 computer which
divided words into two parts, called the "address" part and the
"decrement"; CAR was an instruction to extract the contents of
the address part of a register, and CDR an instruction to extract
the contents of the decrement. By contrast, "cons cells" are named
for the function cons
that creates them, which in turn is named
for its purpose, the construction of cells.
Because cons cells are so central to Lisp, we also have a word for "an object which is not a cons cell". These objects are called atoms.
The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a right parenthesis.
Upon reading, each object inside the parentheses becomes an element
of the list. That is, a cons cell is made for each element. The
CAR slot of the cons cell points to the element, and its CDR
slot points to the next cons cell of the list, which holds the next
element in the list. The CDR slot of the last cons cell is set to
point to nil
.
A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
such an illustration; unlike the textual notation, which can be
understood by both humans and computers, the box illustrations can be
understood only by humans.) This picture represents the three-element
list (rose violet buttercup)
:
--- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup
In this diagram, each box represents a slot that can point to any Lisp object. Each pair of boxes represents a cons cell. Each arrow is a pointer to a Lisp object, either an atom or another cons cell.
In this example, the first box, which holds the CAR of the first
cons cell, points to or "contains" rose
(a symbol). The second
box, holding the CDR of the first cons cell, points to the next
pair of boxes, the second cons cell. The CAR of the second cons
cell is violet
, and its CDR is the third cons cell. The
CDR of the third (and last) cons cell is nil
.
Here is another diagram of the same list, (rose violet
buttercup)
, sketched in a different manner:
--------------- ---------------- ------------------- | car | cdr | | car | cdr | | car | cdr | | rose | o-------->| violet | o-------->| buttercup | nil | | | | | | | | | | --------------- ---------------- -------------------
A list with no elements in it is the empty list; it is identical
to the symbol nil
. In other words, nil
is both a symbol
and a list.
Here are examples of lists written in Lisp syntax:
(A 2 "A") ; A list of three elements. () ; A list of no elements (the empty list). nil ; A list of no elements (the empty list). ("A ()") ; A list of one element: the string"A ()"
. (A ()) ; A list of two elements:A
and the empty list. (A nil) ; Equivalent to the previous. ((A B C)) ; A list of one element ; (which is a list of three elements).
Here is the list (A ())
, or equivalently (A nil)
,
depicted with boxes and arrows:
--- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> A --> nil
Dotted pair notation is an alternative syntax for cons cells
that represents the CAR and CDR explicitly. In this syntax,
(a . b)
stands for a cons cell whose CAR is
the object a, and whose CDR is the object b. Dotted
pair notation is therefore more general than list syntax. In the dotted
pair notation, the list `(1 2 3)' is written as `(1 . (2 . (3
. nil)))'. For nil
-terminated lists, you can use either
notation, but list notation is usually clearer and more convenient.
When printing a list, the dotted pair notation is only used if the
CDR of a cons cell is not a list.
Here's an example using boxes to illustrate dotted pair notation.
This example shows the pair (rose . violet)
:
--- --- | | |--> violet --- --- | | --> rose
You can combine dotted pair notation with list notation to represent
conveniently a chain of cons cells with a non-nil
final CDR.
You write a dot after the last element of the list, followed by the
CDR of the final cons cell. For example, (rose violet
. buttercup)
is equivalent to (rose . (violet . buttercup))
.
The object looks like this:
--- --- --- --- | | |--> | | |--> buttercup --- --- --- --- | | | | --> rose --> violet
The syntax (rose . violet . buttercup)
is invalid because
there is nothing that it could mean. If anything, it would say to put
buttercup
in the CDR of a cons cell whose CDR is already
used for violet
.
The list (rose violet)
is equivalent to (rose . (violet))
,
and looks like this:
--- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> rose --> violet
Similarly, the three-element list (rose violet buttercup)
is equivalent to (rose . (violet . (buttercup)))
.
An association list or alist is a specially-constructed list whose elements are cons cells. In each element, the CAR is considered a key, and the CDR is considered an associated value. (In some cases, the associated value is stored in the CAR of the CDR.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list.
For example,
(setq alist-of-colors '((rose . red) (lily . white) (buttercup . yellow)))
sets the variable alist-of-colors
to an alist of three elements. In the
first element, rose
is the key and red
is the value.
See section Association Lists, for a further explanation of alists and for functions that work on alists.
An array is composed of an arbitrary number of slots for pointing to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.)
Emacs defines four types of array: strings, vectors, bool-vectors, and char-tables.
A string is an array of characters and a vector is an array of
arbitrary objects. A bool-vector can hold only t
or nil
.
These kinds of array may have any length up to the largest integer.
Char-tables are sparse arrays indexed by any valid character code; they
can hold arbitrary objects.
The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed.
All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with an array of arrays.) Each type of array has its own read syntax; see the following sections for details.
The array type is contained in the sequence type and contains the string type, the vector type, the bool-vector type, and the char-table type.
A string is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string.
See section Strings and Characters, for functions that operate on strings.
The read syntax for strings is a double-quote, an arbitrary number of
characters, and another double-quote, "like this"
. To include a
double-quote in a string, precede it with a backslash; thus, "\""
is a string containing just a single double-quote character. Likewise,
you can include a backslash by preceding it with another backslash, like
this: "this \\ is a single embedded backslash"
.
The newline character is not special in the read syntax for strings; if you write a new line between the double-quotes, it becomes a character in the string. But an escaped newline--one that is preceded by `\'---does not become part of the string; i.e., the Lisp reader ignores an escaped newline while reading a string. An escaped space `\ ' is likewise ignored.
"It is useful to include newlines in documentation strings, but the newline is \ ignored if escaped." => "It is useful to include newlines in documentation strings, but the newline is ignored if escaped."
You can include a non-ASCII international character in a string constant by writing it literally. There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. If the string constant is read from a multibyte source, such as a multibyte buffer or string, or a file that would be visited as multibyte, then the character is read as a multibyte character, and that makes the string multibyte. If the string constant is read from a unibyte source, then the character is read as unibyte and that makes the string unibyte.
You can also represent a multibyte non-ASCII character with its character code, using a hex escape, `\xnnnnnnn', with as many digits as necessary. (Multibyte non-ASCII character codes are all greater than 256.) Any character which is not a valid hex digit terminates this construct. If the character that would follow is a hex digit, write `\ ' (backslash and space) to terminate the hex escape--for example, `\x8e0\ ' represents one character, `a' with grave accent. `\ ' in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate the preceding hex escape.
Using a multibyte hex escape forces the string to multibyte. You can represent a unibyte non-ASCII character with its character code, which must be in the range from 128 (0200 octal) to 255 (0377 octal). This forces a unibyte string. See section Text Representations, for more information about the two text representations.
You can use the same backslash escape-sequences in a string constant
as in character literals (but do not use the question mark that begins a
character constant). For example, you can write a string containing the
nonprinting characters tab and C-a, with commas and spaces between
them, like this: "\t, \C-a"
. See section Character Type, for a
description of the read syntax for characters.
However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the ASCII control characters. Strings do not distinguish case in ASCII control characters.
Properly speaking, strings cannot hold meta characters; but when a
string is to be used as a key sequence, there is a special convention
that provides a way to represent meta versions of ASCII characters in a
string. If you use the `\M-' syntax to indicate a meta character
in a string constant, this sets the
bit of the character in the string. If the string is used in
define-key
or lookup-key
, this numeric code is translated
into the equivalent meta character. See section Character Type.
Strings cannot hold characters that have the hyper, super, or alt modifiers.
A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text's properties with no special effort. See section Text Properties, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax:
#("characters" property-data...)
where property-data consists of zero or more elements, in groups of three as follows:
beg end plist
The elements beg and end are integers, and together specify a range of indices in the string; plist is the property list for that range. For example,
#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
represents a string whose textual contents are `foo bar', in which
the first three characters have a face
property with value
bold
, and the last three have a face
property with value
italic
. (The fourth character has no text properties, so its
property list is nil
. It is not actually necessary to mention
ranges with nil
as the property list, since any characters not
mentioned in any range will default to having no properties.)
A vector is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.)
The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation.
[1 "two" (three)] ; A vector of three elements. => [1 "two" (three)]
See section Vectors, for functions that work with vectors.
A char-table is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes--for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set.
The printed representation of a char-table is like a vector except that there is an extra `#^' at the beginning.
See section Char-Tables, for special functions to operate on char-tables. Uses of char-tables include:
A bool-vector is a one-dimensional array of elements that
must be t
or nil
.
The printed representation of a Bool-vector is like a string, except
that it begins with `#&' followed by the length. The string
constant that follows actually specifies the contents of the bool-vector
as a bitmap--each "character" in the string contains 8 bits, which
specify the next 8 elements of the bool-vector (1 stands for t
,
and 0 for nil
). The least significant bits of the character
correspond to the lowest indices in the bool-vector. If the length is not a
multiple of 8, the printed representation shows extra elements, but
these extras really make no difference.
(make-bool-vector 3 t) => #&3"\007" (make-bool-vector 3 nil) => #&3"\0" ;; These are equal since only the first 3 bits are used. (equal #&3"\377" #&3"\007") => t
Just as functions in other programming languages are executable,
Lisp function objects are pieces of executable code. However,
functions in Lisp are primarily Lisp objects, and only secondarily the
text which represents them. These Lisp objects are lambda expressions:
lists whose first element is the symbol lambda
(see section Lambda Expressions).
In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression is also called an anonymous function (see section Anonymous Functions). A named function in Lisp is actually a symbol with a valid function in its function cell (see section Defining Functions).
Most of the time, functions are called when their names are written in
Lisp expressions in Lisp programs. However, you can construct or obtain
a function object at run time and then call it with the primitive
functions funcall
and apply
. See section Calling Functions.
A Lisp macro is a user-defined construct that extends the Lisp
language. It is represented as an object much like a function, but with
different argument-passing semantics. A Lisp macro has the form of a
list whose first element is the symbol macro
and whose CDR
is a Lisp function object, including the lambda
symbol.
Lisp macro objects are usually defined with the built-in
defmacro
function, but any list that begins with macro
is
a macro as far as Emacs is concerned. See section Macros, for an explanation
of how to write a macro.
Warning: Lisp macros and keyboard macros (see section Keyboard Macros) are entirely different things. When we use the word "macro" without qualification, we mean a Lisp macro, not a keyboard macro.
A primitive function is a function callable from Lisp but written in the C programming language. Primitive functions are also called subrs or built-in functions. (The word "subr" is derived from "subroutine".) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a special form (see section Special Forms).
It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, we discourage redefinition of primitive functions.
The term function refers to all Emacs functions, whether written in Lisp or C. See section Function Type, for information about the functions written in Lisp.
Primitive functions have no read syntax and print in hash notation with the name of the subroutine.
(symbol-function 'car) ; Access the function cell ; of the symbol. => #<subr car> (subrp (symbol-function 'car)) ; Is this a primitive function? => t ; Yes.
The byte compiler produces byte-code function objects. Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. See section Byte Compilation, for information about the byte compiler.
The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional `#' before the opening `['.
An autoload object is a list whose first element is the symbol
autoload
. It is stored as the function definition of a symbol as
a placeholder for the real definition; it says that the real definition
is found in a file of Lisp code that should be loaded when necessary.
The autoload object contains the name of the file, plus some other
information about the real definition.
After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user's point of view, the function call works as expected, using the function definition in the loaded file.
An autoload object is usually created with the function
autoload
, which stores the object in the function cell of a
symbol. See section Autoload, for more details.
The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing.
A buffer is an object that holds text that can be edited (see section Buffers). Most buffers hold the contents of a disk file (see section Files) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (see section Windows). But a buffer need not be displayed in any window.
The contents of a buffer are much like a string, but buffers are not used like strings in Emacs Lisp, and the available operations are different. For example, you can insert text efficiently into an existing buffer, whereas "inserting" text into a string requires concatenating substrings, and the result is an entirely new string object.
Each buffer has a designated position called point (see section Positions). At any time, one buffer is the current buffer. Most editing commands act on the contents of the current buffer in the neighborhood of point. Many of the standard Emacs functions manipulate or test the characters in the current buffer; a whole chapter in this manual is devoted to describing these functions (see section Text).
Several other data structures are associated with each buffer:
The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs.
A buffer may be indirect, which means it shares the text of another buffer, but presents it differently. See section Indirect Buffers.
Buffers have no read syntax. They print in hash notation, showing the buffer name.
(current-buffer) => #<buffer objects.texi>
A marker denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer's text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer.
Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer.
(point-marker) => #<marker at 10779 in objects.texi>
See section Markers, for information on how to test, create, copy, and move markers.
A window describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows.
Though many windows may exist simultaneously, at any time one window is designated the selected window. This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer, but this is not necessarily the case.
Windows are grouped on the screen into frames; each window belongs to one and only one frame. See section Frame Type.
Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.
(selected-window) => #<window 1 on objects.texi>
See section Windows, for a description of the functions that work on windows.
A frame is a rectangle on the screen that contains one or more Emacs windows. A frame initially contains a single main window (plus perhaps a minibuffer window) which you can subdivide vertically or horizontally into smaller windows.
Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely).
(selected-frame) => #<frame emacs@psilocin.gnu.org 0xdac80>
See section Frames, for a description of the functions that work on frames.
A window configuration stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later.
Window configurations do not have a read syntax; their print syntax looks like `#<window-configuration>'. See section Window Configurations, for a description of several functions related to window configurations.
A frame configuration stores information about the positions,
sizes, and contents of the windows in all frames. It is actually
a list whose CAR is frame-configuration
and whose
CDR is an alist. Each alist element describes one frame,
which appears as the CAR of that element.
See section Frame Configurations, for a description of several functions related to frame configurations.
The word process usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs.
An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess.
Process objects have no read syntax. They print in hash notation, giving the name of the process:
(process-list) => (#<process shell>)
See section Processes, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes.
A stream is an object that can be used as a source or sink for characters--either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a `*Help*' buffer, or to the echo area.
The object nil
, in addition to its other meanings, may be used
as a stream. It stands for the value of the variable
standard-input
or standard-output
. Also, the object
t
as a stream specifies input using the minibuffer
(see section Minibuffers) or output in the echo area (see section The Echo Area).
Streams have no special printed representation or read syntax, and print as whatever primitive type they are.
See section Reading and Printing Lisp Objects, for a description of functions related to streams, including parsing and printing functions.
A keymap maps keys typed by the user to commands. This mapping
controls how the user's command input is executed. A keymap is actually
a list whose CAR is the symbol keymap
.
See section Keymaps, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings.
An overlay specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays have no read syntax, and print in hash notation, giving the buffer name and range of positions.
See section Overlays, for how to create and use overlays.
The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use.
All built-in functions do check the types of their actual arguments
when appropriate, and signal a wrong-type-argument
error if an
argument is of the wrong type. For example, here is what happens if you
pass an argument to +
that it cannot handle:
(+ 2 'a) error--> Wrong type argument: number-or-marker-p, a
If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a type predicate function. Emacs has a type predicate for each type, as well as some predicates for combinations of types.
A type predicate function takes one argument; it returns t
if
the argument belongs to the appropriate type, and nil
otherwise.
Following a general Lisp convention for predicate functions, most type
predicates' names end with `p'.
Here is an example which uses the predicates listp
to check for
a list and symbolp
to check for a symbol.
(defun add-on (x) (cond ((symbolp x) ;; If X is a symbol, put it on LIST. (setq list (cons x list))) ((listp x) ;; If X is a list, add its elements to LIST. (setq list (append x list))) (t ;; We handle only symbols and lists. (error "Invalid argument %s in add-on" x))))
Here is a table of predefined type predicates, in alphabetical order, with references to further information.
atom
arrayp
bool-vector-p
bufferp
byte-code-function-p
case-table-p
char-or-string-p
char-table-p
commandp
consp
display-table-p
floatp
frame-configuration-p
frame-live-p
framep
functionp
integer-or-marker-p
integerp
keymapp
listp
markerp
wholenump
nlistp
numberp
number-or-marker-p
overlayp
processp
sequencep
stringp
subrp
symbolp
syntax-table-p
user-variable-p
vectorp
window-configuration-p
window-live-p
windowp
The most general way to check the type of an object is to call the
function type-of
. Recall that each object belongs to one and
only one primitive type; type-of
tells you which one (see section Lisp Data Types). But type-of
knows nothing about non-primitive
types. In most cases, it is more convenient to use type predicates than
type-of
.
symbol
,
integer
, float
, string
, cons
, vector
,
char-table
, bool-vector
, subr
,
compiled-function
, marker
, overlay
, window
,
buffer
, frame
, process
, or
window-configuration
.
(type-of 1) => integer (type-of 'nil) => symbol (type-of '()) ;()
isnil
. => symbol (type-of '(x)) => cons
Here we describe two functions that test for equality between any two objects. Other functions test equality between objects of specific types, e.g., strings. For these predicates, see the appropriate chapter describing the data type.
t
if object1 and object2 are
the same object, nil
otherwise. The "same object" means that a
change in one will be reflected by the same change in the other.
eq
returns t
if object1 and object2 are
integers with the same value. Also, since symbol names are normally
unique, if the arguments are symbols with the same name, they are
eq
. For other types (e.g., lists, vectors, strings), two
arguments with the same contents or elements are not necessarily
eq
to each other: they are eq
only if they are the same
object.
(eq 'foo 'foo) => t (eq 456 456) => t (eq "asdf" "asdf") => nil (eq '(1 (2 (3))) '(1 (2 (3)))) => nil (setq foo '(1 (2 (3)))) => (1 (2 (3))) (eq foo foo) => t (eq foo '(1 (2 (3)))) => nil (eq [(1 2) 3] [(1 2) 3]) => nil (eq (point-marker) (point-marker)) => nil
The make-symbol
function returns an uninterned symbol, distinct
from the symbol that is used if you write the name in a Lisp expression.
Distinct symbols with the same name are not eq
. See section Creating and Interning Symbols.
(eq (make-symbol "foo") 'foo) => nil
t
if object1 and object2 have
equal components, nil
otherwise. Whereas eq
tests if its
arguments are the same object, equal
looks inside nonidentical
arguments to see if their elements are the same. So, if two objects are
eq
, they are equal
, but the converse is not always true.
(equal 'foo 'foo) => t (equal 456 456) => t (equal "asdf" "asdf") => t (eq "asdf" "asdf") => nil (equal '(1 (2 (3))) '(1 (2 (3)))) => t (eq '(1 (2 (3))) '(1 (2 (3)))) => nil (equal [(1 2) 3] [(1 2) 3]) => t (eq [(1 2) 3] [(1 2) 3]) => nil (equal (point-marker) (point-marker)) => t (eq (point-marker) (point-marker)) => nil
Comparison of strings is case-sensitive, but does not take account of text properties--it compares only the characters in the strings. A unibyte string never equals a multibyte string unless the contents are entirely ASCII (see section Text Representations).
(equal "asdf" "ASDF") => nil
Two distinct buffers are never equal
, even if their contents
are the same.
The test for equality is implemented recursively, and circular lists may therefore cause infinite recursion (leading to an error).
GNU Emacs supports two numeric data types: integers and floating point numbers. Integers are whole numbers such as -3, 0, 7, 13, and 511. Their values are exact. Floating point numbers are numbers with fractional parts, such as -4.5, 0.0, or 2.71828. They can also be expressed in exponential notation: 1.5e2 equals 150; in this example, `e2' stands for ten to the second power, and that is multiplied by 1.5. Floating point values are not exact; they have a fixed, limited amount of precision.
The range of values for an integer depends on the machine. The minimum range is -134217728 to 134217727 (28 bits; i.e., to but some machines may provide a wider range. Many examples in this chapter assume an integer has 28 bits.
The Lisp reader reads an integer as a sequence of digits with optional initial sign and optional final period.
1 ; The integer 1. 1. ; The integer 1. +1 ; Also the integer 1. -1 ; The integer -1. 268435457 ; Also the integer 1, due to overflow. 0 ; The integer 0. -0 ; The integer 0.
To understand how various functions work on integers, especially the bitwise operators (see section Bitwise Operations on Integers), it is often helpful to view the numbers in their binary form.
In 28-bit binary, the decimal integer 5 looks like this:
0000 0000 0000 0000 0000 0000 0101
(We have inserted spaces between groups of 4 bits, and two spaces between groups of 8 bits, to make the binary integer easier to read.)
The integer -1 looks like this:
1111 1111 1111 1111 1111 1111 1111
-1 is represented as 28 ones. (This is called two's complement notation.)
The negative integer, -5, is creating by subtracting 4 from -1. In binary, the decimal integer 4 is 100. Consequently, -5 looks like this:
1111 1111 1111 1111 1111 1111 1011
In this implementation, the largest 28-bit binary integer value is 134,217,727 in decimal. In binary, it looks like this:
0111 1111 1111 1111 1111 1111 1111
Since the arithmetic functions do not check whether integers go outside their range, when you add 1 to 134,217,727, the value is the negative integer -134,217,728:
(+ 1 134217727) => -134217728 => 1000 0000 0000 0000 0000 0000 0000
Many of the functions described in this chapter accept markers for arguments in place of numbers. (See section Markers.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name number-or-marker. When the argument value is a marker, its position value is used and its buffer is ignored.
Floating point numbers are useful for representing numbers that are
not integral. The precise range of floating point numbers is
machine-specific; it is the same as the range of the C data type
double
on the machine you are using.
The read-syntax for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, `1500.0', `15e2', `15.0e2', `1.5e3', and `.15e4' are five ways of writing a floating point number whose value is 1500. They are all equivalent. You can also use a minus sign to write negative floating point numbers, as in `-1.0'.
Most modern computers support the IEEE floating point standard, which
provides for positive infinity and negative infinity as floating point
values. It also provides for a class of values called NaN or
"not-a-number"; numerical functions return such values in cases where
there is no correct answer. For example, (sqrt -1.0)
returns a
NaN. For practical purposes, there's no significant difference between
different NaN values in Emacs Lisp, and there's no rule for precisely
which NaN value should be used in a particular case, so Emacs Lisp
doesn't try to distinguish them. Here are the read syntaxes for
these special floating point values:
In addition, the value -0.0
is distinguishable from ordinary
zero in IEEE floating point (although equal
and =
consider
them equal values).
You can use logb
to extract the binary exponent of a floating
point number (or estimate the logarithm of an integer):
(logb 10) => 3 (logb 10.0e20) => 69
The functions in this section test whether the argument is a number or
whether it is a certain sort of number. The functions integerp
and floatp
can take any type of Lisp object as argument (the
predicates would not be of much use otherwise); but the zerop
predicate requires a number as its argument. See also
integer-or-marker-p
and number-or-marker-p
, in
section Predicates on Markers.
t
if so, nil
otherwise.
floatp
does not exist in Emacs versions 18 and earlier.
t
if so, nil
otherwise.
t
if so, nil
otherwise.
wholenump
predicate (whose name comes from the phrase
"whole-number-p") tests to see whether its argument is a nonnegative
integer, and returns t
if so, nil
otherwise. 0 is
considered non-negative.
t
if so, nil
otherwise. The argument must be a number.
These two forms are equivalent: (zerop x)
== (= x 0)
.
To test numbers for numerical equality, you should normally use
=
, not eq
. There can be many distinct floating point
number objects with the same numeric value. If you use eq
to
compare them, then you test whether two values are the same
object. By contrast, =
compares only the numeric values
of the objects.
At present, each integer value has a unique Lisp object in Emacs Lisp.
Therefore, eq
is equivalent to =
where integers are
concerned. It is sometimes convenient to use eq
for comparing an
unknown value with an integer, because eq
does not report an
error if the unknown value is not a number--it accepts arguments of any
type. By contrast, =
signals an error if the arguments are not
numbers or markers. However, it is a good idea to use =
if you
can, even for comparing integers, just in case we change the
representation of integers in a future Emacs version.
Sometimes it is useful to compare numbers with equal
; it treats
two numbers as equal if they have the same data type (both integers, or
both floating point) and the same value. By contrast, =
can
treat an integer and a floating point number as equal.
There is another wrinkle: because floating point arithmetic is not exact, it is often a bad idea to check for equality of two floating point values. Usually it is better to test for approximate equality. Here's a function to do this:
(defvar fuzz-factor 1.0e-6) (defun approx-equal (x y) (or (and (= x 0) (= y 0)) (< (/ (abs (- x y)) (max (abs x) (abs y))) fuzz-factor)))
Common Lisp note: Comparing numbers in Common Lisp always requires
=
because Common Lisp implements multi-word integers, and two distinct integer objects can have the same numeric value. Emacs Lisp can have just one integer object for any given value because it has a limited range of integer values.
t
if so, nil
otherwise.
t
if they are not, and nil
if they are.
t
if so, nil
otherwise.
t
if so, nil
otherwise.
t
if so, nil
otherwise.
t
if so, nil
otherwise.
(max 20) => 20 (max 1 2.5) => 2.5 (max 1 3 2.5) => 3
(min -4 1) => -4
To convert an integer to floating point, use the function float
.
float
returns
it unchanged.
There are four functions to convert floating point numbers to integers; they differ in how they round. These functions accept integer arguments also, and return such arguments unchanged.
If divisor is specified, number is divided by divisor
before the floor is taken; this uses the kind of division operation that
corresponds to mod
, rounding downward. An arith-error
results if divisor is 0.
Emacs Lisp provides the traditional four arithmetic operations: addition, subtraction, multiplication, and division. Remainder and modulus functions supplement the division functions. The functions to add or subtract 1 are provided because they are traditional in Lisp and commonly used.
All of these functions except %
return a floating point value
if any argument is floating.
It is important to note that in Emacs Lisp, arithmetic functions
do not check for overflow. Thus (1+ 134217727)
may evaluate to
-134217728, depending on your hardware.
(setq foo 4) => 4 (1+ foo) => 5
This function is not analogous to the C operator ++
---it does not
increment a variable. It just computes a sum. Thus, if we continue,
foo => 4
If you want to increment the variable, you must use setq
,
like this:
(setq foo (1+ foo)) => 5
+
returns 0.
(+) => 0 (+ 1) => 1 (+ 1 2 3 4) => 10
-
function serves two purposes: negation and subtraction.
When -
has a single argument, the value is the negative of the
argument. When there are multiple arguments, -
subtracts each of
the more-numbers-or-markers from number-or-marker,
cumulatively. If there are no arguments, the result is 0.
(- 10 1 2 3 4) => 0 (- 10) => -10 (-) => 0
*
returns 1.
(*) => 1 (* 1) => 1 (* 1 2 3 4) => 24
If all the arguments are integers, then the result is an integer too.
This means the result has to be rounded. On most machines, the result
is rounded towards zero after each division, but some machines may round
differently with negative arguments. This is because the Lisp function
/
is implemented using the C division operator, which also
permits machine-dependent rounding. As a practical matter, all known
machines round in the standard fashion.
If you divide an integer by 0, an arith-error
error is signaled.
(See section Errors.) Floating point division by zero returns either
infinity or a NaN if your machine supports IEEE floating point;
otherwise, it signals an arith-error
error.
(/ 6 2) => 3 (/ 5 2) => 2 (/ 5.0 2) => 2.5 (/ 5 2.0) => 2.5 (/ 5.0 2.0) => 2.5 (/ 25 3 2) => 4 (/ -17 6) => -2
The result of (/ -17 6)
could in principle be -3 on some
machines.
For negative arguments, the remainder is in principle machine-dependent since the quotient is; but in practice, all known machines behave alike.
An arith-error
results if divisor is 0.
(% 9 4) => 1 (% -9 4) => -1 (% 9 -4) => 1 (% -9 -4) => -1
For any two integers dividend and divisor,
(+ (% dividend divisor) (* (/ dividend divisor) divisor))
always equals dividend.
Unlike %
, mod
returns a well-defined result for negative
arguments. It also permits floating point arguments; it rounds the
quotient downward (towards minus infinity) to an integer, and uses that
quotient to compute the remainder.
An arith-error
results if divisor is 0.
(mod 9 4) => 1 (mod -9 4) => 3 (mod 9 -4) => -3 (mod -9 -4) => -1 (mod 5.5 2.5) => .5
For any two numbers dividend and divisor,
(+ (mod dividend divisor) (* (floor dividend divisor) divisor))
always equals dividend, subject to rounding error if either
argument is floating point. For floor
, see section Numeric Conversions.
The functions ffloor
, fceiling
, fround
, and
ftruncate
take a floating point argument and return a floating
point result whose value is a nearby integer. ffloor
returns the
nearest integer below; fceiling
, the nearest integer above;
ftruncate
, the nearest integer in the direction towards zero;
fround
, the nearest integer.
In a computer, an integer is represented as a binary number, a sequence of bits (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, shifting moves the whole sequence left or right one or more places, reproducing the same pattern "moved over".
The bitwise operations in Emacs Lisp apply only to integers.
lsh
, which is an abbreviation for logical shift, shifts the
bits in integer1 to the left count places, or to the right
if count is negative, bringing zeros into the vacated bits. If
count is negative, lsh
shifts zeros into the leftmost
(most-significant) bit, producing a positive result even if
integer1 is negative. Contrast this with ash
, below.
Here are two examples of lsh
, shifting a pattern of bits one
place to the left. We show only the low-order eight bits of the binary
pattern; the rest are all zero.
(lsh 5 1) => 10 ;; Decimal 5 becomes decimal 10. 00000101 => 00001010 (lsh 7 1) => 14 ;; Decimal 7 becomes decimal 14. 00000111 => 00001110
As the examples illustrate, shifting the pattern of bits one place to the left produces a number that is twice the value of the previous number.
Shifting a pattern of bits two places to the left produces results like this (with 8-bit binary numbers):
(lsh 3 2) => 12 ;; Decimal 3 becomes decimal 12. 00000011 => 00001100
On the other hand, shifting one place to the right looks like this:
(lsh 6 -1) => 3 ;; Decimal 6 becomes decimal 3. 00000110 => 00000011 (lsh 5 -1) => 2 ;; Decimal 5 becomes decimal 2. 00000101 => 00000010
As the example illustrates, shifting one place to the right divides the value of a positive integer by two, rounding downward.
The function lsh
, like all Emacs Lisp arithmetic functions, does
not check for overflow, so shifting left can discard significant bits
and change the sign of the number. For example, left shifting
134,217,727 produces -2 on a 28-bit machine:
(lsh 134217727 1) ; left shift => -2
In binary, in the 28-bit implementation, the argument looks like this:
;; Decimal 134,217,727 0111 1111 1111 1111 1111 1111 1111
which becomes the following when left shifted:
;; Decimal -2 1111 1111 1111 1111 1111 1111 1110
ash
(arithmetic shift) shifts the bits in integer1
to the left count places, or to the right if count
is negative.
ash
gives the same results as lsh
except when
integer1 and count are both negative. In that case,
ash
puts ones in the empty bit positions on the left, while
lsh
puts zeros in those bit positions.
Thus, with ash
, shifting the pattern of bits one place to the right
looks like this:
(ash -6 -1) => -3 ;; Decimal -6 becomes decimal -3. 1111 1111 1111 1111 1111 1111 1010 => 1111 1111 1111 1111 1111 1111 1101
In contrast, shifting the pattern of bits one place to the right with
lsh
looks like this:
(lsh -6 -1) => 134217725 ;; Decimal -6 becomes decimal 134,217,725. 1111 1111 1111 1111 1111 1111 1010 => 0111 1111 1111 1111 1111 1111 1101
Here are other examples:
; 28-bit binary values (lsh 5 2) ; 5 = 0000 0000 0000 0000 0000 0000 0101 => 20 ; = 0000 0000 0000 0000 0000 0001 0100 (ash 5 2) => 20 (lsh -5 2) ; -5 = 1111 1111 1111 1111 1111 1111 1011 => -20 ; = 1111 1111 1111 1111 1111 1110 1100 (ash -5 2) => -20 (lsh 5 -2) ; 5 = 0000 0000 0000 0000 0000 0000 0101 => 1 ; = 0000 0000 0000 0000 0000 0000 0001 (ash 5 -2) => 1 (lsh -5 -2) ; -5 = 1111 1111 1111 1111 1111 1111 1011 => 4194302 ; = 0011 1111 1111 1111 1111 1111 1110 (ash -5 -2) ; -5 = 1111 1111 1111 1111 1111 1111 1011 => -2 ; = 1111 1111 1111 1111 1111 1111 1110
For example, using 4-bit binary numbers, the "logical and" of 13 and 12 is 12: 1101 combined with 1100 produces 1100. In both the binary numbers, the leftmost two bits are set (i.e., they are 1's), so the leftmost two bits of the returned value are set. However, for the rightmost two bits, each is zero in at least one of the arguments, so the rightmost two bits of the returned value are 0's.
Therefore,
(logand 13 12) => 12
If logand
is not passed any argument, it returns a value of
-1. This number is an identity element for logand
because its binary representation consists entirely of ones. If
logand
is passed just one argument, it returns that argument.
; 28-bit binary values (logand 14 13) ; 14 = 0000 0000 0000 0000 0000 0000 1110 ; 13 = 0000 0000 0000 0000 0000 0000 1101 => 12 ; 12 = 0000 0000 0000 0000 0000 0000 1100 (logand 14 13 4) ; 14 = 0000 0000 0000 0000 0000 0000 1110 ; 13 = 0000 0000 0000 0000 0000 0000 1101 ; 4 = 0000 0000 0000 0000 0000 0000 0100 => 4 ; 4 = 0000 0000 0000 0000 0000 0000 0100 (logand) => -1 ; -1 = 1111 1111 1111 1111 1111 1111 1111
logior
is
passed just one argument, it returns that argument.
; 28-bit binary values (logior 12 5) ; 12 = 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0000 0000 0000 0000 0000 0000 0101 => 13 ; 13 = 0000 0000 0000 0000 0000 0000 1101 (logior 12 5 7) ; 12 = 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0000 0000 0000 0000 0000 0000 0101 ; 7 = 0000 0000 0000 0000 0000 0000 0111 => 15 ; 15 = 0000 0000 0000 0000 0000 0000 1111
logxor
is passed just one argument, it returns that argument.
; 28-bit binary values (logxor 12 5) ; 12 = 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0000 0000 0000 0000 0000 0000 0101 => 9 ; 9 = 0000 0000 0000 0000 0000 0000 1001 (logxor 12 5 7) ; 12 = 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0000 0000 0000 0000 0000 0000 0101 ; 7 = 0000 0000 0000 0000 0000 0000 0111 => 14 ; 14 = 0000 0000 0000 0000 0000 0000 1110
(lognot 5) => -6 ;; 5 = 0000 0000 0000 0000 0000 0000 0101 ;; becomes ;; -6 = 1111 1111 1111 1111 1111 1111 1010
These mathematical functions allow integers as well as floating point numbers as arguments.
(asin arg)
is a number between -pi/2
and pi/2 (inclusive) whose sine is arg; if, however, arg
is out of range (outside [-1, 1]), then the result is a NaN.
(acos arg)
is a number between 0 and pi
(inclusive) whose cosine is arg; if, however, arg
is out of range (outside [-1, 1]), then the result is a NaN.
(atan arg)
is a number between -pi/2
and pi/2 (exclusive) whose tangent is arg.
(log10 x)
== (log x 10)
, at least approximately.
A deterministic computer program cannot generate true random numbers. For most purposes, pseudo-random numbers suffice. A series of pseudo-random numbers is generated in a deterministic fashion. The numbers are not truly random, but they have certain properties that mimic a random series. For example, all possible values occur equally often in a pseudo-random series.
In Emacs, pseudo-random numbers are generated from a "seed" number.
Starting from any given seed, the random
function always
generates the same sequence of numbers. Emacs always starts with the
same seed value, so the sequence of values of random
is actually
the same in each Emacs run! For example, in one operating system, the
first call to (random)
after you start Emacs always returns
-1457731, and the second one always returns -7692030. This
repeatability is helpful for debugging.
If you want truly unpredictable random numbers, execute (random
t)
. This chooses a new seed based on the current time of day and on
Emacs's process ID number.
If limit is a positive integer, the value is chosen to be nonnegative and less than limit.
If limit is t
, it means to choose a new seed based on the
current time of day and on Emacs's process ID number.
On some machines, any integer representable in Lisp may be the result
of random
. On other machines, the result can never be larger
than a certain maximum or less than a certain (negative) minimum.
A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files, to send messages to users, to hold text being copied between buffers, and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters.
See section Putting Keyboard Events in Strings, for special considerations for strings of keyboard character events.
Strings in Emacs Lisp are arrays that contain an ordered sequence of characters. Characters are represented in Emacs Lisp as integers; whether an integer is a character or not is determined only by how it is used. Thus, strings really contain integers.
The length of a string (like any array) is fixed, and cannot be altered once the string exists. Strings in Lisp are not terminated by a distinguished character code. (By contrast, strings in C are terminated by a character with ASCII code 0.)
Since strings are arrays, and therefore sequences as well, you can
operate on them with the general array and sequence functions.
(See section Sequences, Arrays, and Vectors.) For example, you can access or
change individual characters in a string using the functions aref
and aset
(see section Functions that Operate on Arrays).
There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte (see section Text Representations). ASCII characters always occupy one byte in a string; in fact, there is no real difference between the two representation for a string which is all ASCII. For most Lisp programming, you don't need to be concerned with these two representations.
Sometimes key sequences are represented as strings. When a string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are extremely large integers) rather than character codes in the range 128 to 255.
Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. See section Character Type, for more information about representation of meta and other modifiers for keyboard input characters.
Strings are useful for holding regular expressions. You can also
match regular expressions against strings (see section Regular Expression Searching). The
functions match-string
(see section Simple Match Data Access) and
replace-match
(see section Replacing the Text That Matched) are useful for
decomposing and modifying strings based on regular expression matching.
Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See section Text Properties. All the Lisp primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied.
See section Text, for information about functions that display strings or copy them into buffers. See section Character Type, and section String Type, for information about the syntax of characters and strings. See section Non-ASCII Characters, for functions to convert between text representations and encode and decode character codes.
For more information about general sequence and array predicates, see section Sequences, Arrays, and Vectors, and section Arrays.
t
if object is a string, nil
otherwise.
t
if object is a string or a
character (i.e., an integer), nil
otherwise.
The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.
(make-string 5 ?x) => "xxxxx" (make-string 0 ?x) => ""
Other functions to compare with this one include char-to-string
(see section Conversion of Characters and Strings), make-vector
(see section Vectors), and
make-list
(see section Building Cons Cells and Lists).
(string ?a ?b ?c) => "abc"
(substring "abcdefg" 0 3) => "abc"
Here the index for `a' is 0, the index for `b' is 1, and the
index for `c' is 2. Thus, three letters, `abc', are copied
from the string "abcdefg"
. The index 3 marks the character
position up to which the substring is copied. The character whose index
is 3 is actually the fourth character in the string.
A negative number counts from the end of the string, so that -1 signifies the index of the last character of the string. For example:
(substring "abcdefg" -3 -1) => "ef"
In this example, the index for `e' is -3, the index for `f' is -2, and the index for `g' is -1. Therefore, `e' and `f' are included, and `g' is excluded.
When nil
is used as an index, it stands for the length of the
string. Thus,
(substring "abcdefg" -3 nil) => "efg"
Omitting the argument end is equivalent to specifying nil
.
It follows that (substring string 0)
returns a copy of all
of string.
(substring "abcdefg" 0) => "abcdefg"
But we recommend copy-sequence
for this purpose (see section Sequences).
If the characters copied from string have text properties, the properties are copied into the new string also. See section Text Properties.
substring
also allows vectors for the first argument.
For example:
(substring [a b (c) "d"] 1 3) => [b (c)]
A wrong-type-argument
error is signaled if either start or
end is not an integer or nil
. An args-out-of-range
error is signaled if start indicates a character following
end, or if either integer is out of range for string.
Contrast this function with buffer-substring
(see section Examining Buffer Contents), which returns a string containing a portion of the text in
the current buffer. The beginning of a string is at index 0, but the
beginning of a buffer is at index 1.
concat
receives no arguments, it
returns an empty string.
(concat "abc" "-def")
=> "abc-def"
(concat "abc" (list 120 121) [122])
=> "abcxyz"
;; nil
is an empty sequence.
(concat "abc" nil "-def")
=> "abc-def"
(concat "The " "quick brown " "fox.")
=> "The quick brown fox."
(concat)
=> ""
The concat
function always constructs a new string that is
not eq
to any existing string.
When an argument is an integer (not a sequence of integers), it is
converted to a string of digits making up the decimal printed
representation of the integer. Don't use this feature; we plan
to eliminate it. If you already use this feature, change your programs
now! The proper way to convert an integer to a decimal number in this
way is with format
(see section Formatting Strings) or
number-to-string
(see section Conversion of Characters and Strings).
(concat 137) => "137" (concat 54 321) => "54321"
For information about other concatenation functions, see the
description of mapconcat
in section Mapping Functions,
vconcat
in section Vectors, and append
in section Building Cons Cells and Lists.
nil
(or
omitted), the default is "[ \f\t\n\r\v]+"
.
For example,
(split-string "Soup is good food" "o") => ("S" "up is g" "" "d f" "" "d") (split-string "Soup is good food" "o+") => ("S" "up is g" "d f" "d")
When there is a match adjacent to the beginning or end of the string, this does not cause a null string to appear at the beginning or end of the list:
(split-string "out to moo" "o+") => ("ut t" " m")
Empty matches do count, when not adjacent to another match:
(split-string "Soup is good food" "o*") =>("S" "u" "p" " " "i" "s" " " "g" "d" " " "f" "d") (split-string "Nice doggy!" "") =>("N" "i" "c" "e" " " "d" "o" "g" "g" "y" "!")
The most basic way to alter the contents of an existing string is with
aset
(see section Functions that Operate on Arrays). (aset string
idx char)
stores char into string at index
idx. Each character occupies one or more bytes, and if char
needs a different number of bytes from the character already present at
that index, aset
signals an error.
A more powerful function is store-substring
:
Since it is impossible to change the length of an existing string, it is an error if obj doesn't fit within string's actual length, of if any new character requires a different number of bytes from the character currently present at that point in string.
t
if the arguments represent the same
character, nil
otherwise. This function ignores differences
in case if case-fold-search
is non-nil
.
(char-equal ?x ?x) => t (let ((case-fold-search nil)) (char-equal ?x ?X)) => nil
t
if the characters of the two strings
match exactly; case is significant.
(string= "abc" "abc") => t (string= "abc" "ABC") => nil (string= "ab" "ABC") => nil
The function string=
ignores the text properties of the two
strings. When equal
(see section Equality Predicates) compares two
strings, it uses string=
.
If the strings contain non-ASCII characters, and one is unibyte while the other is multibyte, then they cannot be equal. See section Text Representations.
string-equal
is another name for string=
.
t
. If the lesser character is the one from
string2, then string1 is greater, and this function returns
nil
. If the two strings match entirely, the value is nil
.
Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (see section Text Representations).
(string< "abc" "abd") => t (string< "abd" "abc") => nil (string< "123" "abc") => t
When the strings have different lengths, and they match up to the
length of string1, then the result is t
. If they match up
to the length of string2, the result is nil
. A string of
no characters is less than any other string.
(string< "" "abc") => t (string< "ab" "abc") => t (string< "abc" "") => nil (string< "abc" "ab") => nil (string< "" "") => nil
string-lessp
is another name for string<
.
The strings are both converted to multibyte for the comparison
(see section Text Representations) so that a unibyte string can be equal to
a multibyte string. If ignore-case is non-nil
, then case
is ignored, so that upper case letters can be equal to lower case letters.
If the specified portions of the two strings match, the value is
t
. Otherwise, the value is an integer which indicates how many
leading characters agree, and which string is less. Its absolute value
is one plus the number of characters that agree at the beginning of the
two strings. The sign is negative if string1 (or its specified
portion) is less.
assoc
, except that key must be a
string, and comparison is done using compare-strings
.
Case differences are ignored in this comparison.
assoc
, except that key must be a
string, and comparison is done using compare-strings
.
Case differences are significant.
See also compare-buffer-substrings
in section Comparing Text, for
a way to compare text in buffers. The function string-match
,
which matches a regular expression against a string, can be used
for a kind of string comparison; see section Regular Expression Searching.
This section describes functions for conversions between characters,
strings and integers. format
and prin1-to-string
(see section Output Functions) can also convert Lisp objects into strings.
read-from-string
(see section Input Functions) can "convert" a
string representation of a Lisp object into an object. The functions
string-make-multibyte
and string-make-unibyte
convert the
text representation of a string (see section Converting Text Representations).
See section Documentation, for functions that produce textual descriptions
of text characters and general input events
(single-key-description
and text-char-description
). These
functions are used primarily for making help messages.
string
is more general. See section Creating Strings.
(string-to-char "ABC") => 65 (string-to-char "xyz") => 120 (string-to-char "") => 0 (string-to-char "\000") => 0
This function may be eliminated in the future if it does not seem useful enough to retain.
(number-to-string 256) => "256" (number-to-string -23) => "-23" (number-to-string -23.5) => "-23.5"
int-to-string
is a semi-obsolete alias for this function.
See also the function format
in section Formatting Strings.
nil
, integers are converted
in that base. If base is nil
, then base ten is used.
Floating point conversion always uses base ten; we have not implemented
other radices for floating point numbers, because that would be much
more work and does not seem useful.
The parsing skips spaces and tabs at the beginning of string, then reads as much of string as it can interpret as a number. (On some systems it ignores other whitespace at the beginning, not just spaces and tabs.) If the first character after the ignored whitespace is not a digit or a plus or minus sign, this function returns 0.
(string-to-number "256") => 256 (string-to-number "25 is a perfect square.") => 25 (string-to-number "X256") => 0 (string-to-number "-4.5") => -4.5
Here are some other functions that can convert to or from a string:
concat
concat
can convert a vector or a list into a string.
See section Creating Strings.
vconcat
vconcat
can convert a string into a vector. See section Functions for Vectors.
append
append
can convert a string into a list. See section Building Cons Cells and Lists.
Formatting means constructing a string by substitution of computed values at various places in a constant string. This string controls how the other values are printed as well as where they appear; it is called a format string.
Formatting is often useful for computing messages to be displayed. In
fact, the functions message
and error
provide the same
formatting feature described here; they differ from format
only
in how they use the result of formatting.
A format specification is a sequence of characters beginning with a
`%'. Thus, if there is a `%d' in string, the
format
function replaces it with the printed representation of
one of the values to be formatted (one of the arguments objects).
For example:
(format "The value of fill-column is %d." fill-column) => "The value of fill-column is 72."
If string contains more than one format specification, the format specifications correspond with successive values from objects. Thus, the first format specification in string uses the first such value, the second format specification uses the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause unpredictable behavior. Any extra values to be formatted are ignored.
Certain format specifications require values of particular types. If you supply a value that doesn't fit the requirements, an error is signaled.
Here is a table of valid format specifications:
princ
, not
prin1
---see section Output Functions). Thus, strings are represented
by their contents alone, with no `"' characters, and symbols appear
without `\' characters.
If there is no corresponding object, the empty string is used.
prin1
---see section Output Functions). Thus, strings are enclosed in `"' characters, and
`\' characters appear where necessary before special characters.
If there is no corresponding object, the empty string is used.
(format "%%
%d" 30)
returns "% 30"
.
Any other format character results in an `Invalid format operation' error.
Here are several examples:
(format "The name of this buffer is %s." (buffer-name)) => "The name of this buffer is strings.texi." (format "The buffer object prints as %s." (current-buffer)) => "The buffer object prints as strings.texi." (format "The octal value of %d is %o, and the hex value is %x." 18 18 18) => "The octal value of 18 is 22, and the hex value is 12."
All the specification characters allow an optional numeric prefix between the `%' and the character. The optional numeric prefix defines the minimum width for the object. If the printed representation of the object contains fewer characters than this, then it is padded. The padding is on the left if the prefix is positive (or starts with zero) and on the right if the prefix is negative. The padding character is normally a space, but if the numeric prefix starts with a zero, zeros are used for padding. Here are some examples of padding:
(format "%06d is padded on the left with zeros" 123) => "000123 is padded on the left with zeros" (format "%-6d is padded on the right" 123) => "123 is padded on the right"
format
never truncates an object's printed representation, no
matter what width you specify. Thus, you can use a numeric prefix to
specify a minimum spacing between columns with no risk of losing
information.
In the following three examples, `%7s' specifies a minimum width
of 7. In the first case, the string inserted in place of `%7s' has
only 3 letters, so 4 blank spaces are inserted for padding. In the
second case, the string "specification"
is 13 letters wide but is
not truncated. In the third case, the padding is on the right.
(format "The word `%7s' actually has %d letters in it." "foo" (length "foo")) => "The word ` foo' actually has 3 letters in it." (format "The word `%7s' actually has %d letters in it." "specification" (length "specification")) => "The word `specification' actually has 13 letters in it." (format "The word `%-7s' actually has %d letters in it." "foo" (length "foo")) => "The word `foo ' actually has 3 letters in it."
The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters `A' through `Z' and `a' through `z', as well as non-ASCII letters); other characters are not altered. (You can specify a different case conversion mapping by specifying a case table---see section The Case Table.)
These functions do not modify the strings that are passed to them as arguments.
The examples below use the characters `X' and `x' which have ASCII codes 88 and 120 respectively.
When the argument to downcase
is a string, the function creates
and returns a new string in which each letter in the argument that is
upper case is converted to lower case. When the argument to
downcase
is a character, downcase
returns the
corresponding lower case character. This value is an integer. If the
original character is lower case, or is not a letter, then the value
equals the original character.
(downcase "The cat in the hat") => "the cat in the hat" (downcase ?X) => 120
When the argument to upcase
is a string, the function creates
and returns a new string in which each letter in the argument that is
lower case is converted to upper case.
When the argument to upcase
is a character, upcase
returns the corresponding upper case character. This value is an integer.
If the original character is upper case, or is not a letter, then the
value equals the original character.
(upcase "The cat in the hat") => "THE CAT IN THE HAT" (upcase ?x) => 88
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (See section Table of Syntax Classes).
When the argument to capitalize
is a character, capitalize
has the same result as upcase
.
(capitalize "The cat in the hat") => "The Cat In The Hat" (capitalize "THE 77TH-HATTED CAT") => "The 77th-Hatted Cat" (capitalize ?x) => 88
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (See section Table of Syntax Classes).
(upcase-initials "The CAT in the hAt") => "The CAT In The HAt"
See section Comparison of Characters and Strings, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences.
You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see section Case Changes). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers.
A case table is a char-table (see section Char-Tables) whose subtype is
case-table
. This char-table maps each character into the
corresponding lower case character. It has three extra slots, which
hold related tables:
In simple cases, all you need to specify is the mapping to lower-case; the three related tables will be calculated automatically from that one.
For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both lower case and upper case.
The extra table canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. For example, since `a' and `A' are related by case-conversion, they should have the same canonical equivalent character (which should be either `a' for both of them, or `A' for both of them).
The extra table equivalences is a map that cyclicly permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map `a' into `A' and `A' into `a', and likewise for each set of equivalent characters.)
When you construct a case table, you can provide nil
for
canonicalize; then Emacs fills in this slot from the lower case
and upper case mappings. You can also provide nil
for
equivalences; then Emacs fills in this slot from
canonicalize. In a case table that is actually in use, those
components are non-nil
. Do not try to specify equivalences
without also specifying canonicalize.
Here are the functions for working with case tables:
nil
if object is a valid case
table.
The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify the specified case table case-table; they also modify the standard syntax table. See section Syntax Tables. Normally you would use these functions to change the standard case table.
A list represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list.
Lists in Lisp are not a primitive data type; they are built up from cons cells. A cons cell is a data object that represents an ordered pair. It holds, or "points to," two Lisp objects, one labeled as the CAR, and the other labeled as the CDR. These names are traditional; see section Cons Cell and List Types. CDR is pronounced "could-er."
A list is a series of cons cells chained together, one cons cell per
element of the list. By convention, the CARs of the cons cells are
the elements of the list, and the CDRs are used to chain the list:
the CDR of each cons cell is the following cons cell. The CDR
of the last cons cell is nil
. This asymmetry between the
CAR and the CDR is entirely a matter of convention; at the
level of cons cells, the CAR and CDR slots have the same
characteristics.
Because most cons cells are used as part of lists, the phrase list structure has come to mean any structure made out of cons cells.
The symbol nil
is considered a list as well as a symbol; it is
the list with no elements. For convenience, the symbol nil
is
considered to have nil
as its CDR (and also as its
CAR).
The CDR of any nonempty list l is a list containing all the elements of l except the first.
A cons cell can be illustrated as a pair of boxes. The first box
represents the CAR and the second box represents the CDR.
Here is an illustration of the two-element list, (tulip lily)
,
made from two cons cells:
--------------- --------------- | car | cdr | | car | cdr | | tulip | o---------->| lily | nil | | | | | | | --------------- ---------------
Each pair of boxes represents a cons cell. Each box "refers to",
"points to" or "contains" a Lisp object. (These terms are
synonymous.) The first box, which describes the CAR of the first
cons cell, contains the symbol tulip
. The arrow from the
CDR box of the first cons cell to the second cons cell indicates
that the CDR of the first cons cell is the second cons cell.
The same list can be illustrated in a different sort of box notation like this:
--- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> tulip --> lily
Here is a more complex illustration, showing the three-element list,
((pine needles) oak maple)
, the first element of which is a
two-element list:
--- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | | --> oak --> maple | | --- --- --- --- --> | | |--> | | |--> nil --- --- --- --- | | | | --> pine --> needles
The same list represented in the first box notation looks like this:
-------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | o | o------->| oak | o------->| maple | nil | | | | | | | | | | | -- | --------- -------------- -------------- | | | -------------- ---------------- | | car | cdr | | car | cdr | ------>| pine | o------->| needles | nil | | | | | | | -------------- ----------------
See section Cons Cell and List Types, for the read and print syntax of cons cells and lists, and for more "box and arrow" illustrations of lists.
The following predicates test whether a Lisp object is an atom, is a
cons cell or is a list, or whether it is the distinguished object
nil
. (Many of these predicates can be defined in terms of the
others, but they are used so often that it is worth having all of them.)
t
if object is a cons cell, nil
otherwise. nil
is not a cons cell, although it is a list.
t
if object is an atom, nil
otherwise. All objects except cons cells are atoms. The symbol
nil
is an atom and is also a list; it is the only Lisp object
that is both.
(atom object) == (not (consp object))
t
if object is a cons cell or
nil
. Otherwise, it returns nil
.
(listp '(1)) => t (listp '()) => t
listp
: it returns t
if
object is not a list. Otherwise, it returns nil
.
(listp object) == (not (nlistp object))
t
if object is nil
, and
returns nil
otherwise. This function is identical to not
,
but as a matter of clarity we use null
when object is
considered a list and not
when it is considered a truth value
(see not
in section Constructs for Combining Conditions).
(null '(1)) => nil (null '()) => t
As a special case, if cons-cell is nil
, then car
is defined to return nil
; therefore, any list is a valid argument
for car
. An error is signaled if the argument is not a cons cell
or nil
.
(car '(a b c)) => a (car '()) => nil
As a special case, if cons-cell is nil
, then cdr
is defined to return nil
; therefore, any list is a valid argument
for cdr
. An error is signaled if the argument is not a cons cell
or nil
.
(cdr '(a b c)) => (b c) (cdr '()) => nil
nil
otherwise. This is in contrast
to car
, which signals an error if object is not a list.
(car-safe object) == (let ((x object)) (if (consp x) (car x) nil))
nil
otherwise.
This is in contrast to cdr
, which signals an error if
object is not a list.
(cdr-safe object) == (let ((x object)) (if (consp x) (cdr x) nil))
nil
.
If n is negative, nth
returns the first element of
list.
(nth 2 '(1 2 3 4)) => 3 (nth 10 '(1 2 3 4)) => nil (nth -3 '(1 2 3 4)) => 1 (nth n x) == (car (nthcdr n x))
The function elt
is similar, but applies to any kind of sequence.
For historical reasons, it takes its arguments in the opposite order.
See section Sequences.
If n is zero or negative, nthcdr
returns all of
list. If the length of list is n or less,
nthcdr
returns nil
.
(nthcdr 1 '(1 2 3 4)) => (2 3 4) (nthcdr 10 '(1 2 3 4)) => nil (nthcdr -3 '(1 2 3 4)) => (1 2 3 4)
If list is not really a list, safe-length
returns 0. If
list is circular, it returns a finite value which is at least the
number of distinct elements.
The most common way to compute the length of a list, when you are not
worried that it may be circular, is with length
. See section Sequences.
(car (car cons-cell))
.
(car (cdr cons-cell))
or (nth 1 cons-cell)
.
(cdr (car cons-cell))
.
(cdr (cdr cons-cell))
or (nthcdr 2 cons-cell)
.
Many functions build lists, as lists reside at the very heart of Lisp.
cons
is the fundamental list-building function; however, it is
interesting to note that list
is used more times in the source
code for Emacs than cons
.
(cons 1 '(2)) => (1 2) (cons 1 '()) => (1) (cons 1 2) => (1 . 2)
cons
is often used to add a single element to the front of a
list. This is called consing the element onto the list. For
example:
(setq list (cons newelt list))
Note that there is no conflict between the variable named list
used in this example and the function named list
described below;
any symbol can serve both purposes.
nil
-terminated. If no objects
are given, the empty list is returned.
(list 1 2 3 4 5) => (1 2 3 4 5) (list 1 2 '(3 4 5) 'foo) => (1 2 (3 4 5) foo) (list) => nil
make-list
with make-string
(see section Creating Strings).
(make-list 3 'pigs) => (pigs pigs pigs) (make-list 0 'pigs) => nil
nconc
in section Functions that Rearrange Lists, for a way to join
lists with no copying.)
More generally, the final argument to append
may be any Lisp
object. The final argument is not copied or converted; it becomes the
CDR of the last cons cell in the new list. If the final argument
is itself a list, then its elements become in effect elements of the
result list. If the final element is not a list, the result is a
"dotted list" since its final CDR is not nil
as required
in a true list.
The append
function also allows integers as arguments. It
converts them to strings of digits, making up the decimal print
representation of the integer, and then uses the strings instead of the
original integers. Don't use this feature; we plan to eliminate
it. If you already use this feature, change your programs now! The
proper way to convert an integer to a decimal number in this way is with
format
(see section Formatting Strings) or number-to-string
(see section Conversion of Characters and Strings).
Here is an example of using append
:
(setq trees '(pine oak)) => (pine oak) (setq more-trees (append '(maple birch) trees)) => (maple birch pine oak) trees => (pine oak) more-trees => (maple birch pine oak) (eq trees (cdr (cdr more-trees))) => t
You can see how append
works by looking at a box diagram. The
variable trees
is set to the list (pine oak)
and then the
variable more-trees
is set to the list (maple birch pine
oak)
. However, the variable trees
continues to refer to the
original list:
more-trees trees | | | --- --- --- --- -> --- --- --- --- --> | | |--> | | |--> | | |--> | | |--> nil --- --- --- --- --- --- --- --- | | | | | | | | --> maple -->birch --> pine --> oak
An empty sequence contributes nothing to the value returned by
append
. As a consequence of this, a final nil
argument
forces a copy of the previous argument:
trees => (pine oak) (setq wood (append trees nil)) => (pine oak) wood => (pine oak) (eq wood trees) => nil
This once was the usual way to copy a list, before the function
copy-sequence
was invented. See section Sequences, Arrays, and Vectors.
Here we show the use of vectors and strings as arguments to append
:
(append [a b] "cd" nil) => (a b 99 100)
With the help of apply
(see section Calling Functions), we can append
all the lists in a list of lists:
(apply 'append '((a b c) nil (x y z) nil)) => (a b c x y z)
If no sequences are given, nil
is returned:
(append) => nil
Here are some examples where the final argument is not a list:
(append '(x y) 'z) => (x y . z) (append '(x y) [z]) => (x y . [z])
The second example shows that when the final argument is a sequence but not a list, the sequence's elements do not become elements of the resulting list. Instead, the sequence becomes the final CDR, like any other non-list final argument.
(setq x '(1 2 3 4)) => (1 2 3 4) (reverse x) => (4 3 2 1) x => (1 2 3 4)
You can modify the CAR and CDR contents of a cons cell with the
primitives setcar
and setcdr
. We call these "destructive"
operations because they change existing list structure.
Common Lisp note: Common Lisp uses functions
rplaca
andrplacd
to alter list structure; they change structure the same way assetcar
andsetcdr
, but the Common Lisp functions return the cons cell whilesetcar
andsetcdr
return the new CAR or CDR.
setcar
Changing the CAR of a cons cell is done with setcar
. When
used on a list, setcar
replaces one element of a list with a
different element.
(setq x '(1 2)) => (1 2) (setcar x 4) => 4 x => (4 2)
When a cons cell is part of the shared structure of several lists, storing a new CAR into the cons changes one element of each of these lists. Here is an example:
;; Create two lists that are partly shared. (setq x1 '(a b c)) => (a b c) (setq x2 (cons 'z (cdr x1))) => (z b c) ;; Replace the CAR of a shared link. (setcar (cdr x1) 'foo) => foo x1 ; Both lists are changed. => (a foo c) x2 => (z foo c) ;; Replace the CAR of a link that is not shared. (setcar x1 'baz) => baz x1 ; Only one list is changed. => (baz foo c) x2 => (z foo c)
Here is a graphical depiction of the shared structure of the two lists
in the variables x1
and x2
, showing why replacing b
changes them both:
--- --- --- --- --- --- x1---> | | |----> | | |--> | | |--> nil --- --- --- --- --- --- | --> | | | | | | --> a | --> b --> c | --- --- | x2--> | | |-- --- --- | | --> z
Here is an alternative form of box diagram, showing the same relationship:
x1: -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | a | o------->| b | o------->| c | nil | | | | -->| | | | | | -------------- | -------------- -------------- | x2: | -------------- | | car | cdr | | | z | o---- | | | --------------
The lowest-level primitive for modifying a CDR is setcdr
:
Here is an example of replacing the CDR of a list with a different list. All but the first element of the list are removed in favor of a different sequence of elements. The first element is unchanged, because it resides in the CAR of the list, and is not reached via the CDR.
(setq x '(1 2 3)) => (1 2 3) (setcdr x '(4)) => (4) x => (1 4)
You can delete elements from the middle of a list by altering the
CDRs of the cons cells in the list. For example, here we delete
the second element, b
, from the list (a b c)
, by changing
the CDR of the first cons cell:
(setq x1 '(a b c)) => (a b c) (setcdr x1 (cdr (cdr x1))) => (c) x1 => (a c)
Here is the result in box notation:
-------------------- | | -------------- | -------------- | -------------- | car | cdr | | | car | cdr | -->| car | cdr | | a | o----- | b | o-------->| c | nil | | | | | | | | | | -------------- -------------- --------------
The second cons cell, which previously held the element b
, still
exists and its CAR is still b
, but it no longer forms part
of this list.
It is equally easy to insert a new element by changing CDRs:
(setq x1 '(a b c)) => (a b c) (setcdr x1 (cons 'd (cdr x1))) => (d b c) x1 => (a d b c)
Here is this result in box notation:
-------------- ------------- ------------- | car | cdr | | car | cdr | | car | cdr | | a | o | -->| b | o------->| c | nil | | | | | | | | | | | | --------- | -- | ------------- ------------- | | ----- -------- | | | --------------- | | | car | cdr | | -->| d | o------ | | | ---------------
Here are some functions that rearrange lists "destructively" by modifying the CDRs of their component cons cells. We call these functions "destructive" because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value.
The function delq
in the following section is another example
of destructive list manipulation.
append
(see section Building Cons Cells and Lists), the lists are
not copied. Instead, the last CDR of each of the
lists is changed to refer to the following list. The last of the
lists is not altered. For example:
(setq x '(1 2 3)) => (1 2 3) (nconc x '(4 5)) => (1 2 3 4 5) x => (1 2 3 4 5)
Since the last argument of nconc
is not itself modified, it is
reasonable to use a constant list, such as '(4 5)
, as in the
above example. For the same reason, the last argument need not be a
list:
(setq x '(1 2 3)) => (1 2 3) (nconc x 'z) => (1 2 3 . z) x => (1 2 3 . z)
However, the other arguments (all but the last) must be lists.
A common pitfall is to use a quoted constant list as a non-last
argument to nconc
. If you do this, your program will change
each time you run it! Here is what happens:
(defun add-foo (x) ; We want this function to add
(nconc '(foo) x)) ; foo
to the front of its arg.
(symbol-function 'add-foo)
=> (lambda (x) (nconc (quote (foo)) x))
(setq xx (add-foo '(1 2))) ; It seems to work.
=> (foo 1 2)
(setq xy (add-foo '(3 4))) ; What happened?
=> (foo 1 2 3 4)
(eq xx xy)
=> t
(symbol-function 'add-foo)
=> (lambda (x) (nconc (quote (foo 1 2 3 4) x)))
reverse
, nreverse
alters its argument by reversing
the CDRs in the cons cells forming the list. The cons cell that
used to be the last one in list becomes the first cons cell of the
value.
For example:
(setq x '(1 2 3 4)) => (1 2 3 4) x => (1 2 3 4) (nreverse x) => (4 3 2 1) ;; The cons cell that was first is now last. x => (1)
To avoid confusion, we usually store the result of nreverse
back in the same variable which held the original list:
(setq x (nreverse x))
Here is the nreverse
of our favorite example, (a b c)
,
presented graphically:
Original list head: Reversed list: ------------- ------------- ------------ | car | cdr | | car | cdr | | car | cdr | | a | nil |<-- | b | o |<-- | c | o | | | | | | | | | | | | | | ------------- | --------- | - | -------- | - | | | | ------------- ------------
The argument predicate must be a function that accepts two
arguments. It is called with two elements of list. To get an
increasing order sort, the predicate should return t
if the
first element is "less than" the second, or nil
if not.
The comparison function predicate must give reliable results for
any given pair of arguments, at least within a single call to
sort
. It must be antisymmetric; that is, if a is
less than b, b must not be less than a. It must be
transitive---that is, if a is less than b, and b
is less than c, then a must be less than c. If you
use a comparison function which does not meet these requirements, the
result of sort
is unpredictable.
The destructive aspect of sort
is that it rearranges the cons
cells forming list by changing CDRs. A nondestructive sort
function would create new cons cells to store the elements in their
sorted order. If you wish to make a sorted copy without destroying the
original, copy it first with copy-sequence
and then sort.
Sorting does not change the CARs of the cons cells in list;
the cons cell that originally contained the element a
in
list still has a
in its CAR after sorting, but it now
appears in a different position in the list due to the change of
CDRs. For example:
(setq nums '(1 3 2 6 5 4 0)) => (1 3 2 6 5 4 0) (sort nums '<) => (0 1 2 3 4 5 6) nums => (1 2 3 4 5 6)
Warning: Note that the list in nums
no longer contains
0; this is the same cons cell that it was before, but it is no longer
the first one in the list. Don't assume a variable that formerly held
the argument now holds the entire sorted list! Instead, save the result
of sort
and use that. Most often we store the result back into
the variable that held the original list:
(setq nums (sort nums '<))
See section Sorting Text, for more functions that perform sorting.
See documentation
in section Access to Documentation Strings, for a
useful example of sort
.
A list can represent an unordered mathematical set--simply consider a
value an element of a set if it appears in the list, and ignore the
order of the list. To form the union of two sets, use append
(as
long as you don't mind having duplicate elements). Other useful
functions for sets include memq
and delq
, and their
equal
versions, member
and delete
.
Common Lisp note: Common Lisp has functions
union
(which avoids duplicate elements) andintersection
for set operations, but GNU Emacs Lisp does not have them. You can write them in Lisp if you wish.
memq
returns a list starting with the
first occurrence of object. Otherwise, it returns nil
.
The letter `q' in memq
says that it uses eq
to
compare object against the elements of the list. For example:
(memq 'b '(a b c b a)) => (b c b a) (memq '(2) '((1) (2))) ;(2)
and(2)
are noteq
. => nil
eq
to
object from list. The letter `q' in delq
says
that it uses eq
to compare object against the elements of
the list, like memq
.
When delq
deletes elements from the front of the list, it does so
simply by advancing down the list and returning a sublist that starts
after those elements:
(delq 'a '(a b c)) == (cdr '(a b c))
When an element to be deleted appears in the middle of the list, removing it involves changing the CDRs (see section Altering the CDR of a List).
(setq sample-list '(a b c (4))) => (a b c (4)) (delq 'a sample-list) => (b c (4)) sample-list => (a b c (4)) (delq 'c sample-list) => (a b (4)) sample-list => (a b (4))
Note that (delq 'c sample-list)
modifies sample-list
to
splice out the third element, but (delq 'a sample-list)
does not
splice anything--it just returns a shorter list. Don't assume that a
variable which formerly held the argument list now has fewer
elements, or that it still holds the original list! Instead, save the
result of delq
and use that. Most often we store the result back
into the variable that held the original list:
(setq flowers (delq 'rose flowers))
In the following example, the (4)
that delq
attempts to match
and the (4)
in the sample-list
are not eq
:
(delq '(4) sample-list) => (a c (4))
The following two functions are like memq
and delq
but use
equal
rather than eq
to compare elements. See section Equality Predicates.
member
tests to see whether object is a member
of list, comparing members with object using equal
.
If object is a member, member
returns a list starting with
its first occurrence in list. Otherwise, it returns nil
.
Compare this with memq
:
(member '(2) '((1) (2))) ;(2)
and(2)
areequal
. => ((2)) (memq '(2) '((1) (2))) ;(2)
and(2)
are noteq
. => nil ;; Two strings with the same contents areequal
. (member "foo" '("foo" "bar")) => ("foo" "bar")
equal
to
object from list. It is to delq
as member
is
to memq
: it uses equal
to compare elements with
object, like member
; when it finds an element that matches,
it removes the element just as delq
would. For example:
(delete '(2) '((2) (1) (2))) => ((1))
Common Lisp note: The functions
member
anddelete
in GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not useequal
to compare elements.
See also the function add-to-list
, in section How to Alter a Variable Value,
for another way to add an element to a list stored in a variable.
An association list, or alist for short, records a mapping from keys to values. It is a list of cons cells called associations: the CAR of each cons cell is the key, and the CDR is the associated value.(1)
Here is an example of an alist. The key pine
is associated with
the value cones
; the key oak
is associated with
acorns
; and the key maple
is associated with seeds
.
'((pine . cones) (oak . acorns) (maple . seeds))
The associated values in an alist may be any Lisp objects; so may the
keys. For example, in the following alist, the symbol a
is
associated with the number 1
, and the string "b"
is
associated with the list (2 3)
, which is the CDR of
the alist element:
((a . 1) ("b" 2 3))
Sometimes it is better to design an alist to store the associated value in the CAR of the CDR of the element. Here is an example:
'((rose red) (lily white) (buttercup yellow))
Here we regard red
as the value associated with rose
. One
advantage of this kind of alist is that you can store other related
information--even a list of other items--in the CDR of the
CDR. One disadvantage is that you cannot use rassq
(see
below) to find the element containing a given value. When neither of
these considerations is important, the choice is a matter of taste, as
long as you are consistent about it for any given alist.
Note that the same alist shown above could be regarded as having the
associated value in the CDR of the element; the value associated
with rose
would be the list (red)
.
Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one.
In Emacs Lisp, it is not an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases.
Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. See section Property Lists, for a comparison of property lists and association lists.
equal
(see section Equality Predicates). It returns nil
if no
association in alist has a CAR equal
to key.
For example:
(setq trees '((pine . cones) (oak . acorns) (maple . seeds))) => ((pine . cones) (oak . acorns) (maple . seeds)) (assoc 'oak trees) => (oak . acorns) (cdr (assoc 'oak trees)) => acorns (assoc 'birch trees) => nil
Here is another example, in which the keys and values are not symbols:
(setq needles-per-cluster '((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine"))) (cdr (assoc 3 needles-per-cluster)) => ("Pitch Pine") (cdr (assoc 2 needles-per-cluster)) => ("Austrian Pine" "Red Pine")
The functions assoc-ignore-representation
and
assoc-ignore-case
are much like assoc
except using
compare-strings
to do the comparison. See section Comparison of Characters and Strings.
nil
if no association in alist has
a CDR equal
to value.
rassoc
is like assoc
except that it compares the CDR of
each alist association instead of the CAR. You can think of
this as "reverse assoc
", finding the key for a given value.
assoc
in that it returns the first
association for key in alist, but it makes the comparison
using eq
instead of equal
. assq
returns nil
if no association in alist has a CAR eq
to key.
This function is used more often than assoc
, since eq
is
faster than equal
and most alists use symbols as keys.
See section Equality Predicates.
(setq trees '((pine . cones) (oak . acorns) (maple . seeds))) => ((pine . cones) (oak . acorns) (maple . seeds)) (assq 'pine trees) => (pine . cones)
On the other hand, assq
is not usually useful in alists where the
keys may not be symbols:
(setq leaves '(("simple leaves" . oak) ("compound leaves" . horsechestnut))) (assq "simple leaves" leaves) => nil (assoc "simple leaves" leaves) => ("simple leaves" . oak)
nil
if no association in alist has
a CDR eq
to value.
rassq
is like assq
except that it compares the CDR of
each alist association instead of the CAR. You can think of
this as "reverse assq
", finding the key for a given value.
For example:
(setq trees '((pine . cones) (oak . acorns) (maple . seeds))) (rassq 'acorns trees) => (oak . acorns) (rassq 'spores trees) => nil
Note that rassq
cannot search for a value stored in the CAR
of the CDR of an element:
(setq colors '((rose red) (lily white) (buttercup yellow))) (rassq 'white colors) => nil
In this case, the CDR of the association (lily white)
is not
the symbol white
, but rather the list (white)
. This
becomes clearer if the association is written in dotted pair notation:
(lily white) == (lily . (white))
string-match
with an alist that contains
regular expressions (see section Regular Expression Searching). If test is omitted
or nil
, equal
is used for comparison.
If an alist element matches key by this criterion,
then assoc-default
returns a value based on this element.
If the element is a cons, then the value is the element's CDR.
Otherwise, the return value is default.
If no alist element matches key, assoc-default
returns
nil
.
(setq needles-per-cluster '((2 . ("Austrian Pine" "Red Pine")) (3 . ("Pitch Pine")) (5 . ("White Pine")))) => ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (setq copy (copy-alist needles-per-cluster)) => ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (eq needles-per-cluster copy) => nil (equal needles-per-cluster copy) => t (eq (car needles-per-cluster) (car copy)) => nil (cdr (car (cdr needles-per-cluster))) => ("Pitch Pine") (eq (cdr (car (cdr needles-per-cluster))) (cdr (car (cdr copy)))) => t
This example shows how copy-alist
makes it possible to change
the associations of one copy without affecting the other:
(setcdr (assq 3 copy) '("Martian Vacuum Pine")) (cdr (assq 3 needles-per-cluster)) => ("Pitch Pine")
Recall that the sequence type is the union of two other Lisp types: lists and arrays. In other words, any list is a sequence, and any array is a sequence. The common property that all sequences have is that each is an ordered collection of elements.
An array is a single primitive object that has a slot for each of its elements. All the elements are accessible in constant time, but the length of an existing array cannot be changed. Strings, vectors, char-tables and bool-vectors are the four types of arrays.
A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. Finding the nth element requires looking through n cons cells, so elements farther from the beginning of the list take longer to access. But it is possible to add elements to the list, or remove elements.
The following diagram shows the relationship between these types:
_____________________________________________ | | | Sequence | | ______ ________________________________ | | | | | | | | | List | | Array | | | | | | ________ ________ | | | |______| | | | | | | | | | | Vector | | String | | | | | |________| |________| | | | | ____________ _____________ | | | | | | | | | | | | | Char-table | | Bool-vector | | | | | |____________| |_____________| | | | |________________________________| | |_____________________________________________|
The elements of vectors and lists may be any Lisp objects. The elements of strings are all characters.
In Emacs Lisp, a sequence is either a list or an array. The common property of all sequences is that they are ordered collections of elements. This section describes functions that accept any kind of sequence.
t
if object is a list, vector, or
string, nil
otherwise.
nil
), a wrong-type-argument
error is
signaled.
See section Accessing Elements of Lists, for the related function safe-length
.
(length '(1 2 3)) => 3 (length ()) => 0 (length "foobar") => 6 (length [1 2 3]) => 3 (length (make-bool-vector 5 nil)) => 5
nil
;
otherwise, they trigger an args-out-of-range
error.
(elt [1 2 3 4] 2) => 3 (elt '(1 2 3 4) 2) => 3 ;; We usestring
to show clearly which characterelt
returns. (string (elt "1234" 2)) => "3" (elt [1 2 3 4] 4) error--> Args out of range: [1 2 3 4], 4 (elt [1 2 3 4] -1) error--> Args out of range: [1 2 3 4], -1
This function generalizes aref
(see section Functions that Operate on Arrays) and
nth
(see section Accessing Elements of Lists).
Storing a new element into the copy does not affect the original
sequence, and vice versa. However, the elements of the new
sequence are not copies; they are identical (eq
) to the elements
of the original. Therefore, changes made within these elements, as
found via the copied sequence, are also visible in the original
sequence.
If the sequence is a string with text properties, the property list in the copy is itself a copy, not shared with the original's property list. However, the actual values of the properties are shared. See section Text Properties.
See also append
in section Building Cons Cells and Lists, concat
in
section Creating Strings, and vconcat
in section Vectors, for others
ways to copy sequences.
(setq bar '(1 2)) => (1 2) (setq x (vector 'foo bar)) => [foo (1 2)] (setq y (copy-sequence x)) => [foo (1 2)] (eq x y) => nil (equal x y) => t (eq (elt x 1) (elt y 1)) => t ;; Replacing an element of one sequence. (aset x 0 'quux) x => [quux (1 2)] y => [foo (1 2)] ;; Modifying the inside of a shared element. (setcar (aref x 1) 69) x => [quux (69 2)] y => [foo (69 2)]
An array object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, an element of a list requires access time that is proportional to the position of the element in the list.
Emacs defines four types of array, all one-dimensional: strings, vectors, bool-vectors and char-tables. A vector is a general array; its elements can be any Lisp objects. A string is a specialized array; its elements must be characters (i.e., integers between 0 and 255). Each type of array has its own read syntax. See section String Type, and section Vector Type.
All four kinds of array share these characteristics:
aref
and aset
, respectively (see section Functions that Operate on Arrays).
When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes.
In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:
By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are outside the range that will fit in a string. See section Key Sequence Input.
In this section, we describe the functions that accept all types of arrays.
t
if object is an array (i.e., a
vector, a string, a bool-vector or a char-table).
(arrayp [a]) => t (arrayp "asdf") => t (arrayp (syntax-table)) ;; A char-table. => t
(setq primes [2 3 5 7 11 13]) => [2 3 5 7 11 13] (aref primes 4) => 11 (aref "abcdefg" 1) => 98 ; `b' is ASCII code 98.
See also the function elt
, in section Sequences.
(setq w [foo bar baz]) => [foo bar baz] (aset w 0 'fu) => fu w => [fu bar baz] (setq x "asdfasfd") => "asdfasfd" (aset x 3 ?Z) => 90 x => "asdZasfd"
If array is a string and object is not a character, a
wrong-type-argument
error results. If array is a string
and object is character, but object does not use the same
number of bytes as the character currently stored in (aref
object index)
, that is also an error. See section Splitting Characters.
(setq a [a b c d e f g]) => [a b c d e f g] (fillarray a 0) => [0 0 0 0 0 0 0] a => [0 0 0 0 0 0 0] (setq s "When in the course") => "When in the course" (fillarray s ?-) => "------------------"
If array is a string and object is not a character, a
wrong-type-argument
error results.
The general sequence functions copy-sequence
and length
are often useful for objects known to be arrays. See section Sequences.
Arrays in Lisp, like arrays in most languages, are blocks of memory whose elements can be accessed in constant time. A vector is a general-purpose array of specified length; its elements can be any Lisp objects. (By contrast, a string can hold only characters as elements.) Vectors in Emacs are used for obarrays (vectors of symbols), and as part of keymaps (vectors of commands). They are also used internally as part of the representation of a byte-compiled function; if you print such a function, you will see a vector in it.
In Emacs Lisp, the indices of the elements of a vector start from zero and count up from there.
Vectors are printed with square brackets surrounding the elements.
Thus, a vector whose elements are the symbols a
, b
and
a
is printed as [a b a]
. You can write vectors in the
same way in Lisp input.
A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. See section Self-Evaluating Forms.
Here are examples illustrating these principles:
(setq avector [1 two '(three) "four" [five]]) => [1 two (quote (three)) "four" [five]] (eval avector) => [1 two (quote (three)) "four" [five]] (eq avector (eval avector)) => t
Here are some functions that relate to vectors:
t
if object is a vector.
(vectorp [a]) => t (vectorp "asdf") => nil
(vector 'foo 23 [bar baz] "rats") => [foo 23 [bar baz] "rats"] (vector) => []
(setq sleepy (make-vector 9 'Z)) => [Z Z Z Z Z Z Z Z Z]
The value is a newly constructed vector that is not eq
to any
existing vector.
(setq a (vconcat '(A B C) '(D E F))) => [A B C D E F] (eq a (vconcat a)) => nil (vconcat) => [] (vconcat [A B C] "aa" '(foo (6 7))) => [A B C 97 97 foo (6 7)]
The vconcat
function also allows byte-code function objects as
arguments. This is a special feature to make it easy to access the entire
contents of a byte-code function object. See section Byte-Code Function Objects.
The vconcat
function also allows integers as arguments. It
converts them to strings of digits, making up the decimal print
representation of the integer, and then uses the strings instead of the
original integers. Don't use this feature; we plan to eliminate
it. If you already use this feature, change your programs now! The
proper way to convert an integer to a decimal number in this way is with
format
(see section Formatting Strings) or number-to-string
(see section Conversion of Characters and Strings).
For other concatenation functions, see mapconcat
in section Mapping Functions, concat
in section Creating Strings, and append
in section Building Cons Cells and Lists.
The append
function provides a way to convert a vector into a
list with the same elements (see section Building Cons Cells and Lists):
(setq avector [1 two (quote (three)) "four" [five]]) => [1 two (quote (three)) "four" [five]] (append avector nil) => (1 two (quote (three)) "four" [five])
A char-table is much like a vector, except that it is indexed by
character codes. Any valid character code, without modifiers, can be
used as an index in a char-table. You can access a char-table's
elements with aref
and aset
, as with any array. In
addition, a char-table can have extra slots to hold additional
data not associated with particular character codes. Char-tables are
constants when evaluated.
Each char-table has a subtype which is a symbol. The subtype
has two purposes: to distinguish char-tables meant for different uses,
and to control the number of extra slots. For example, display tables
are char-tables with display-table
as the subtype, and syntax
tables are char-tables with syntax-table
as the subtype. A valid
subtype must have a char-table-extra-slots
property which is an
integer between 0 and 10. This integer specifies the number of
extra slots in the char-table.
A char-table can have a parent. which is another char-table. If
it does, then whenever the char-table specifies nil
for a
particular character c, it inherits the value specified in the
parent. In other words, (aref char-table c)
returns
the value from the parent of char-table if char-table itself
specifies nil
.
A char-table can also have a default value. If so, then
(aref char-table c)
returns the default value
whenever the char-table does not specify any other non-nil
value.
nil
. You
cannot alter the subtype of a char-table after the char-table is
created.
There is no argument to specify the length of the char-table, because all char-tables have room for any valid character code as an index.
t
if object is a char-table,
otherwise nil
.
There is no special function to access the default value of a char-table.
To do that, use (char-table-range char-table nil)
.
nil
or another char-table.
A char-table can specify an element value for a single character code; it can also specify a value for an entire character set.
nil
nil
t
char-table-range
---either
a valid character or a generic character--and the value is
(char-table-range char-table key)
.
Overall, the key-value pairs passed to function describe all the values stored in char-table.
The return value is always nil
; to make this function useful,
function should have side effects. For example,
here is how to examine each element of the syntax table:
(let (accumulator) (map-char-table #'(lambda (key value) (setq accumulator (cons (list key value) accumulator))) (syntax-table)) accumulator) => ((475008 nil) (474880 nil) (474752 nil) (474624 nil) ... (5 (3)) (4 (3)) (3 (3)) (2 (3)) (1 (3)) (0 (3)))
A bool-vector is much like a vector, except that it stores only the
values t
and nil
. If you try to store any non-nil
value into an element of the bool-vector, the effect is to store
t
there. As with all arrays, bool-vector indices start from 0,
and the length cannot be changed once the bool-vector is created.
Bool-vectors are constants when evaluated.
There are two special functions for working with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays.
t
if object is a bool-vector,
and nil
otherwise.
A symbol is an object with a unique name. This chapter describes symbols, their components, their property lists, and how they are created and interned. Separate chapters describe the use of symbols as variables and as function names; see section Variables, and section Functions. For the precise read syntax for symbols, see section Symbol Type.
You can test whether an arbitrary Lisp object is a symbol
with symbolp
:
t
if object is a symbol, nil
otherwise.
Each symbol has four components (or "cells"), each of which references another object:
symbol-name
in section Creating and Interning Symbols.
symbol-value
in
section Accessing Variable Values.
symbol-function
in section Accessing Function Cell Contents.
symbol-plist
in section Property Lists.
The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object.
The print name cell holds the string that is the name of the symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. (In GNU Emacs Lisp, this lookup uses a hashing algorithm and an obarray; see section Creating and Interning Symbols.)
In normal usage, the function cell usually contains a function
(see section Functions) or a macro (see section Macros), as that is what the
Lisp interpreter expects to see there (see section Evaluation). Keyboard
macros (see section Keyboard Macros), keymaps (see section Keymaps) and autoload
objects (see section Autoloading) are also sometimes stored in the function
cells of symbols. We often refer to "the function foo
" when we
really mean the function stored in the function cell of the symbol
foo
. We make the distinction only when necessary.
The property list cell normally should hold a correctly formatted property list (see section Property Lists), as a number of functions expect to see a property list there.
The function cell or the value cell may be void, which means
that the cell does not reference any object. (This is not the same
thing as holding the symbol void
, nor the same as holding the
symbol nil
.) Examining a function or value cell that is void
results in an error, such as `Symbol's value as variable is void'.
The four functions symbol-name
, symbol-value
,
symbol-plist
, and symbol-function
return the contents of
the four cells of a symbol. Here as an example we show the contents of
the four cells of the symbol buffer-file-name
:
(symbol-name 'buffer-file-name) => "buffer-file-name" (symbol-value 'buffer-file-name) => "/gnu/elisp/symbols.texi" (symbol-plist 'buffer-file-name) => (variable-documentation 29529) (symbol-function 'buffer-file-name) => #<subr buffer-file-name>
Because this symbol is the variable which holds the name of the file
being visited in the current buffer, the value cell contents we see are
the name of the source file of this chapter of the Emacs Lisp Manual.
The property list cell contains the list (variable-documentation
29529)
which tells the documentation functions where to find the
documentation string for the variable buffer-file-name
in the
`DOC-version' file. (29529 is the offset from the beginning
of the `DOC-version' file to where that documentation string
begins--see section Documentation Basics.) The function cell contains
the function for returning the name of the file.
buffer-file-name
names a primitive function, which has no read
syntax and prints in hash notation (see section Primitive Function Type). A
symbol naming a function written in Lisp would have a lambda expression
(or a byte-code object) in this cell.
A definition in Lisp is a special form that announces your intention to use a certain symbol in a particular way. In Emacs Lisp, you can define a symbol as a variable, or define it as a function (or macro), or both independently.
A definition construct typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable.
defvar
and defconst
are special forms that define a
symbol as a global variable. They are documented in detail in
section Defining Global Variables. For defining user option variables that can
be customized, use defcustom
(see section Writing Customization Definitions).
defun
defines a symbol as a function, creating a lambda
expression and storing it in the function cell of the symbol. This
lambda expression thus becomes the function definition of the symbol.
(The term "function definition", meaning the contents of the function
cell, is derived from the idea that defun
gives the symbol its
definition as a function.) defsubst
and defalias
are two
other ways of defining a function. See section Functions.
defmacro
defines a symbol as a macro. It creates a macro
object and stores it in the function cell of the symbol. Note that a
given symbol can be a macro or a function, but not both at once, because
both macro and function definitions are kept in the function cell, and
that cell can hold only one Lisp object at any given time.
See section Macros.
In Emacs Lisp, a definition is not required in order to use a symbol
as a variable or function. Thus, you can make a symbol a global
variable with setq
, whether you define it first or not. The real
purpose of definitions is to guide programmers and programming tools.
They inform programmers who read the code that certain symbols are
intended to be used as variables, or as functions. In addition,
utilities such as `etags' and `make-docfile' recognize
definitions, and add appropriate information to tag tables and the
`DOC-version' file. See section Access to Documentation Strings.
To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion.
When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it "hashes" those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code.
If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called interning it, and the symbol is then called an interned symbol.
Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray.
No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called uninterned symbols. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable.
In Emacs Lisp, an obarray is actually a vector. Each element of the
vector is a bucket; its value is either an interned symbol whose name
hashes to that bucket, or 0 if the bucket is empty. Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket. Because these links are invisible, there is no way to
find all the symbols in an obarray except using mapatoms
(below).
The order of symbols in a bucket is not significant.
In an empty obarray, every element is 0, and you can create an obarray
with (make-vector length 0)
. This is the only
valid way to create an obarray. Prime numbers as lengths tend
to result in good hashing; lengths one less than a power of two are also
good.
Do not try to put symbols in an obarray yourself. This does
not work--only intern
can enter a symbol in an obarray properly.
Common Lisp note: In Common Lisp, a single symbol may be interned in several obarrays.
Most of the functions below take a name and sometimes an obarray as
arguments. A wrong-type-argument
error is signaled if the name
is not a string, or if the obarray is not a vector.
(symbol-name 'foo) => "foo"
Warning: Changing the string by substituting characters does change the name of the symbol, but fails to update the obarray, so don't do it!
nil
. In the example below,
the value of sym
is not eq
to foo
because it is a
distinct uninterned symbol whose name is also `foo'.
(setq sym (make-symbol "foo")) => foo (eq sym 'foo) => nil
intern
creates a new one, adds it to the obarray, and returns it. If
obarray is omitted, the value of the global variable
obarray
is used.
(setq sym (intern "foo")) => foo (eq sym 'foo) => t (setq sym1 (intern "foo" other-obarray)) => foo (eq sym 'foo) => nil
Common Lisp note: In Common Lisp, you can intern an existing symbol in an obarray. In Emacs Lisp, you cannot do this, because the argument to
intern
must be a string, not a symbol.
nil
if obarray has no symbol with that name.
Therefore, you can use intern-soft
to test whether a symbol with
a given name is already interned. If obarray is omitted, the
value of the global variable obarray
is used.
(intern-soft "frazzle") ; No such symbol exists. => nil (make-symbol "frazzle") ; Create an uninterned one. => frazzle (intern-soft "frazzle") ; That one cannot be found. => nil (setq sym (intern "frazzle")) ; Create an interned one. => frazzle (intern-soft "frazzle") ; That one can be found! => frazzle (eq sym 'frazzle) ; And it is the same one. => t
intern
and
read
.
nil
. If obarray is
omitted, it defaults to the value of obarray
, the standard
obarray for ordinary symbols.
(setq count 0) => 0 (defun count-syms (s) (setq count (1+ count))) => count-syms (mapatoms 'count-syms) => nil count => 1871
See documentation
in section Access to Documentation Strings, for another
example using mapatoms
.
symbol
is not actually in the obarray, unintern
does
nothing. If obarray is nil
, the current obarray is used.
If you provide a string instead of a symbol as symbol, it stands
for a symbol name. Then unintern
deletes the symbol (if any) in
the obarray which has that name. If there is no such symbol,
unintern
does nothing.
If unintern
does delete a symbol, it returns t
. Otherwise
it returns nil
.
A property list (plist for short) is a list of paired elements stored in the property list cell of a symbol. Each of the pairs associates a property name (usually a symbol) with a property or value. Property lists are generally used to record information about a symbol, such as its documentation as a variable, the name of the file where it was defined, or perhaps even the grammatical class of the symbol (representing a word) in a language-understanding system.
Character positions in a string or buffer can also have property lists. See section Text Properties.
The property names and values in a property list can be any Lisp
objects, but the names are usually symbols. Property list functions
compare the property names using eq
. Here is an example of a
property list, found on the symbol progn
when the compiler is
loaded:
(lisp-indent-function 0 byte-compile byte-compile-progn)
Here lisp-indent-function
and byte-compile
are property
names, and the other two elements are the corresponding values.
Association lists (see section Association Lists) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant since the property names must be distinct.
Property lists are better than association lists for attaching
information to various Lisp function names or variables. If your
program keeps all of its associations in one association list, it will
typically need to search that entire list each time it checks for an
association. This could be slow. By contrast, if you keep the same
information in the property lists of the function names or variables
themselves, each search will scan only the length of one property list,
which is usually short. This is why the documentation for a variable is
recorded in a property named variable-documentation
. The byte
compiler likewise uses properties to record those functions needing
special treatment.
However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by beginning the property name with the program's usual name-prefix for variables and functions.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list.
(setplist 'foo '(a 1 b (2 3) c nil)) => (a 1 b (2 3) c nil) (symbol-plist 'foo) => (a 1 b (2 3) c nil)
For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (see section Abbrevs And Abbrev Expansion).
nil
is returned. Thus, there is no distinction between a value of
nil
and the absence of the property.
The name property is compared with the existing property names
using eq
, so any object is a legitimate property.
See put
for an example.
put
function returns value.
(put 'fly 'verb 'transitive) =>'transitive (put 'fly 'noun '(a buzzing little bug)) => (a buzzing little bug) (get 'fly 'verb) => transitive (symbol-plist 'fly) => (verb transitive noun (a buzzing little bug))
These two functions are useful for manipulating property lists that are stored in places other than symbols:
(plist-get '(foo 4) 'foo) => 4
(setq my-plist '(bar t foo 4)) => (bar t foo 4) (setq my-plist (plist-put my-plist 'foo 69)) => (bar t foo 69) (setq my-plist (plist-put my-plist 'quux '(a))) => (bar t foo 69 quux (a))
You could define put
in terms of plist-put
as follows:
(defun put (symbol prop value) (setplist symbol (plist-put (symbol-plist symbol) prop value)))
The evaluation of expressions in Emacs Lisp is performed by the
Lisp interpreter---a program that receives a Lisp object as input
and computes its value as an expression. How it does this depends
on the data type of the object, according to rules described in this
chapter. The interpreter runs automatically to evaluate portions of
your program, but can also be called explicitly via the Lisp primitive
function eval
.
A Lisp object that is intended for evaluation is called an expression or a form. The fact that expressions are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often.
It is very common to read a Lisp expression and then evaluate the
expression, but reading and evaluation are separate activities, and
either can be performed alone. Reading per se does not evaluate
anything; it converts the printed representation of a Lisp object to the
object itself. It is up to the caller of read
whether this
object is a form to be evaluated, or serves some entirely different
purpose. See section Input Functions.
Do not confuse evaluation with command key interpretation. The
editor command loop translates keyboard input into a command (an
interactively callable function) using the active keymaps, and then
uses call-interactively
to invoke the command. The execution of
the command itself involves evaluation if the command is written in
Lisp, but that is not a part of command key interpretation itself.
See section Command Loop.
Evaluation is a recursive process. That is, evaluation of a form may
call eval
to evaluate parts of the form. For example, evaluation
of a function call first evaluates each argument of the function call,
and then evaluates each form in the function body. Consider evaluation
of the form (car x)
: the subform x
must first be evaluated
recursively, so that its value can be passed as an argument to the
function car
.
Evaluation of a function call ultimately calls the function specified in it. See section Functions. The execution of the function may itself work by evaluating the function definition; or the function may be a Lisp primitive implemented in C, or it may be a byte-compiled function (see section Byte Compilation).
The evaluation of forms takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables.(2) Whenever a form refers to a variable without creating a new binding for it, the value of the variable's binding in the current environment is used. See section Variables.
Evaluation of a form may create new environments for recursive
evaluation by binding variables (see section Local Variables). These
environments are temporary and vanish by the time evaluation of the form
is complete. The form may also make changes that persist; these changes
are called side effects. An example of a form that produces side
effects is (setq foo 1)
.
The details of what evaluation means for each kind of form are described below (see section Kinds of Forms).
A Lisp object that is intended to be evaluated is called a form. How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and "all other types". This section describes all three kinds, one by one, starting with the "all other types" which are self-evaluating forms.
A self-evaluating form is any form that is not a list or symbol.
Self-evaluating forms evaluate to themselves: the result of evaluation
is the same object that was evaluated. Thus, the number 25 evaluates to
25, and the string "foo"
evaluates to the string "foo"
.
Likewise, evaluation of a vector does not cause evaluation of the
elements of the vector--it returns the same vector with its contents
unchanged.
'123 ; A number, shown without evaluation. => 123 123 ; Evaluated as usual---result is the same. => 123 (eval '123) ; Evaluated ``by hand''---result is the same. => 123 (eval (eval '123)) ; Evaluating twice changes nothing. => 123
It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there's no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example:
;; Build an expression containing a buffer object. (setq print-exp (list 'print (current-buffer))) => (print #<buffer eval.texi>) ;; Evaluate it. (eval print-exp) -| #<buffer eval.texi> => #<buffer eval.texi>
When a symbol is evaluated, it is treated as a variable. The result is the variable's value, if it has one. If it has none (if its value cell is void), an error is signaled. For more information on the use of variables, see section Variables.
In the following example, we set the value of a symbol with
setq
. Then we evaluate the symbol, and get back the value that
setq
stored.
(setq a 123) => 123 (eval 'a) => 123 a => 123
The symbols nil
and t
are treated specially, so that the
value of nil
is always nil
, and the value of t
is
always t
; you cannot set or bind them to any other values. Thus,
these two symbols act like self-evaluating forms, even though
eval
treats them like any other symbol. A symbol whose name
starts with `:' also self-evaluates in the same way; likewise,
its value ordinarily cannot be changed. See section Variables That Never Change.
A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The remaining list elements constitute the arguments for the function, macro, or special form.
The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is not evaluated, as it would be in some Lisp dialects such as Scheme.
If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called symbol function indirection, is repeated until it obtains a non-symbol. See section Naming a Function, for more information about using a symbol as a name for a function stored in the function cell of the symbol.
One possible consequence of this process is an infinite loop, in the
event that a symbol's function cell refers to the same symbol. Or a
symbol may have a void function cell, in which case the subroutine
symbol-function
signals a void-function
error. But if
neither of these things happens, we eventually obtain a non-symbol,
which ought to be a function or other suitable object.
More precisely, we should now have a Lisp function (a lambda
expression), a byte-code function, a primitive function, a Lisp macro, a
special form, or an autoload object. Each of these types is a case
described in one of the following sections. If the object is not one of
these types, the error invalid-function
is signaled.
The following example illustrates the symbol indirection process. We
use fset
to set the function cell of a symbol and
symbol-function
to get the function cell contents
(see section Accessing Function Cell Contents). Specifically, we store the symbol car
into the function cell of first
, and the symbol first
into
the function cell of erste
.
;; Build this function cell linkage: ;; ------------- ----- ------- ------- ;; | #<subr car> | <-- | car | <-- | first | <-- | erste | ;; ------------- ----- ------- -------
(symbol-function 'car)
=> #<subr car>
(fset 'first 'car)
=> car
(fset 'erste 'first)
=> first
(erste '(1 2 3)) ; Call the function referenced by erste
.
=> 1
By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol.
((lambda (arg) (erste arg)) '(1 2 3)) => 1
Executing the function itself evaluates its body; this does involve
symbol function indirection when calling erste
.
The built-in function indirect-function
provides an easy way to
perform symbol function indirection explicitly.
Here is how you could define indirect-function
in Lisp:
(defun indirect-function (function) (if (symbolp function) (indirect-function (symbol-function function)) function))
If the first element of a list being evaluated is a Lisp function
object, byte-code object or primitive function object, then that list is
a function call. For example, here is a call to the function
+
:
(+ 1 x)
The first step in evaluating a function call is to evaluate the
remaining elements of the list from left to right. The results are the
actual argument values, one value for each list element. The next step
is to call the function with this list of arguments, effectively using
the function apply
(see section Calling Functions). If the function
is written in Lisp, the arguments are used to bind the argument
variables of the function (see section Lambda Expressions); then the forms
in the function body are evaluated in order, and the value of the last
body form becomes the value of the function call.
If the first element of a list being evaluated is a macro object, then the list is a macro call. When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the expansion of the macro, to be evaluated in place of the original form. The expansion may be any sort of form: a self-evaluating constant, a symbol, or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results.
Ordinary evaluation of a macro call finishes by evaluating the expansion. However, the macro expansion is not necessarily evaluated right away, or at all, because other programs also expand macro calls, and they may or may not evaluate the expansions.
Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are computed when the expansion is evaluated.
For example, given a macro defined as follows:
(defmacro cadr (x) (list 'car (list 'cdr x)))
an expression such as (cadr (assq 'handler list))
is a macro
call, and its expansion is:
(car (cdr (assq 'handler list)))
Note that the argument (assq 'handler list)
appears in the
expansion.
See section Macros, for a complete description of Emacs Lisp macros.
A special form is a primitive function specially marked so that its arguments are not all evaluated. Most special forms define control structures or perform variable bindings--things which functions cannot do.
Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments.
Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described.
and
catch
catch
and throw
cond
condition-case
defconst
defmacro
defun
defvar
function
if
interactive
let
let*
or
prog1
prog2
progn
quote
save-current-buffer
save-excursion
save-restriction
save-window-excursion
setq
setq-default
track-mouse
unwind-protect
while
with-output-to-temp-buffer
Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.
setq
,if
, andcatch
are special forms in both Emacs Lisp and Common Lisp.defun
is a special form in Emacs Lisp, but a macro in Common Lisp.save-excursion
is a special form in Emacs Lisp, but doesn't exist in Common Lisp.throw
is a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values).
The autoload feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs. It specifies which file contains the definition. When an autoload object appears as a symbol's function definition, calling that symbol as a function automatically loads the specified file; then it calls the real definition loaded from that file. See section Autoload.
The special form quote
returns its single argument, as written,
without evaluating it. This provides a way to include constant symbols
and lists, which are not self-evaluating objects, in a program. (It is
not necessary to quote self-evaluating objects such as numbers, strings,
and vectors.)
Because quote
is used so often in programs, Lisp provides a
convenient read syntax for it. An apostrophe character (`'')
followed by a Lisp object (in read syntax) expands to a list whose first
element is quote
, and whose second element is the object. Thus,
the read syntax 'x
is an abbreviation for (quote x)
.
Here are some examples of expressions that use quote
:
(quote (+ 1 2)) => (+ 1 2) (quote foo) => foo 'foo => foo ''foo => (quote foo) '(quote foo) => (quote foo) ['foo] => [(quote foo)]
Other quoting constructs include function
(see section Anonymous Functions), which causes an anonymous lambda expression written in Lisp
to be compiled, and ``' (see section Backquote), which is used to quote
only part of a list, while computing and substituting other parts.
Most often, forms are evaluated automatically, by virtue of their
occurrence in a program being run. On rare occasions, you may need to
write code that evaluates a form that is computed at run time, such as
after reading a form from text being edited or getting one from a
property list. On these occasions, use the eval
function.
The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (see section Loading).
Note: it is generally cleaner and more flexible to store a
function in a data structure, and call it with funcall
or
apply
, than to store an expression in the data structure and
evaluate it. Using functions provides the ability to pass information
to them as arguments.
Since eval
is a function, the argument expression that appears
in a call to eval
is evaluated twice: once as preparation before
eval
is called, and again by the eval
function itself.
Here is an example:
(setq foo 'bar) => bar (setq bar 'baz) => baz ;; Hereeval
receives argumentfoo
(eval 'foo) => bar ;; Hereeval
receives argumentbar
, which is the value offoo
(eval foo) => baz
The number of currently active calls to eval
is limited to
max-lisp-eval-depth
(see below).
eval
on them until the end of the region is
reached, or until an error is signaled and not handled.
If stream is non-nil
, the values that result from
evaluating the expressions in the region are printed using stream.
See section Output Streams.
If read-function is non-nil
, it should be a function, which
is used instead of read
to read expressions one by one. This
function is called with one argument, the stream for reading input. You
can also use the variable load-read-function
(see section How Programs Do Loading) to specify this function, but it is more robust to use the
read-function argument.
eval-region
always returns nil
.
eval-region
except that it operates on the whole
buffer.
eval
,
apply
, and funcall
before an error is signaled (with error
message "Lisp nesting exceeds max-lisp-eval-depth"
). This limit,
with the associated error when it is exceeded, is one way that Lisp
avoids infinite recursion on an ill-defined function.
The depth limit counts internal uses of eval
, apply
, and
funcall
, such as for calling the functions mentioned in Lisp
expressions, and recursive evaluation of function call arguments and
function body forms, as well as explicit calls in Lisp code.
The default value of this variable is 300. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
max-specpdl-size
provides another limit on nesting.
See section Local Variables.
(setq x 1) => 1 (list 'A (1+ 2) auto-save-default) => (A 3 t) values => ((A 3 t) 1 ...)
This variable is useful for referring back to values of forms recently
evaluated. It is generally a bad idea to print the value of
values
itself, since this may be very long. Instead, examine
particular elements, like this:
;; Refer to the most recent evaluation result. (nth 0 values) => (A 3 t) ;; That put a new element on, ;; so all elements move back one. (nth 1 values) => (A 3 t) ;; This gets the element that was next-to-most-recent ;; before this example. (nth 3 values) => 1
A Lisp program consists of expressions or forms (see section Kinds of Forms). We control the order of execution of the forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.
The simplest order of execution is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code--the forms are executed in the order written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b, and the function's value is the value of b.
Explicit control structures make possible an order of execution other than sequential.
Emacs Lisp provides several kinds of control structure, including other varieties of sequencing, conditionals, iteration, and (controlled) jumps--all discussed below. The built-in control structures are special forms since their subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see section Macros).
Evaluating forms in the order they appear is the most common way
control passes from one form to another. In some contexts, such as in a
function body, this happens automatically. Elsewhere you must use a
control structure construct to do this: progn
, the simplest
control construct of Lisp.
A progn
special form looks like this:
(progn a b c ...)
and it says to execute the forms a, b, c and so on, in
that order. These forms are called the body of the progn
form.
The value of the last form in the body becomes the value of the entire
progn
.
In the early days of Lisp, progn
was the only way to execute
two or more forms in succession and use the value of the last of them.
But programmers found they often needed to use a progn
in the
body of a function, where (at that time) only one form was allowed. So
the body of a function was made into an "implicit progn
":
several forms are allowed just as in the body of an actual progn
.
Many other control structures likewise contain an implicit progn
.
As a result, progn
is not used as often as it used to be. It is
needed now most often inside an unwind-protect
, and
,
or
, or in the then-part of an if
.
(progn (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" => "The third form"
Two other control constructs likewise evaluate a series of forms but return a different value:
(prog1 (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" => "The first form"
Here is a way to remove the first element from a list in the variable
x
, then return the value of that former element:
(prog1 (car x) (setq x (cdr x)))
(prog2 (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" => "The second form"
Conditional control structures choose among alternatives. Emacs Lisp
has four conditional forms: if
, which is much the same as in
other languages; when
and unless
, which are variants of
if
; and cond
, which is a generalized case statement.
if
chooses between the then-form and the else-forms
based on the value of condition. If the evaluated condition is
non-nil
, then-form is evaluated and the result returned.
Otherwise, the else-forms are evaluated in textual order, and the
value of the last one is returned. (The else part of if
is
an example of an implicit progn
. See section Sequencing.)
If condition has the value nil
, and no else-forms are
given, if
returns nil
.
if
is a special form because the branch that is not selected is
never evaluated--it is ignored. Thus, in the example below,
true
is not printed because print
is never called.
(if nil (print 'true) 'very-false) => very-false
if
where there are no else-forms,
and possibly several then-forms. In particular,
(when condition a b c)
is entirely equivalent to
(if condition (progn a b c) nil)
if
where there is no then-form:
(unless condition a b c)
is entirely equivalent to
(if condition nil a b c)
cond
chooses among an arbitrary number of alternatives. Each
clause in the cond
must be a list. The CAR of this
list is the condition; the remaining elements, if any, the
body-forms. Thus, a clause looks like this:
(condition body-forms...)
cond
tries the clauses in textual order, by evaluating the
condition of each clause. If the value of condition is
non-nil
, the clause "succeeds"; then cond
evaluates its
body-forms, and the value of the last of body-forms becomes
the value of the cond
. The remaining clauses are ignored.
If the value of condition is nil
, the clause "fails", so
the cond
moves on to the following clause, trying its
condition.
If every condition evaluates to nil
, so that every clause
fails, cond
returns nil
.
A clause may also look like this:
(condition)
Then, if condition is non-nil
when tested, the value of
condition becomes the value of the cond
form.
The following example has four clauses, which test for the cases where
the value of x
is a number, string, buffer and symbol,
respectively:
(cond ((numberp x) x) ((stringp x) x) ((bufferp x) (setq temporary-hack x) ; multiple body-forms (buffer-name x)) ; in one clause ((symbolp x) (symbol-value x)))
Often we want to execute the last clause whenever none of the previous
clauses was successful. To do this, we use t
as the
condition of the last clause, like this: (t
body-forms)
. The form t
evaluates to t
, which is
never nil
, so this clause never fails, provided the cond
gets to it at all.
For example,
(cond ((eq a 'hack) 'foo) (t "default")) => "default"
This expression is a cond
which returns foo
if the value
of a
is hack
, and returns the string "default"
otherwise.
Any conditional construct can be expressed with cond
or with
if
. Therefore, the choice between them is a matter of style.
For example:
(if a b c) == (cond (a b) (t c))
This section describes three constructs that are often used together
with if
and cond
to express complicated conditions. The
constructs and
and or
can also be used individually as
kinds of multiple conditional constructs.
t
if condition is nil
, and nil
otherwise.
The function not
is identical to null
, and we recommend
using the name null
if you are testing for an empty list.
and
special form tests whether all the conditions are
true. It works by evaluating the conditions one by one in the
order written.
If any of the conditions evaluates to nil
, then the result
of the and
must be nil
regardless of the remaining
conditions; so and
returns right away, ignoring the
remaining conditions.
If all the conditions turn out non-nil
, then the value of
the last of them becomes the value of the and
form.
Here is an example. The first condition returns the integer 1, which is
not nil
. Similarly, the second condition returns the integer 2,
which is not nil
. The third condition is nil
, so the
remaining condition is never evaluated.
(and (print 1) (print 2) nil (print 3)) -| 1 -| 2 => nil
Here is a more realistic example of using and
:
(if (and (consp foo) (eq (car foo) 'x)) (message "foo is a list starting with x"))
Note that (car foo)
is not executed if (consp foo)
returns
nil
, thus avoiding an error.
and
can be expressed in terms of either if
or cond
.
For example:
(and arg1 arg2 arg3) == (if arg1 (if arg2 arg3)) == (cond (arg1 (cond (arg2 arg3))))
or
special form tests whether at least one of the
conditions is true. It works by evaluating all the
conditions one by one in the order written.
If any of the conditions evaluates to a non-nil
value, then
the result of the or
must be non-nil
; so or
returns
right away, ignoring the remaining conditions. The value it
returns is the non-nil
value of the condition just evaluated.
If all the conditions turn out nil
, then the or
expression returns nil
.
For example, this expression tests whether x
is either 0 or
nil
:
(or (eq x nil) (eq x 0))
Like the and
construct, or
can be written in terms of
cond
. For example:
(or arg1 arg2 arg3) == (cond (arg1) (arg2) (arg3))
You could almost write or
in terms of if
, but not quite:
(if arg1 arg1 (if arg2 arg2 arg3))
This is not completely equivalent because it can evaluate arg1 or
arg2 twice. By contrast, (or arg1 arg2
arg3)
never evaluates any argument more than once.
Iteration means executing part of a program repetitively. For
example, you might want to repeat some computation once for each element
of a list, or once for each integer from 0 to n. You can do this
in Emacs Lisp with the special form while
:
while
first evaluates condition. If the result is
non-nil
, it evaluates forms in textual order. Then it
reevaluates condition, and if the result is non-nil
, it
evaluates forms again. This process repeats until condition
evaluates to nil
.
There is no limit on the number of iterations that may occur. The loop
will continue until either condition evaluates to nil
or
until an error or throw
jumps out of it (see section Nonlocal Exits).
The value of a while
form is always nil
.
(setq num 0) => 0 (while (< num 4) (princ (format "Iteration %d." num)) (setq num (1+ num))) -| Iteration 0. -| Iteration 1. -| Iteration 2. -| Iteration 3. => nil
If you would like to execute something on each iteration before the
end-test, put it together with the end-test in a progn
as the
first argument of while
, as shown here:
(while (progn (forward-line 1) (not (looking-at "^$"))))
This moves forward one line and continues moving by lines until it
reaches an empty line. It is peculiar in that the while
has no
body, just the end test (which also does the real work of moving point).
A nonlocal exit is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited.
catch
and throw
Most control constructs affect only the flow of control within the
construct itself. The function throw
is the exception to this
rule of normal program execution: it performs a nonlocal exit on
request. (There are other exceptions, but they are for error handling
only.) throw
is used inside a catch
, and jumps back to
that catch
. For example:
(defun foo-outer () (catch 'foo (foo-inner))) (defun foo-inner () ... (if x (throw 'foo t)) ...)
The throw
form, if executed, transfers control straight back to
the corresponding catch
, which returns immediately. The code
following the throw
is not executed. The second argument of
throw
is used as the return value of the catch
.
The function throw
finds the matching catch
based on the
first argument: it searches for a catch
whose first argument is
eq
to the one specified in the throw
. If there is more
than one applicable catch
, the innermost one takes precedence.
Thus, in the above example, the throw
specifies foo
, and
the catch
in foo-outer
specifies the same symbol, so that
catch
is the applicable one (assuming there is no other matching
catch
in between).
Executing throw
exits all Lisp constructs up to the matching
catch
, including function calls. When binding constructs such as
let
or function calls are exited in this way, the bindings are
unbound, just as they are when these constructs exit normally
(see section Local Variables). Likewise, throw
restores the buffer
and position saved by save-excursion
(see section Excursions), and
the narrowing status saved by save-restriction
and the window
selection saved by save-window-excursion
(see section Window Configurations). It also runs any cleanups established with the
unwind-protect
special form when it exits that form
(see section Cleaning Up from Nonlocal Exits).
The throw
need not appear lexically within the catch
that it jumps to. It can equally well be called from another function
called within the catch
. As long as the throw
takes place
chronologically after entry to the catch
, and chronologically
before exit from it, it has access to that catch
. This is why
throw
can be used in commands such as exit-recursive-edit
that throw back to the editor command loop (see section Recursive Editing).
Common Lisp note: Most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially:
return
,return-from
, andgo
, for example. Emacs Lisp has onlythrow
.
catch
establishes a return point for the throw
function.
The return point is distinguished from other such return points by
tag, which may be any Lisp object except nil
. The argument
tag is evaluated normally before the return point is established.
With the return point in effect, catch
evaluates the forms of the
body in textual order. If the forms execute normally, without
error or nonlocal exit, the value of the last body form is returned from
the catch
.
If a throw
is done within body specifying the same value
tag, the catch
exits immediately; the value it returns is
whatever was specified as the second argument of throw
.
throw
is to return from a return point previously
established with catch
. The argument tag is used to choose
among the various existing return points; it must be eq
to the value
specified in the catch
. If multiple return points match tag,
the innermost one is used.
The argument value is used as the value to return from that
catch
.
If no return point is in effect with tag tag, then a no-catch
error is signaled with data (tag value)
.
catch
and throw
One way to use catch
and throw
is to exit from a doubly
nested loop. (In most languages, this would be done with a "go to".)
Here we compute (foo i j)
for i and j
varying from 0 to 9:
(defun search-foo () (catch 'loop (let ((i 0)) (while (< i 10) (let ((j 0)) (while (< j 10) (if (foo i j) (throw 'loop (list i j))) (setq j (1+ j)))) (setq i (1+ i))))))
If foo
ever returns non-nil
, we stop immediately and return a
list of i and j. If foo
always returns nil
, the
catch
returns normally, and the value is nil
, since that
is the result of the while
.
Here are two tricky examples, slightly different, showing two
return points at once. First, two return points with the same tag,
hack
:
(defun catch2 (tag) (catch tag (throw 'hack 'yes))) => catch2 (catch 'hack (print (catch2 'hack)) 'no) -| yes => no
Since both return points have tags that match the throw
, it goes to
the inner one, the one established in catch2
. Therefore,
catch2
returns normally with value yes
, and this value is
printed. Finally the second body form in the outer catch
, which is
'no
, is evaluated and returned from the outer catch
.
Now let's change the argument given to catch2
:
(defun catch2 (tag) (catch tag (throw 'hack 'yes))) => catch2 (catch 'hack (print (catch2 'quux)) 'no) => yes
We still have two return points, but this time only the outer one has
the tag hack
; the inner one has the tag quux
instead.
Therefore, throw
makes the outer catch
return the value
yes
. The function print
is never called, and the
body-form 'no
is never evaluated.
When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.
When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.
In complicated programs, simple termination may not be what you want.
For example, the program may have made temporary changes in data
structures, or created temporary buffers that should be deleted before
the program is finished. In such cases, you would use
unwind-protect
to establish cleanup expressions to be
evaluated in case of error. (See section Cleaning Up from Nonlocal Exits.) Occasionally, you may
wish the program to continue execution despite an error in a subroutine.
In these cases, you would use condition-case
to establish
error handlers to recover control in case of error.
Resist the temptation to use error handling to transfer control from
one part of the program to another; use catch
and throw
instead. See section Explicit Nonlocal Exits: catch
and throw
.
Most errors are signaled "automatically" within Lisp primitives
which you call for other purposes, such as if you try to take the
CAR of an integer or move forward a character at the end of the
buffer; you can also signal errors explicitly with the functions
error
and signal
.
Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See section Quitting.
format
(see section Conversion of Characters and Strings) to
format-string and args.
These examples show typical uses of error
:
(error "That is an error -- try something else") error--> That is an error -- try something else (error "You have committed %d errors" 10) error--> You have committed 10 errors
error
works by calling signal
with two arguments: the
error symbol error
, and a list containing the string returned by
format
.
Warning: If you want to use your own string as an error message
verbatim, don't just write (error string)
. If string
contains `%', it will be interpreted as a format specifier, with
undesirable results. Instead, use (error "%s" string)
.
The argument error-symbol must be an error symbol---a symbol
bearing a property error-conditions
whose value is a list of
condition names. This is how Emacs Lisp classifies different sorts of
errors.
The number and significance of the objects in data depends on
error-symbol. For example, with a wrong-type-arg
error,
there should be two objects in the list: a predicate that describes the type
that was expected, and the object that failed to fit that type.
See section Error Symbols and Condition Names, for a description of error symbols.
Both error-symbol and data are available to any error
handlers that handle the error: condition-case
binds a local
variable to a list of the form (error-symbol .
data)
(see section Writing Code to Handle Errors). If the error is not handled,
these two values are used in printing the error message.
The function signal
never returns (though in older Emacs versions
it could sometimes return).
(signal 'wrong-number-of-arguments '(x y)) error--> Wrong number of arguments: x, y (signal 'no-such-error '("My unknown error condition")) error--> peculiar error: "My unknown error condition"
Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.
When an error is signaled, signal
searches for an active
handler for the error. A handler is a sequence of Lisp
expressions designated to be executed if an error happens in part of the
Lisp program. If the error has an applicable handler, the handler is
executed, and control resumes following the handler. The handler
executes in the environment of the condition-case
that
established it; all functions called within that condition-case
have already been exited, and the handler cannot return to them.
If there is no applicable handler for the error, the current command is terminated and control returns to the editor command loop, because the command loop has an implicit handler for all kinds of errors. The command loop's handler uses the error symbol and associated data to print an error message.
An error that has no explicit handler may call the Lisp debugger. The
debugger is enabled if the variable debug-on-error
(see section Entering the Debugger on an Error) is non-nil
. Unlike error handlers, the debugger runs
in the environment of the error, so that you can examine values of
variables precisely as they were at the time of the error.
The usual effect of signaling an error is to terminate the command
that is running and return immediately to the Emacs editor command loop.
You can arrange to trap errors occurring in a part of your program by
establishing an error handler, with the special form
condition-case
. A simple example looks like this:
(condition-case nil (delete-file filename) (error nil))
This deletes the file named filename, catching any error and
returning nil
if an error occurs.
The second argument of condition-case
is called the
protected form. (In the example above, the protected form is a
call to delete-file
.) The error handlers go into effect when
this form begins execution and are deactivated when this form returns.
They remain in effect for all the intervening time. In particular, they
are in effect during the execution of functions called by this form, in
their subroutines, and so on. This is a good thing, since, strictly
speaking, errors can be signaled only by Lisp primitives (including
signal
and error
) called by the protected form, not by the
protected form itself.
The arguments after the protected form are handlers. Each handler
lists one or more condition names (which are symbols) to specify
which errors it will handle. The error symbol specified when an error
is signaled also defines a list of condition names. A handler applies
to an error if they have any condition names in common. In the example
above, there is one handler, and it specifies one condition name,
error
, which covers all errors.
The search for an applicable handler checks all the established handlers
starting with the most recently established one. Thus, if two nested
condition-case
forms offer to handle the same error, the inner of
the two will actually handle it.
If an error is handled by some condition-case
form, this
ordinarily prevents the debugger from being run, even if
debug-on-error
says this error should invoke the debugger.
See section Entering the Debugger on an Error. If you want to be able to debug errors that are
caught by a condition-case
, set the variable
debug-on-signal
to a non-nil
value.
When an error is handled, control returns to the handler. Before this
happens, Emacs unbinds all variable bindings made by binding constructs
that are being exited and executes the cleanups of all
unwind-protect
forms that are exited. Once control arrives at
the handler, the body of the handler is executed.
After execution of the handler body, execution returns from the
condition-case
form. Because the protected form is exited
completely before execution of the handler, the handler cannot resume
execution at the point of the error, nor can it examine variable
bindings that were made within the protected form. All it can do is
clean up and proceed.
The condition-case
construct is often used to trap errors that
are predictable, such as failure to open a file in a call to
insert-file-contents
. It is also used to trap errors that are
totally unpredictable, such as when the program evaluates an expression
read from the user.
Error signaling and handling have some resemblance to throw
and
catch
, but they are entirely separate facilities. An error
cannot be caught by a catch
, and a throw
cannot be handled
by an error handler (though using throw
when there is no suitable
catch
signals an error that can be handled).
condition-case
form; in this case, the condition-case
has
no effect. The condition-case
form makes a difference when an
error occurs during protected-form.
Each of the handlers is a list of the form (conditions
body...)
. Here conditions is an error condition name
to be handled, or a list of condition names; body is one or more
Lisp expressions to be executed when this handler handles an error.
Here are examples of handlers:
(error nil) (arith-error (message "Division by zero")) ((arith-error file-error) (message "Either division by zero or failure to open a file"))
Each error that occurs has an error symbol that describes what
kind of error it is. The error-conditions
property of this
symbol is a list of condition names (see section Error Symbols and Condition Names). Emacs
searches all the active condition-case
forms for a handler that
specifies one or more of these condition names; the innermost matching
condition-case
handles the error. Within this
condition-case
, the first applicable handler handles the error.
After executing the body of the handler, the condition-case
returns normally, using the value of the last form in the handler body
as the overall value.
The argument var is a variable. condition-case
does not
bind this variable when executing the protected-form, only when it
handles an error. At that time, it binds var locally to an
error description, which is a list giving the particulars of the
error. The error description has the form (error-symbol
. data)
. The handler can refer to this list to decide what to
do. For example, if the error is for failure opening a file, the file
name is the second element of data---the third element of the
error description.
If var is nil
, that means no variable is bound. Then the
error symbol and associated data are not available to the handler.
Here is an example of using condition-case
to handle the error
that results from dividing by zero. The handler displays the error
message (but without a beep), then returns a very large number.
(defun safe-divide (dividend divisor) (condition-case err ;; Protected form. (/ dividend divisor) ;; The handler. (arith-error ; Condition. ;; Display the usual message for this error. (message "%s" (error-message-string err)) 1000000))) => safe-divide (safe-divide 5 0) -| Arithmetic error: (arith-error) => 1000000
The handler specifies condition name arith-error
so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not by this condition-case
. Thus,
(safe-divide nil 3) error--> Wrong type argument: number-or-marker-p, nil
Here is a condition-case
that catches all kinds of errors,
including those signaled with error
:
(setq baz 34)
=> 34
(condition-case err
(if (eq baz 35)
t
;; This is a call to the function error
.
(error "Rats! The variable %s was %s, not 35" 'baz baz))
;; This is the handler; it is not a form.
(error (princ (format "The error was: %s" err))
2))
-| The error was: (error "Rats! The variable baz was 34, not 35")
=> 2
When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.
These narrow classifications are grouped into a hierarchy of wider
classes called error conditions, identified by condition
names. The narrowest such classes belong to the error symbols
themselves: each error symbol is also a condition name. There are also
condition names for more extensive classes, up to the condition name
error
which takes in all kinds of errors. Thus, each error has
one or more condition names: error
, the error symbol if that
is distinct from error
, and perhaps some intermediate
classifications.
In order for a symbol to be an error symbol, it must have an
error-conditions
property which gives a list of condition names.
This list defines the conditions that this kind of error belongs to.
(The error symbol itself, and the symbol error
, should always be
members of this list.) Thus, the hierarchy of condition names is
defined by the error-conditions
properties of the error symbols.
In addition to the error-conditions
list, the error symbol
should have an error-message
property whose value is a string to
be printed when that error is signaled but not handled. If the
error-message
property exists, but is not a string, the error
message `peculiar error' is used.
Here is how we define a new error symbol, new-error
:
(put 'new-error 'error-conditions '(error my-own-errors new-error)) => (error my-own-errors new-error) (put 'new-error 'error-message "A new error") => "A new error"
This error has three condition names: new-error
, the narrowest
classification; my-own-errors
, which we imagine is a wider
classification; and error
, which is the widest of all.
The error string should start with a capital letter but it should
not end with a period. This is for consistency with the rest of Emacs.
Naturally, Emacs will never signal new-error
on its own; only
an explicit call to signal
(see section How to Signal an Error) in your
code can do this:
(signal 'new-error '(x y)) error--> A new error: x, y
This error can be handled through any of the three condition names.
This example handles new-error
and any other errors in the class
my-own-errors
:
(condition-case foo (bar nil t) (my-own-errors nil))
The significant way that errors are classified is by their condition
names--the names used to match errors with handlers. An error symbol
serves only as a convenient way to specify the intended error message
and list of condition names. It would be cumbersome to give
signal
a list of condition names rather than one error symbol.
By contrast, using only error symbols without condition names would
seriously decrease the power of condition-case
. Condition names
make it possible to categorize errors at various levels of generality
when you write an error handler. Using error symbols alone would
eliminate all but the narrowest level of classification.
See section Standard Errors, for a list of all the standard error symbols and their conditions.
The unwind-protect
construct is essential whenever you
temporarily put a data structure in an inconsistent state; it permits
you to make the data consistent again in the event of an error or throw.
unwind-protect
executes the body with a guarantee that the
cleanup-forms will be evaluated if control leaves body, no
matter how that happens. The body may complete normally, or
execute a throw
out of the unwind-protect
, or cause an
error; in all cases, the cleanup-forms will be evaluated.
If the body forms finish normally, unwind-protect
returns
the value of the last body form, after it evaluates the
cleanup-forms. If the body forms do not finish,
unwind-protect
does not return any value in the normal sense.
Only the body is actually protected by the unwind-protect
.
If any of the cleanup-forms themselves exits nonlocally (e.g., via
a throw
or an error), unwind-protect
is not
guaranteed to evaluate the rest of them. If the failure of one of the
cleanup-forms has the potential to cause trouble, then protect it
with another unwind-protect
around that form.
The number of currently active unwind-protect
forms counts,
together with the number of local variable bindings, against the limit
max-specpdl-size
(see section Local Variables).
For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:
(save-excursion (let ((buffer (get-buffer-create " *temp*"))) (set-buffer buffer) (unwind-protect body (kill-buffer buffer))))
You might think that we could just as well write (kill-buffer
(current-buffer))
and dispense with the variable buffer
.
However, the way shown above is safer, if body happens to get an
error after switching to a different buffer! (Alternatively, you could
write another save-excursion
around the body, to ensure that the
temporary buffer becomes current again in time to kill it.)
Emacs includes a standard macro called with-temp-buffer
which
expands into more or less the code shown above (see section The Current Buffer).
Several of the macros defined in this manual use unwind-protect
in this way.
Here is an actual example taken from the file `ftp.el'. It
creates a process (see section Processes) to try to establish a connection
to a remote machine. As the function ftp-login
is highly
susceptible to numerous problems that the writer of the function cannot
anticipate, it is protected with a form that guarantees deletion of the
process in the event of failure. Otherwise, Emacs might fill up with
useless subprocesses.
(let ((win nil)) (unwind-protect (progn (setq process (ftp-setup-buffer host file)) (if (setq win (ftp-login process host user password)) (message "Logged in") (error "Ftp login failed"))) (or win (and process (delete-process process)))))
This example actually has a small bug: if the user types C-g to
quit, and the quit happens immediately after the function
ftp-setup-buffer
returns but before the variable process
is
set, the process will not be killed. There is no easy way to fix this bug,
but at least it is very unlikely.
A variable is a name used in a program to stand for a value. Nearly all programming languages have variables of some sort. In the text of a Lisp program, variables are written using the syntax for symbols.
In Lisp, unlike most programming languages, programs are represented primarily as Lisp objects and only secondarily as text. The Lisp objects used for variables are symbols: the symbol name is the variable name, and the variable's value is stored in the value cell of the symbol. The use of a symbol as a variable is independent of its use as a function name. See section Symbol Components.
The Lisp objects that constitute a Lisp program determine the textual form of the program--it is simply the read syntax for those Lisp objects. This is why, for example, a variable in a textual Lisp program is written using the read syntax for the symbol that represents the variable.
The simplest way to use a variable is globally. This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable.
You specify a value for a symbol with setq
. For example,
(setq x '(a b))
gives the variable x
the value (a b)
. Note that
setq
does not evaluate its first argument, the name of the
variable, but it does evaluate the second argument, the new value.
Once the variable has a value, you can refer to it by using the symbol by itself as an expression. Thus,
x => (a b)
assuming the setq
form shown above has already been executed.
If you do set the same variable again, the new value replaces the old one:
x => (a b) (setq x 4) => 4 x => 4
In Emacs Lisp, certain symbols normally evaluate to themselves. These
include nil
and t
, as well as any symbol whose name starts
with `:'. These symbols cannot be rebound, nor can their values be
changed. Any attempt to set or bind nil
or t
signals a
setting-constant
error. The same is true for a symbol whose name
starts with `:', except that you are allowed to set such a symbol to
itself.
nil == 'nil => nil (setq nil 500) error--> Attempt to set constant symbol: nil
nil
, you are allowed to set and bind symbols
whose names start with `:' as you wish. This is to make it
possible to run old Lisp programs which do that.
Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to create variable values that exist temporarily--only until a certain part of the program finishes. These values are called local, and the variables so used are called local variables.
For example, when a function is called, its argument variables receive
new local values that last until the function exits. The let
special form explicitly establishes new local values for specified
variables; these last until exit from the let
form.
Establishing a local value saves away the previous value (or lack of one) of the variable. When the life span of the local value is over, the previous value is restored. In the mean time, we say that the previous value is shadowed and not visible. Both global and local values may be shadowed (see section Scope).
If you set a variable (such as with setq
) while it is local,
this replaces the local value; it does not alter the global value, or
previous local values, that are shadowed. To model this behavior, we
speak of a local binding of the variable as well as a local value.
The local binding is a conceptual place that holds a local value.
Entry to a function, or a special form such as let
, creates the
local binding; exit from the function or from the let
removes the
local binding. As long as the local binding lasts, the variable's value
is stored within it. Use of setq
or set
while there is a
local binding stores a different value into the local binding; it does
not create a new binding.
We also speak of the global binding, which is where (conceptually) the global value is kept.
A variable can have more than one local binding at a time (for
example, if there are nested let
forms that bind it). In such a
case, the most recently created local binding that still exists is the
current binding of the variable. (This rule is called
dynamic scoping; see section Scoping Rules for Variable Bindings.) If there are no
local bindings, the variable's global binding is its current binding.
We sometimes call the current binding the most-local existing
binding, for emphasis. Ordinary evaluation of a symbol always returns
the value of its current binding.
The special forms let
and let*
exist to create
local bindings.
let
-form
returns the value of the last form in forms.
Each of the bindings is either (i) a symbol, in which case
that symbol is bound to nil
; or (ii) a list of the form
(symbol value-form)
, in which case symbol is
bound to the result of evaluating value-form. If value-form
is omitted, nil
is used.
All of the value-forms in bindings are evaluated in the
order they appear and before binding any of the symbols to them.
Here is an example of this: Z
is bound to the old value of
Y
, which is 2, not the new value of Y
, which is 1.
(setq Y 2) => 2 (let ((Y 1) (Z Y)) (list Y Z)) => (1 2)
let
, but it binds each variable right
after computing its local value, before computing the local value for
the next variable. Therefore, an expression in bindings can
reasonably refer to the preceding symbols bound in this let*
form. Compare the following example with the example above for
let
.
(setq Y 2)
=> 2
(let* ((Y 1)
(Z Y)) ; Use the just-established value of Y
.
(list Y Z))
=> (1 1)
Here is a complete list of the other facilities that create local bindings:
condition-case
(see section Errors).
Variables can also have buffer-local bindings (see section Buffer-Local Variables) and frame-local bindings (see section Frame-Local Variables); a few variables have terminal-local bindings (see section Multiple Displays). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on "where" you are in Emacs, rather than localized in time.
unwind-protect
cleanups (see section Nonlocal Exits)
that are allowed before signaling an error (with data "Variable
binding depth exceeds max-specpdl-size"
).
This limit, with the associated error when it is exceeded, is one way
that Lisp avoids infinite recursion on an ill-defined function.
max-lisp-eval-depth
provides another limit on depth of nesting.
See section Eval.
The default value is 600. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
If you have never given a symbol any value as a global variable, we
say that that symbol's global value is void. In other words, the
symbol's value cell does not have any Lisp object in it. If you try to
evaluate the symbol, you get a void-variable
error rather than
a value.
Note that a value of nil
is not the same as void. The symbol
nil
is a Lisp object and can be the value of a variable just as any
other object can be; but it is a value. A void variable does not
have any value.
After you have given a variable a value, you can make it void once more
using makunbound
.
void-variable
, unless and until you set it again.
makunbound
returns symbol.
(makunbound 'x) ; Make the global value of x
void.
=> x
x
error--> Symbol's value as variable is void: x
If symbol is locally bound, makunbound
affects the most
local existing binding. This is the only way a symbol can have a void
local binding, since all the constructs that create local bindings
create them with values. In this case, the voidness lasts at most as
long as the binding does; when the binding is removed due to exit from
the construct that made it, the previous local or global binding is
reexposed as usual, and the variable is no longer void unless the newly
reexposed binding was void all along.
(setq x 1) ; Put a value in the global binding.
=> 1
(let ((x 2)) ; Locally bind it.
(makunbound 'x) ; Void the local binding.
x)
error--> Symbol's value as variable is void: x
x ; The global binding is unchanged.
=> 1
(let ((x 2)) ; Locally bind it.
(let ((x 3)) ; And again.
(makunbound 'x) ; Void the innermost-local binding.
x)) ; And refer: it's void.
error--> Symbol's value as variable is void: x
(let ((x 2))
(let ((x 3))
(makunbound 'x)) ; Void inner binding, then remove it.
x) ; Now outer let
binding is visible.
=> 2
A variable that has been made void with makunbound
is
indistinguishable from one that has never received a value and has
always been void.
You can use the function boundp
to test whether a variable is
currently void.
boundp
returns t
if variable (a symbol) is not void;
more precisely, if its current binding is not void. It returns
nil
otherwise.
(boundp 'abracadabra) ; Starts out void. => nil (let ((abracadabra 5)) ; Locally bind it. (boundp 'abracadabra)) => t (boundp 'abracadabra) ; Still globally void. => nil (setq abracadabra 5) ; Make it globally nonvoid. => 5 (boundp 'abracadabra) => t
You may announce your intention to use a symbol as a global variable
with a variable definition: a special form, either defconst
or defvar
.
In Emacs Lisp, definitions serve three purposes. First, they inform
people who read the code that certain symbols are intended to be
used a certain way (as variables). Second, they inform the Lisp system
of these things, supplying a value and documentation. Third, they
provide information to utilities such as etags
and
make-docfile
, which create data bases of the functions and
variables in a program.
The difference between defconst
and defvar
is primarily
a matter of intent, serving to inform human readers of whether the value
should ever change. Emacs Lisp does not restrict the ways in which a
variable can be used based on defconst
or defvar
declarations. However, it does make a difference for initialization:
defconst
unconditionally initializes the variable, while
defvar
initializes it only if it is void.
defvar
.
If symbol is void and value is specified, defvar
evaluates it and sets symbol to the result. But if symbol
already has a value (i.e., it is not void), value is not even
evaluated, and symbol's value remains unchanged. If value
is omitted, the value of symbol is not changed in any case.
If symbol has a buffer-local binding in the current buffer,
defvar
operates on the default value, which is buffer-independent,
not the current (buffer-local) binding. It sets the default value if
the default value is void. See section Buffer-Local Variables.
When you evaluate a top-level defvar
form with C-M-x in
Emacs Lisp mode (eval-defun
), a special feature of
eval-defun
arranges to set the variable unconditionally, without
testing whether its value is void.
If the doc-string argument appears, it specifies the documentation
for the variable. (This opportunity to specify documentation is one of
the main benefits of defining the variable.) The documentation is
stored in the symbol's variable-documentation
property. The
Emacs help functions (see section Documentation) look for this property.
If the first character of doc-string is `*', it means that
this variable is considered a user option. This lets users set the
variable conveniently using the commands set-variable
and
edit-options
. However, it is better to use defcustom
instead of defvar
for user option variables, so you can specify
customization information. See section Writing Customization Definitions.
Here are some examples. This form defines foo
but does not
initialize it:
(defvar foo) => foo
This example initializes the value of bar
to 23
, and gives
it a documentation string:
(defvar bar 23 "The normal weight of a bar.") => bar
The following form changes the documentation string for bar
,
making it a user option, but does not change the value, since bar
already has a value. (The addition (1+ nil)
would get an error
if it were evaluated, but since it is not evaluated, there is no error.)
(defvar bar (1+ nil) "*The normal weight of a bar.") => bar bar => 23
Here is an equivalent expression for the defvar
special form:
(defvar symbol value doc-string) == (progn (if (not (boundp 'symbol)) (setq symbol value)) (if 'doc-string (put 'symbol 'variable-documentation 'doc-string)) 'symbol)
The defvar
form returns symbol, but it is normally used
at top level in a file where its value does not matter.
defconst
.
defconst
always evaluates value, and sets the value of
symbol to the result if value is given. If symbol
does have a buffer-local binding in the current buffer, defconst
sets the default value, not the buffer-local value. (But you should not
be making buffer-local bindings for a symbol that is defined with
defconst
.)
Here, pi
is a constant that presumably ought not to be changed
by anyone (attempts by the Indiana State Legislature notwithstanding).
As the second form illustrates, however, this is only advisory.
(defconst pi 3.1415 "Pi to five places.") => pi (setq pi 3) => pi pi => 3
t
if variable is a user option--a
variable intended to be set by the user for customization--and
nil
otherwise. (Variables other than user options exist for the
internal purposes of Lisp programs, and users need not know about them.)
User option variables are distinguished from other variables by the
first character of the variable-documentation
property. If the
property exists and is a string, and its first character is `*',
then the variable is a user option.
If a user option variable has a variable-interactive
property,
the set-variable
command uses that value to control reading the
new value for the variable. The property's value is used as if it were
to interactive
(see section Using interactive
). However, this feature
is largely obsoleted by defcustom
(see section Writing Customization Definitions).
Warning: If the defconst
and defvar
special
forms are used while the variable has a local binding, they set the
local binding's value; the global binding is not changed. This is not
what we really want. To prevent it, use these special forms at top
level in a file, where normally no local binding is in effect, and make
sure to load the file before making a local binding for the variable.
When defining and initializing a variable that holds a complicated
value (such as a keymap with bindings in it), it's best to put the
entire computation of the value into the defvar
, like this:
(defvar my-mode-map (let ((map (make-sparse-keymap))) (define-key map "\C-c\C-a" 'my-command) ... map) docstring)
This method has several benefits. First, if the user quits while
loading the file, the variable is either still uninitialized or
initialized properly, never in-between. If it is still uninitialized,
reloading the file will initialize it properly. Second, reloading the
file once the variable is initialized will not alter it; that is
important if the user has run hooks to alter part of the contents (such
as, to rebind keys). Third, evaluating the defvar
form with
C-M-x will reinitialize the map completely.
Putting so much code in the defvar
form has one disadvantage:
it puts the documentation string far away from the line which names the
variable. Here's a safe way to avoid that:
(defvar my-mode-map nil docstring) (if my-mode-map nil (let ((map (make-sparse-keymap))) (define-key my-mode-map "\C-c\C-a" 'my-command) ... (setq my-mode-map map)))
This has all the same advantages as putting the initialization inside
the defvar
, except that you must type C-M-x twice, once on
each form, if you do want to reinitialize the variable.
But be careful not to write the code like this:
(defvar my-mode-map nil docstring) (if my-mode-map nil (setq my-mode-map (make-sparse-keymap)) (define-key my-mode-map "\C-c\C-a" 'my-command) ...)
This code sets the variable, then alters it, but it does so in more than
one step. If the user quits just after the setq
, that leaves the
variable neither correctly initialized nor void nor nil
. Once
that happens, reloading the file will not initialize the variable; it
will remain incomplete.
The usual way to reference a variable is to write the symbol which
names it (see section Symbol Forms). This requires you to specify the
variable name when you write the program. Usually that is exactly what
you want to do. Occasionally you need to choose at run time which
variable to reference; then you can use symbol-value
.
(setq abracadabra 5) => 5 (setq foo 9) => 9 ;; Here the symbolabracadabra
;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value 'abracadabra)) => foo ;; Here the value ofabracadabra
, ;; which isfoo
, ;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value abracadabra)) => 9 (symbol-value 'abracadabra) => 5
A void-variable
error is signaled if the current binding of
symbol is void.
The usual way to change the value of a variable is with the special
form setq
. When you need to compute the choice of variable at
run time, use the function set
.
setq
does not evaluate symbol; it sets the symbol that you
write. We say that this argument is automatically quoted. The
`q' in setq
stands for "quoted."
The value of the setq
form is the value of the last form.
(setq x (1+ 2)) => 3 x ;x
now has a global value. => 3 (let ((x 5)) (setq x 6) ; The local binding ofx
is set. x) => 6 x ; The global value is unchanged. => 3
Note that the first form is evaluated, then the first symbol is set, then the second form is evaluated, then the second symbol is set, and so on:
(setq x 10 ; Notice thatx
is set before y (1+ x)) ; the value ofy
is computed. => 11
set
is a function, the expression written for
symbol is evaluated to obtain the symbol to set.
The most-local existing binding of the variable is the binding that is set; shadowed bindings are not affected.
(set one 1) error--> Symbol's value as variable is void: one (set 'one 1) => 1 (set 'two 'one) => one (set two 2) ;two
evaluates to symbolone
. => 2 one ; So it isone
that was set. => 2 (let ((one 1)) ; This binding ofone
is set, (set 'one 3) ; not the global value. one) => 3 one => 2
If symbol is not actually a symbol, a wrong-type-argument
error is signaled.
(set '(x y) 'z) error--> Wrong type argument: symbolp, (x y)
Logically speaking, set
is a more fundamental primitive than
setq
. Any use of setq
can be trivially rewritten to use
set
; setq
could even be defined as a macro, given the
availability of set
. However, set
itself is rarely used;
beginners hardly need to know about it. It is useful only for choosing
at run time which variable to set. For example, the command
set-variable
, which reads a variable name from the user and then
sets the variable, needs to use set
.
Common Lisp note: In Common Lisp,
set
always changes the symbol's "special" or dynamic value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are dynamic, soset
always affects the most local existing binding.
One other function for setting a variable is designed to add an element to a list if it is not already present in the list.
The argument symbol is not implicitly quoted; add-to-list
is an ordinary function, like set
and unlike setq
. Quote
the argument yourself if that is what you want.
Here's a scenario showing how to use add-to-list
:
(setq foo '(a b)) => (a b) (add-to-list 'foo 'c) ;; Addc
. => (c a b) (add-to-list 'foo 'b) ;; No effect. => (c a b) foo ;;foo
was changed. => (c a b)
An equivalent expression for (add-to-list 'var
value)
is this:
(or (member value var) (setq var (cons value var)))
A given symbol foo
can have several local variable bindings,
established at different places in the Lisp program, as well as a global
binding. The most recently established binding takes precedence over
the others.
Local bindings in Emacs Lisp have indefinite scope and dynamic extent. Scope refers to where textually in the source code the binding can be accessed. Indefinite scope means that any part of the program can potentially access the variable binding. Extent refers to when, as the program is executing, the binding exists. Dynamic extent means that the binding lasts as long as the activation of the construct that established it.
The combination of dynamic extent and indefinite scope is called dynamic scoping. By contrast, most programming languages use lexical scoping, in which references to a local variable must be located textually within the function or block that binds the variable.
Common Lisp note: Variables declared "special" in Common Lisp are dynamically scoped, like all variables in Emacs Lisp.
Emacs Lisp uses indefinite scope for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions:
(defun binder (x) ;x
is bound inbinder
. (foo 5)) ;foo
is some other function. (defun user () ;x
is used ``free'' inuser
. (list x))
In a lexically scoped language, the binding of x
in
binder
would never be accessible in user
, because
user
is not textually contained within the function
binder
. However, in dynamically scoped Emacs Lisp, user
may or may not refer to the binding of x
established in
binder
, depending on circumstances:
user
directly without calling binder
at all,
then whatever binding of x
is found, it cannot come from
binder
.
foo
as follows and then call binder
, then the
binding made in binder
will be seen in user
:
(defun foo (lose) (user))
foo
as follows and then call binder
,
then the binding made in binder
will not be seen in
user
:
(defun foo (x) (user))Here, when
foo
is called by binder
, it binds x
.
(The binding in foo
is said to shadow the one made in
binder
.) Therefore, user
will access the x
bound
by foo
instead of the one bound by binder
.
Emacs Lisp uses dynamic scoping because simple implementations of lexical scoping are slow. In addition, every Lisp system needs to offer dynamic scoping at least as an option; if lexical scoping is the norm, there must be a way to specify dynamic scoping instead for a particular variable. It might not be a bad thing for Emacs to offer both, but implementing it with dynamic scoping only was much easier.
Extent refers to the time during program execution that a variable name is valid. In Emacs Lisp, a variable is valid only while the form that bound it is executing. This is called dynamic extent. "Local" or "automatic" variables in most languages, including C and Pascal, have dynamic extent.
One alternative to dynamic extent is indefinite extent. This means that a variable binding can live on past the exit from the form that made the binding. Common Lisp and Scheme, for example, support this, but Emacs Lisp does not.
To illustrate this, the function below, make-add
, returns a
function that purports to add n to its own argument m. This
would work in Common Lisp, but it does not do the job in Emacs Lisp,
because after the call to make-add
exits, the variable n
is no longer bound to the actual argument 2.
(defun make-add (n) (function (lambda (m) (+ n m)))) ; Return a function. => make-add (fset 'add2 (make-add 2)) ; Define functionadd2
; with(make-add 2)
. => (lambda (m) (+ n m)) (add2 4) ; Try to add 2 to 4. error--> Symbol's value as variable is void: n
Some Lisp dialects have "closures", objects that are like functions but record additional variable bindings. Emacs Lisp does not have closures.
A simple sample implementation (which is not how Emacs Lisp actually works) may help you understand dynamic binding. This technique is called deep binding and was used in early Lisp systems.
Suppose there is a stack of bindings, which are variable-value pairs.
At entry to a function or to a let
form, we can push bindings
onto the stack for the arguments or local variables created there. We
can pop those bindings from the stack at exit from the binding
construct.
We can find the value of a variable by searching the stack from top to bottom for a binding for that variable; the value from that binding is the value of the variable. To set the variable, we search for the current binding, then store the new value into that binding.
As you can see, a function's bindings remain in effect as long as it continues execution, even during its calls to other functions. That is why we say the extent of the binding is dynamic. And any other function can refer to the bindings, if it uses the same variables while the bindings are in effect. That is why we say the scope is indefinite.
The actual implementation of variable scoping in GNU Emacs Lisp uses a technique called shallow binding. Each variable has a standard place in which its current value is always found--the value cell of the symbol.
In shallow binding, setting the variable works by storing a value in the value cell. Creating a new binding works by pushing the old value (belonging to a previous binding) onto a stack, and storing the new local value in the value cell. Eliminating a binding works by popping the old value off the stack, into the value cell.
We use shallow binding because it has the same results as deep binding, but runs faster, since there is never a need to search for a binding.
Binding a variable in one function and using it in another is a powerful technique, but if used without restraint, it can make programs hard to understand. There are two clean ways to use this technique:
case-fold-search
is defined as "non-nil
means ignore case
when searching"; various search and replace functions refer to it
directly or through their subroutines, but do not bind or set it.
Then you can bind the variable in other programs, knowing reliably what
the effect will be.
In either case, you should define the variable with defvar
.
This helps other people understand your program by telling them to look
for inter-function usage. It also avoids a warning from the byte
compiler. Choose the variable's name to avoid name conflicts--don't
use short names like x
.
Global and local variable bindings are found in most programming languages in one form or another. Emacs also supports additional, unusual kinds of variable binding: buffer-local bindings, which apply only in one buffer, and frame-local bindings, which apply only in one frame. Having different values for a variable in different buffers and/or frames is an important customization method.
This section describes buffer-local bindings; for frame-local bindings, see the following section, section Frame-Local Variables. (A few variables have bindings that are local to each terminal; see section Multiple Displays.)
A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it.
The variable's ordinary binding, which is not associated with any specific buffer, is called the default binding. In most cases, this is the global binding.
A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don't have their own bindings for the variable. (This includes all newly created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding (assuming there are no frame-local bindings to complicate the matter), so the new value is visible in all the buffers that see the default binding.
The most common use of buffer-local bindings is for major modes to change
variables that control the behavior of commands. For example, C mode and
Lisp mode both set the variable paragraph-start
to specify that only
blank lines separate paragraphs. They do this by making the variable
buffer-local in the buffer that is being put into C mode or Lisp mode, and
then setting it to the new value for that mode. See section Major Modes.
The usual way to make a buffer-local binding is with
make-local-variable
, which is what major mode commands typically
use. This affects just the current buffer; all other buffers (including
those yet to be created) will continue to share the default value unless
they are explicitly given their own buffer-local bindings.
A more powerful operation is to mark the variable as
automatically buffer-local by calling
make-variable-buffer-local
. You can think of this as making the
variable local in all buffers, even those yet to be created. More
precisely, the effect is that setting the variable automatically makes
the variable local to the current buffer if it is not already so. All
buffers start out by sharing the default value of the variable as usual,
but setting the variable creates a buffer-local binding for the current
buffer. The new value is stored in the buffer-local binding, leaving
the default binding untouched. This means that the default value cannot
be changed with setq
in any buffer; the only way to change it is
with setq-default
.
Warning: When a variable has buffer-local values in one or
more buffers, you can get Emacs very confused by binding the variable
with let
, changing to a different current buffer in which a
different binding is in effect, and then exiting the let
. This
can scramble the values of the buffer-local and default bindings.
To preserve your sanity, avoid using a variable in that way. If you
use save-excursion
around each piece of code that changes to a
different current buffer, you will not have this problem
(see section Excursions). Here is an example of what to avoid:
(setq foo 'b)
(set-buffer "a")
(make-local-variable 'foo)
(setq foo 'a)
(let ((foo 'temp))
(set-buffer "b")
body...)
foo => 'a ; The old buffer-local value from buffer `a'
; is now the default value.
(set-buffer "a")
foo => 'temp ; The local let
value that should be gone
; is now the buffer-local value in buffer `a'.
But save-excursion
as shown here avoids the problem:
(let ((foo 'temp)) (save-excursion (set-buffer "b") body...))
Note that references to foo
in body access the
buffer-local binding of buffer `b'.
When a file specifies local variable values, these become buffer-local values when you visit the file. See section `File Variables' in The GNU Emacs Manual.
The buffer-local value of variable starts out as the same value variable previously had. If variable was void, it remains void.
;; In buffer `b1': (setq foo 5) ; Affects all buffers. => 5 (make-local-variable 'foo) ; Now it is local in `b1'. => foo foo ; That did not change => 5 ; the value. (setq foo 6) ; Change the value => 6 ; in `b1'. foo => 6 ;; In buffer `b2', the value hasn't changed. (save-excursion (set-buffer "b2") foo) => 5
Making a variable buffer-local within a let
-binding for that
variable does not work reliably, unless the buffer in which you do this
is not current either on entry to or exit from the let
. This is
because let
does not distinguish between different kinds of
bindings; it knows only which variable the binding was made for.
If the variable is terminal-local, this function signals an error. Such variables cannot have buffer-local bindings as well. See section Multiple Displays.
Note: do not use make-local-variable
for a hook
variable. Instead, use make-local-hook
. See section Hooks.
A peculiar wrinkle of this feature is that binding the variable (with
let
or other binding constructs) does not create a buffer-local
binding for it. Only setting the variable (with set
or
setq
) does so.
The value returned is variable.
Warning: Don't assume that you should use
make-variable-buffer-local
for user-option variables, simply
because users might want to customize them differently in
different buffers. Users can make any variable local, when they wish
to. It is better to leave the choice to them.
The time to use make-variable-buffer-local
is when it is crucial
that no two buffers ever share the same binding. For example, when a
variable is used for internal purposes in a Lisp program which depends
on having separate values in separate buffers, then using
make-variable-buffer-local
can be the best solution.
t
if variable is buffer-local in buffer
buffer (which defaults to the current buffer); otherwise,
nil
.
(make-local-variable 'foobar) (makunbound 'foobar) (make-local-variable 'bind-me) (setq bind-me 69) (setq lcl (buffer-local-variables)) ;; First, built-in variables local in all buffers: => ((mark-active . nil) (buffer-undo-list . nil) (mode-name . "Fundamental") ... ;; Next, non-built-in buffer-local variables. ;; This one is buffer-local and void: foobar ;; This one is buffer-local and nonvoid: (bind-me . 69))
Note that storing new values into the CDRs of cons cells in this list does not change the buffer-local values of the variables.
If you kill the buffer-local binding of a variable that automatically becomes buffer-local when set, this makes the default value visible in the current buffer. However, if you set the variable again, that will once again create a buffer-local binding for it.
kill-local-variable
returns variable.
This function is a command because it is sometimes useful to kill one buffer-local variable interactively, just as it is useful to create buffer-local variables interactively.
This function also resets certain other information pertaining to the
buffer: it sets the local keymap to nil
, the syntax table to the
value of (standard-syntax-table)
, the case table to
(standard-case-table)
, and the abbrev table to the value of
fundamental-mode-abbrev-table
.
The very first thing this function does is run the normal hook
change-major-mode-hook
(see below).
Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent.
kill-all-local-variables
returns nil
.
kill-all-local-variables
runs this normal hook
before it does anything else. This gives major modes a way to arrange
for something special to be done if the user switches to a different
major mode. For best results, make this variable buffer-local, so that
it will disappear after doing its job and will not interfere with the
subsequent major mode. See section Hooks.
A buffer-local variable is permanent if the variable name (a
symbol) has a permanent-local
property that is non-nil
.
Permanent locals are appropriate for data pertaining to where the file
came from or how to save it, rather than with how to edit the contents.
The global value of a variable with buffer-local bindings is also called the default value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable.
The functions default-value
and setq-default
access and
change a variable's default value regardless of whether the current
buffer has a buffer-local binding. For example, you could use
setq-default
to change the default setting of
paragraph-start
for most buffers; and this would work even when
you are in a C or Lisp mode buffer that has a buffer-local value for
this variable.
The special forms defvar
and defconst
also set the
default value (if they set the variable at all), rather than any
buffer-local or frame-local value.
symbol-value
(see section Accessing Variable Values).
default-boundp
tells you whether symbol's
default value is nonvoid. If (default-boundp 'foo)
returns
nil
, then (default-value 'foo)
would get an error.
default-boundp
is to default-value
as boundp
is to
symbol-value
.
setq-default
form is the value of the last form.
If a symbol is not buffer-local for the current buffer, and is not
marked automatically buffer-local, setq-default
has the same
effect as setq
. If symbol is buffer-local for the current
buffer, then this changes the value that other buffers will see (as long
as they don't have a buffer-local value), but not the value that the
current buffer sees.
;; In buffer `foo': (make-local-variable 'buffer-local) => buffer-local (setq buffer-local 'value-in-foo) => value-in-foo (setq-default buffer-local 'new-default) => new-default buffer-local => value-in-foo (default-value 'buffer-local) => new-default ;; In (the new) buffer `bar': buffer-local => new-default (default-value 'buffer-local) => new-default (setq buffer-local 'another-default) => another-default (default-value 'buffer-local) => another-default ;; Back in buffer `foo': buffer-local => value-in-foo (default-value 'buffer-local) => another-default
setq-default
, except that symbol is
an ordinary evaluated argument.
(set-default (car '(a b c)) 23) => 23 (default-value 'a) => 23
Just as variables can have buffer-local bindings, they can also have
frame-local bindings. These bindings belong to one frame, and are in
effect when that frame is selected. Frame-local bindings are actually
frame parameters: you create a frame-local binding in a specific frame
by calling modify-frame-parameters
and specifying the variable
name as the parameter name.
To enable frame-local bindings for a certain variable, call the function
make-variable-frame-local
.
If the variable is terminal-local, this function signals an error, because such variables cannot have frame-local bindings as well. See section Multiple Displays. A few variables that are implemented specially in Emacs can be (and usually are) buffer-local, but can never be frame-local.
Buffer-local bindings take precedence over frame-local bindings. Thus,
consider a variable foo
: if the current buffer has a buffer-local
binding for foo
, that binding is active; otherwise, if the
selected frame has a frame-local binding for foo
, that binding is
active; otherwise, the default binding of foo
is active.
Here is an example. First we prepare a few bindings for foo
:
(setq f1 (selected-frame)) (make-variable-frame-local 'foo) ;; Make a buffer-local binding forfoo
in `b1'. (set-buffer (get-buffer-create "b1")) (make-local-variable 'foo) (setq foo '(b 1)) ;; Make a frame-local binding forfoo
in a new frame. ;; Store that frame inf2
. (setq f2 (make-frame)) (modify-frame-parameters f2 '((foo . (f 2))))
Now we examine foo
in various contexts. Whenever the
buffer `b1' is current, its buffer-local binding is in effect,
regardless of the selected frame:
(select-frame f1) (set-buffer (get-buffer-create "b1")) foo => (b 1) (select-frame f2) (set-buffer (get-buffer-create "b1")) foo => (b 1)
Otherwise, the frame gets a chance to provide the binding; when frame
f2
is selected, its frame-local binding is in effect:
(select-frame f2) (set-buffer (get-buffer "*scratch*")) foo => (f 2)
When neither the current buffer nor the selected frame provides a binding, the default binding is used:
(select-frame f1) (set-buffer (get-buffer "*scratch*")) foo => nil
When the active binding of a variable is a frame-local binding, setting
the variable changes that binding. You can observe the result with
frame-parameters
:
(select-frame f2) (set-buffer (get-buffer "*scratch*")) (setq foo 'nobody) (assq 'foo (frame-parameters f2)) => (foo . nobody)
We have considered the idea of bindings that are local to a category
of frames--for example, all color frames, or all frames with dark
backgrounds. We have not implemented them because it is not clear that
this feature is really useful. You can get more or less the same
results by adding a function to after-make-frame-hook
, set up to
define a particular frame parameter according to the appropriate
conditions for each frame.
It would also be possible to implement window-local bindings. We don't know of many situations where they would be useful, and it seems that indirect buffers (see section Indirect Buffers) with buffer-local bindings offer a way to handle these situations more robustly.
If sufficient application is found for either of these two kinds of local bindings, we will provide it in a subsequent Emacs version.
A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them.
In a general sense, a function is a rule for carrying on a computation given several values called arguments. The result of the computation is called the value of the function. The computation can also have side effects: lasting changes in the values of variables or the contents of data structures.
Here are important terms for functions in Emacs Lisp and for other function-like objects.
car
or append
. These functions are also called
built-in functions or subrs. (Special forms are also
considered primitives.)
Usually the reason we implement a function as a primitive is either
because it is fundamental, because it provides a low-level interface to
operating system services, or because it needs to run fast. Primitives
can be modified or added only by changing the C sources and recompiling
the editor. See section Writing Emacs Primitives.
command-execute
can invoke; it
is a possible definition for a key sequence. Some functions are
commands; a function written in Lisp is a command if it contains an
interactive declaration (see section Defining Commands). Such a function
can be called from Lisp expressions like other functions; in this case,
the fact that the function is a command makes no difference.
Keyboard macros (strings and vectors) are commands also, even though
they are not functions. A symbol is a command if its function
definition is a command; such symbols can be invoked with M-x.
The symbol is a function as well if the definition is a function.
See section Command Loop Overview.
t
if object is any kind of function,
or a special form or macro.
t
if object is a built-in function
(i.e., a Lisp primitive).
(subrp 'message) ; message
is a symbol,
=> nil ; not a subr object.
(subrp (symbol-function 'message))
=> t
t
if object is a byte-code
function. For example:
(byte-code-function-p (symbol-function 'next-line)) => t
A function written in Lisp is a list that looks like this:
(lambda (arg-variables...) [documentation-string] [interactive-declaration] body-forms...)
Such a list is called a lambda expression. In Emacs Lisp, it actually is valid as an expression--it evaluates to itself. In some other Lisp dialects, a lambda expression is not a valid expression at all. In either case, its main use is not to be evaluated as an expression, but to be called as a function.
The first element of a lambda expression is always the symbol
lambda
. This indicates that the list represents a function. The
reason functions are defined to start with lambda
is so that
other lists, intended for other uses, will not accidentally be valid as
functions.
The second element is a list of symbols--the argument variable names. This is called the lambda list. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. See section Local Variables.
The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. See section Documentation Strings of Functions.
The interactive declaration is a list of the form (interactive
code-string)
. This declares how to provide arguments if the
function is used interactively. Functions with this declaration are called
commands; they can be called using M-x or bound to a key.
Functions not intended to be called in this way should not have interactive
declarations. See section Defining Commands, for how to write an interactive
declaration.
The rest of the elements are the body of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, "a list of Lisp forms to evaluate"). The value returned by the function is the value returned by the last element of the body.
Consider for example the following function:
(lambda (a b c) (+ a b c))
We can call this function by writing it as the CAR of an expression, like this:
((lambda (a b c) (+ a b c)) 1 2 3)
This call evaluates the body of the lambda expression with the variable
a
bound to 1, b
bound to 2, and c
bound to 3.
Evaluation of the body adds these three numbers, producing the result 6;
therefore, this call to the function returns the value 6.
Note that the arguments can be the results of other function calls, as in this example:
((lambda (a b c) (+ a b c)) 1 (* 2 3) (- 5 4))
This evaluates the arguments 1
, (* 2 3)
, and (- 5
4)
from left to right. Then it applies the lambda expression to the
argument values 1, 6 and 1 to produce the value 8.
It is not often useful to write a lambda expression as the CAR of
a form in this way. You can get the same result, of making local
variables and giving them values, using the special form let
(see section Local Variables). And let
is clearer and easier to use.
In practice, lambda expressions are either stored as the function
definitions of symbols, to produce named functions, or passed as
arguments to other functions (see section Anonymous Functions).
However, calls to explicit lambda expressions were very useful in the
old days of Lisp, before the special form let
was invented. At
that time, they were the only way to bind and initialize local
variables.
Our simple sample function, (lambda (a b c) (+ a b c))
,
specifies three argument variables, so it must be called with three
arguments: if you try to call it with only two arguments or four
arguments, you get a wrong-number-of-arguments
error.
It is often convenient to write a function that allows certain
arguments to be omitted. For example, the function substring
accepts three arguments--a string, the start index and the end
index--but the third argument defaults to the length of the
string if you omit it. It is also convenient for certain functions to
accept an indefinite number of arguments, as the functions list
and +
do.
To specify optional arguments that may be omitted when a function
is called, simply include the keyword &optional
before the optional
arguments. To specify a list of zero or more extra arguments, include the
keyword &rest
before one final argument.
Thus, the complete syntax for an argument list is as follows:
(required-vars... [&optional optional-vars...] [&rest rest-var])
The square brackets indicate that the &optional
and &rest
clauses, and the variables that follow them, are optional.
A call to the function requires one actual argument for each of the
required-vars. There may be actual arguments for zero or more of
the optional-vars, and there cannot be any actual arguments beyond
that unless the lambda list uses &rest
. In that case, there may
be any number of extra actual arguments.
If actual arguments for the optional and rest variables are omitted,
then they always default to nil
. There is no way for the
function to distinguish between an explicit argument of nil
and
an omitted argument. However, the body of the function is free to
consider nil
an abbreviation for some other meaningful value.
This is what substring
does; nil
as the third argument to
substring
means to use the length of the string supplied.
Common Lisp note: Common Lisp allows the function to specify what default value to use when an optional argument is omitted; Emacs Lisp always uses
nil
. Emacs Lisp does not support "supplied-p" variables that tell you whether an argument was explicitly passed.
For example, an argument list that looks like this:
(a b &optional c d &rest e)
binds a
and b
to the first two actual arguments, which are
required. If one or two more arguments are provided, c
and
d
are bound to them respectively; any arguments after the first
four are collected into a list and e
is bound to that list. If
there are only two arguments, c
is nil
; if two or three
arguments, d
is nil
; if four arguments or fewer, e
is nil
.
There is no way to have required arguments following optional
ones--it would not make sense. To see why this must be so, suppose
that c
in the example were optional and d
were required.
Suppose three actual arguments are given; which variable would the third
argument be for? Similarly, it makes no sense to have any more
arguments (either required or optional) after a &rest
argument.
Here are some examples of argument lists and proper calls:
((lambda (n) (1+ n)) ; One required: 1) ; requires exactly one argument. => 2 ((lambda (n &optional n1) ; One required and one optional: (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments. 1 2) => 3 ((lambda (n &rest ns) ; One required and one rest: (+ n (apply '+ ns))) ; 1 or more arguments. 1 2 3 4 5) => 15
A lambda expression may optionally have a documentation string just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. See section Documentation, for how the documentation-string is accessed.
It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access.
The first line of the documentation string should stand on its own,
because apropos
displays just this first line. It should consist
of one or two complete sentences that summarize the function's purpose.
The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. This is a mistake. The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands.
You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation.
In most computer languages, every function has a name; the idea of a
function without a name is nonsensical. In Lisp, a function in the
strictest sense has no name. It is simply a list whose first element is
lambda
, a byte-code function object, or a primitive subr-object.
However, a symbol can serve as the name of a function. This happens when you put the function in the symbol's function cell (see section Symbol Components). Then the symbol itself becomes a valid, callable function, equivalent to the list or subr-object that its function cell refers to. The contents of the function cell are also called the symbol's function definition. The procedure of using a symbol's function definition in place of the symbol is called symbol function indirection; see section Symbol Function Indirection.
In practice, nearly all functions are given names in this way and
referred to through their names. For example, the symbol car
works
as a function and does what it does because the primitive subr-object
#<subr car>
is stored in its function cell.
We give functions names because it is convenient to refer to them by
their names in Lisp expressions. For primitive subr-objects such as
#<subr car>
, names are the only way you can refer to them: there
is no read syntax for such objects. For functions written in Lisp, the
name is more convenient to use in a call than an explicit lambda
expression. Also, a function with a name can refer to itself--it can
be recursive. Writing the function's name in its own definition is much
more convenient than making the function definition point to itself
(something that is not impossible but that has various disadvantages in
practice).
We often identify functions with the symbols used to name them. For
example, we often speak of "the function car
", not
distinguishing between the symbol car
and the primitive
subr-object that is its function definition. For most purposes, there
is no need to distinguish.
Even so, keep in mind that a function need not have a unique name. While
a given function object usually appears in the function cell of only
one symbol, this is just a matter of convenience. It is easy to store
it in several symbols using fset
; then each of the symbols is
equally well a name for the same function.
A symbol used as a function name may also be used as a variable; these two uses of a symbol are independent and do not conflict. (Some Lisp dialects, such as Scheme, do not distinguish between a symbol's value and its function definition; a symbol's value as a variable is also its function definition.) If you have not given a symbol a function definition, you cannot use it as a function; whether the symbol has a value as a variable makes no difference to this.
We usually give a name to a function when it is first created. This
is called defining a function, and it is done with the
defun
special form.
defun
is the usual way to define new Lisp functions. It
defines the symbol name as a function that looks like this:
(lambda argument-list . body-forms)
defun
stores this lambda expression in the function cell of
name. It returns the value name, but usually we ignore this
value.
As described previously (see section Lambda Expressions),
argument-list is a list of argument names and may include the
keywords &optional
and &rest
. Also, the first two of the
body-forms may be a documentation string and an interactive
declaration.
There is no conflict if the same symbol name is also used as a variable, since the symbol's value cell is independent of the function cell. See section Symbol Components.
Here are some examples:
(defun foo () 5) => foo (foo) => 5 (defun bar (a &optional b &rest c) (list a b c)) => bar (bar 1 2 3 4 5) => (1 2 (3 4 5)) (bar 1) => (1 nil nil) (bar) error--> Wrong number of arguments. (defun capitalize-backwards () "Upcase the last letter of a word." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) => capitalize-backwards
Be careful not to redefine existing functions unintentionally.
defun
redefines even primitive functions such as car
without any hesitation or notification. Redefining a function already
defined is often done deliberately, and there is no way to distinguish
deliberate redefinition from unintentional redefinition.
The proper place to use defalias
is where a specific function
name is being defined--especially where that name appears explicitly in
the source file being loaded. This is because defalias
records
which file defined the function, just like defun
(see section Unloading).
By contrast, in programs that manipulate function definitions for other
purposes, it is better to use fset
, which does not keep such
records.
See also defsubst
, which defines a function like defun
and tells the Lisp compiler to open-code it. See section Inline Functions.
Defining functions is only half the battle. Functions don't do anything until you call them, i.e., tell them to run. Calling a function is also known as invocation.
The most common way of invoking a function is by evaluating a list.
For example, evaluating the list (concat "a" "b")
calls the
function concat
with arguments "a"
and "b"
.
See section Evaluation, for a description of evaluation.
When you write a list as an expression in your program, the function
name it calls is written in your program. This means that you choose
which function to call, and how many arguments to give it, when you
write the program. Usually that's just what you want. Occasionally you
need to compute at run time which function to call. To do that, use the
function funcall
. When you also need to determine at run time
how many arguments to pass, use apply
.
funcall
calls function with arguments, and returns
whatever function returns.
Since funcall
is a function, all of its arguments, including
function, are evaluated before funcall
is called. This
means that you can use any expression to obtain the function to be
called. It also means that funcall
does not see the expressions
you write for the arguments, only their values. These values are
not evaluated a second time in the act of calling function;
funcall
enters the normal procedure for calling a function at the
place where the arguments have already been evaluated.
The argument function must be either a Lisp function or a
primitive function. Special forms and macros are not allowed, because
they make sense only when given the "unevaluated" argument
expressions. funcall
cannot provide these because, as we saw
above, it never knows them in the first place.
(setq f 'list) => list (funcall f 'x 'y 'z) => (x y z) (funcall f 'x 'y '(z)) => (x y (z)) (funcall 'and t nil) error--> Invalid function: #<subr and>
Compare these example with the examples of apply
.
apply
calls function with arguments, just like
funcall
but with one difference: the last of arguments is a
list of objects, which are passed to function as separate
arguments, rather than a single list. We say that apply
spreads this list so that each individual element becomes an
argument.
apply
returns the result of calling function. As with
funcall
, function must either be a Lisp function or a
primitive function; special forms and macros do not make sense in
apply
.
(setq f 'list) => list (apply f 'x 'y 'z) error--> Wrong type argument: listp, z (apply '+ 1 2 '(3 4)) => 10 (apply '+ '(1 2 3 4)) => 10 (apply 'append '((a b c) nil (x y z) nil)) => (a b c x y z)
For an interesting example of using apply
, see the description of
mapcar
, in section Mapping Functions.
It is common for Lisp functions to accept functions as arguments or
find them in data structures (especially in hook variables and property
lists) and call them using funcall
or apply
. Functions
that accept function arguments are often called functionals.
Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function:
nil
.
A mapping function applies a given function to each element of a
list or other collection. Emacs Lisp has several such functions;
mapcar
and mapconcat
, which scan a list, are described
here. See section Creating and Interning Symbols, for the function mapatoms
which
maps over the symbols in an obarray.
These mapping functions do not allow char-tables because a char-table
is a sparse array whose nominal range of indices is very large. To map
over a char-table in a way that deals properly with its sparse nature,
use the function map-char-table
(see section Char-Tables).
mapcar
applies function to each element of sequence
in turn, and returns a list of the results.
The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. The result is always a list. The length of the result is the same as the length of sequence.
For example:
(mapcar 'car '((a b) (c d) (e f)))
=> (a c e)
(mapcar '1+ [1 2 3])
=> (2 3 4)
(mapcar 'char-to-string "abc")
=> ("a" "b" "c")
;; Call each function in my-hooks
.
(mapcar 'funcall my-hooks)
(defun mapcar* (function &rest args)
"Apply FUNCTION to successive cars of all ARGS.
Return the list of results."
;; If no list is exhausted,
(if (not (memq 'nil args))
;; apply function to CARs.
(cons (apply function (mapcar 'car args))
(apply 'mapcar* function
;; Recurse for rest of elements.
(mapcar 'cdr args)))))
(mapcar* 'cons '(a b c) '(1 2 3 4))
=> ((a . 1) (b . 2) (c . 3))
mapconcat
applies function to each element of
sequence: the results, which must be strings, are concatenated.
Between each pair of result strings, mapconcat
inserts the string
separator. Usually separator contains a space or comma or
other suitable punctuation.
The argument function must be a function that can take one argument and return a string. The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string.
(mapconcat 'symbol-name '(The cat in the hat) " ") => "The cat in the hat" (mapconcat (function (lambda (x) (format "%c" (1+ x)))) "HAL-8000" "") => "IBM.9111"
In Lisp, a function is a list that starts with lambda
, a
byte-code function compiled from such a list, or alternatively a
primitive subr-object; names are "extra". Although usually functions
are defined with defun
and given names at the same time, it is
occasionally more concise to use an explicit lambda expression--an
anonymous function. Such a list is valid wherever a function name is.
Any method of creating such a list makes a valid function. Even this:
(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) => (lambda (x) (+ 12 x))
This computes a list that looks like (lambda (x) (+ 12 x))
and
makes it the value (not the function definition!) of
silly
.
Here is how we might call this function:
(funcall silly 1) => 13
(It does not work to write (silly 1)
, because this function
is not the function definition of silly
. We have not given
silly
any function definition, just a value as a variable.)
Most of the time, anonymous functions are constants that appear in
your program. For example, you might want to pass one as an argument to
the function mapcar
, which applies any given function to each
element of a list.
Here we define a function change-property
which
uses a function as its third argument:
(defun change-property (symbol prop function) (let ((value (get symbol prop))) (put symbol prop (funcall function value))))
Here we define a function that uses change-property
,
passing it a function to double a number:
(defun double-property (symbol prop) (change-property symbol prop '(lambda (x) (* 2 x))))
In such cases, we usually use the special form function
instead
of simple quotation to quote the anonymous function, like this:
(defun double-property (symbol prop) (change-property symbol prop (function (lambda (x) (* 2 x)))))
Using function
instead of quote
makes a difference if you
compile the function double-property
. For example, if you
compile the second definition of double-property
, the anonymous
function is compiled as well. By contrast, if you compile the first
definition which uses ordinary quote
, the argument passed to
change-property
is the precise list shown:
(lambda (x) (* x 2))
The Lisp compiler cannot assume this list is a function, even though it
looks like one, since it does not know what change-property
will
do with the list. Perhaps it will check whether the CAR of the third
element is the symbol *
! Using function
tells the
compiler it is safe to go ahead and compile the constant function.
We sometimes write function
instead of quote
when
quoting the name of a function, but this usage is just a sort of
comment:
(function symbol) == (quote symbol) == 'symbol
The read syntax #'
is a short-hand for using function
.
For example,
#'(lambda (x) (* x x))
is equivalent to
(function (lambda (x) (* x x)))
quote
. However, it serves as a
note to the Emacs Lisp compiler that function-object is intended
to be used only as a function, and therefore can safely be compiled.
Contrast this with quote
, in section Quoting.
See documentation
in section Access to Documentation Strings, for a
realistic example using function
and an anonymous function.
The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.
See also the function indirect-function
in section Symbol Function Indirection.
void-function
error is
signaled.
This function does not check that the returned object is a legitimate function.
(defun bar (n) (+ n 2)) => bar (symbol-function 'bar) => (lambda (n) (+ n 2)) (fset 'baz 'bar) => bar (symbol-function 'baz) => bar
If you have never given a symbol any function definition, we say that
that symbol's function cell is void. In other words, the function
cell does not have any Lisp object in it. If you try to call such a symbol
as a function, it signals a void-function
error.
Note that void is not the same as nil
or the symbol
void
. The symbols nil
and void
are Lisp objects,
and can be stored into a function cell just as any other object can be
(and they can be valid functions if you define them in turn with
defun
). A void function cell contains no object whatsoever.
You can test the voidness of a symbol's function definition with
fboundp
. After you have given a symbol a function definition, you
can make it void once more using fmakunbound
.
t
if the symbol has an object in its
function cell, nil
otherwise. It does not check that the object
is a legitimate function.
void-function
error. (See also makunbound
, in section When a Variable is "Void".)
(defun foo (x) x) => foo (foo 1) =>1 (fmakunbound 'foo) => foo (foo 1) error--> Symbol's function definition is void: foo
There are three normal uses of this function:
defalias
instead of
fset
; see section Defining Functions.)
defun
. For example, you can use fset
to give a symbol s1
a function definition which is another symbol
s2
; then s1
serves as an alias for whatever definition
s2
presently has. (Once again use defalias
instead of
fset
if you think of this as the definition of s1
.)
defun
were not a primitive, it could be written in Lisp (as a macro) using
fset
.
Here are examples of these uses:
;; Savefoo
's definition inold-foo
. (fset 'old-foo (symbol-function 'foo)) ;; Make the symbolcar
the function definition ofxfirst
. ;; (Most likely,defalias
would be better thanfset
here.) (fset 'xfirst 'car) => car (xfirst '(1 2 3)) => 1 (symbol-function 'xfirst) => car (symbol-function (symbol-function 'xfirst)) => #<subr car> ;; Define a named keyboard macro. (fset 'kill-two-lines "\^u2\^k") => "\^u2\^k" ;; Here is a function that alters other functions. (defun copy-function-definition (new old) "Define NEW with the same function definition as OLD." (fset new (symbol-function old)))
When writing a function that extends a previously defined function, the following idiom is sometimes used:
(fset 'old-foo (symbol-function 'foo)) (defun foo () "Just like old-foo, except more so." (old-foo) (more-so))
This does not work properly if foo
has been defined to autoload.
In such a case, when foo
calls old-foo
, Lisp attempts
to define old-foo
by loading a file. Since this presumably
defines foo
rather than old-foo
, it does not produce the
proper results. The only way to avoid this problem is to make sure the
file is loaded before moving aside the old definition of foo
.
But it is unmodular and unclean, in any case, for a Lisp file to redefine a function defined elsewhere. It is cleaner to use the advice facility (see section Advising Emacs Lisp Functions).
You can define an inline function by using defsubst
instead
of defun
. An inline function works just like an ordinary
function except for one thing: when you compile a call to the function,
the function's definition is open-coded into the caller.
Making a function inline makes explicit calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them. Since the flexibility of redefining functions is an important feature of Emacs, you should not make a function inline unless its speed is really crucial.
Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline.
It's possible to define a macro to expand into the same code that an
inline function would execute. (See section Macros.) But the macro would be
limited to direct use in expressions--a macro cannot be called with
apply
, mapcar
and so on. Also, it takes some work to
convert an ordinary function into a macro. To convert it into an inline
function is very easy; simply replace defun
with defsubst
.
Since each argument of an inline function is evaluated exactly once, you
needn't worry about how many times the body uses the arguments, as you
do for macros. (See section Evaluating Macro Arguments Repeatedly.)
Inline functions can be used and open-coded later on in the same file, following the definition, just like macros.
Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here.
apply
autoload
call-interactively
commandp
documentation
eval
funcall
function
ignore
indirect-function
interactive
interactive
.
interactive-p
mapatoms
mapcar
map-char-table
mapconcat
undefined
Macros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro.
Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.
If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See section Inline Functions.
Suppose we would like to define a Lisp construct to increment a
variable value, much like the ++
operator in C. We would like to
write (inc x)
and have the effect of (setq x (1+ x))
.
Here's a macro definition that does the job:
(defmacro inc (var) (list 'setq var (list '1+ var)))
When this is called with (inc x)
, the argument var is the
symbol x
---not the value of x
, as it would
be in a function. The body of the macro uses this to construct the
expansion, which is (setq x (1+ x))
. Once the macro definition
returns this expansion, Lisp proceeds to evaluate it, thus incrementing
x
.
A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro.
Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list.
Having obtained the arguments, Lisp invokes the macro definition just
as a function is invoked. The argument variables of the macro are bound
to the argument values from the macro call, or to a list of them in the
case of a &rest
argument. And the macro body executes and
returns its value just as a function body does.
The second crucial difference between macros and functions is that the value returned by the macro body is not the value of the macro call. Instead, it is an alternate expression for computing that value, also known as the expansion of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro.
Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual.
You can see the expansion of a given macro call by calling
macroexpand
.
macroexpand
. If form is not a macro call to begin with, it
is returned as given.
Note that macroexpand
does not look at the subexpressions of
form (although some macro definitions may do so). Even if they
are macro calls themselves, macroexpand
does not expand them.
The function macroexpand
does not expand calls to inline functions.
Normally there is no need for that, since a call to an inline function is
no harder to understand than a call to an ordinary function.
If environment is provided, it specifies an alist of macro definitions that shadow the currently defined macros. Byte compilation uses this feature.
(defmacro inc (var)
(list 'setq var (list '1+ var)))
=> inc
(macroexpand '(inc r))
=> (setq r (1+ r))
(defmacro inc2 (var1 var2)
(list 'progn (list 'inc var1) (list 'inc var2)))
=> inc2
(macroexpand '(inc2 r s))
=> (progn (inc r) (inc s)) ; inc
not expanded here.
You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation.
When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself--they would be computed at compile time, which is not useful.
In order for compilation of macro calls to work, the macros must
already be defined in Lisp when the calls to them are compiled. The
compiler has a special feature to help you do this: if a file being
compiled contains a defmacro
form, the macro is defined
temporarily for the rest of the compilation of that file. To make this
feature work, you must put the defmacro
in the same file where it
is used, and before its first use.
Byte-compiling a file executes any require
calls at top-level
in the file. This is in case the file needs the required packages for
proper compilation. One way to ensure that necessary macro definitions
are available during compilation is to require the files that define
them (see section Features). To avoid loading the macro definition files
when someone runs the compiled program, write
eval-when-compile
around the require
calls (see section Evaluation During Compilation).
A Lisp macro is a list whose CAR is macro
. Its CDR should
be a function; expansion of the macro works by applying the function
(with apply
) to the list of unevaluated argument-expressions
from the macro call.
It is possible to use an anonymous Lisp macro just like an anonymous
function, but this is never done, because it does not make sense to pass
an anonymous macro to functionals such as mapcar
. In practice,
all Lisp macros have names, and they are usually defined with the
special form defmacro
.
defmacro
defines the symbol name as a macro that looks
like this:
(macro lambda argument-list . body-forms)
(Note that the CDR of this list is a function--a lambda expression.)
This macro object is stored in the function cell of name. The
value returned by evaluating the defmacro
form is name, but
usually we ignore this value.
The shape and meaning of argument-list is the same as in a
function, and the keywords &rest
and &optional
may be used
(see section Other Features of Argument Lists). Macros may have a documentation string, but
any interactive
declaration is ignored since macros cannot be
called interactively.
Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ``' syntax (usually called backquote).
Backquote allows you to quote a list, but selectively evaluate
elements of that list. In the simplest case, it is identical to the
special form quote
(see section Quoting). For example, these
two forms yield identical results:
`(a list of (+ 2 3) elements) => (a list of (+ 2 3) elements) '(a list of (+ 2 3) elements) => (a list of (+ 2 3) elements)
The special marker `,' inside of the argument to backquote indicates a value that isn't constant. Backquote evaluates the argument of `,' and puts the value in the list structure:
(list 'a 'list 'of (+ 2 3) 'elements) => (a list of 5 elements) `(a list of ,(+ 2 3) elements) => (a list of 5 elements)
Substitution with `,' is allowed at deeper levels of the list structure also. For example:
(defmacro t-becomes-nil (variable) `(if (eq ,variable t) (setq ,variable nil))) (t-becomes-nil foo) == (if (eq foo t) (setq foo nil))
You can also splice an evaluated value into the resulting list, using the special marker `,@'. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ``' is often unreadable. Here are some examples:
(setq some-list '(2 3)) => (2 3) (cons 1 (append some-list '(4) some-list)) => (1 2 3 4 2 3) `(1 ,@some-list 4 ,@some-list) => (1 2 3 4 2 3) (setq list '(hack foo bar)) => (hack foo bar) (cons 'use (cons 'the (cons 'words (append (cdr list) '(as elements))))) => (use the words foo bar as elements) `(use the words ,@(cdr list) as elements) => (use the words foo bar as elements)
In old Emacs versions, before version 19.29, ``' used a different syntax which required an extra level of parentheses around the entire backquote construct. Likewise, each `,' or `,@' substitution required an extra level of parentheses surrounding both the `,' or `,@' and the following expression. The old syntax required whitespace between the ``', `,' or `,@' and the following expression.
This syntax is still accepted, for compatibility with old Emacs versions, but we recommend not using it in new programs.
The basic facts of macro expansion have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble.
When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a simple "for" loop such as one might find in Pascal.
(defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." (list 'let (list (list var init)) (cons 'while (cons (list '<= var final) (append body (list (list 'inc var))))))) => for (for i from 1 to 3 do (setq square (* i i)) (princ (format "\n%d %d" i square))) ==> (let ((i 1)) (while (<= i 3) (setq square (* i i)) (princ (format "%d %d" i square)) (inc i))) -|1 1 -|2 4 -|3 9 => nil
The arguments from
, to
, and do
in this macro are
"syntactic sugar"; they are entirely ignored. The idea is that you
will write noise words (such as from
, to
, and do
)
in those positions in the macro call.
Here's an equivalent definition simplified through use of backquote:
(defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." `(let ((,var ,init)) (while (<= ,var ,final) ,@body (inc ,var))))
Both forms of this definition (with backquote and without) suffer from
the defect that final is evaluated on every iteration. If
final is a constant, this is not a problem. If it is a more
complex form, say (long-complex-calculation x)
, this can slow
down the execution significantly. If final has side effects,
executing it more than once is probably incorrect.
A well-designed macro definition takes steps to avoid this problem by
producing an expansion that evaluates the argument expressions exactly
once unless repeated evaluation is part of the intended purpose of the
macro. Here is a correct expansion for the for
macro:
(let ((i 1) (max 3)) (while (<= i max) (setq square (* i i)) (princ (format "%d %d" i square)) (inc i)))
Here is a macro definition that creates this expansion:
(defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." `(let ((,var ,init) (max ,final)) (while (<= ,var max) ,@body (inc ,var))))
Unfortunately, this fix introduces another problem, described in the following section.
The new definition of for
has a new problem: it introduces a
local variable named max
which the user does not expect. This
causes trouble in examples such as the following:
(let ((max 0)) (for x from 0 to 10 do (let ((this (frob x))) (if (< max this) (setq max this)))))
The references to max
inside the body of the for
, which
are supposed to refer to the user's binding of max
, really access
the binding made by for
.
The way to correct this is to use an uninterned symbol instead of
max
(see section Creating and Interning Symbols). The uninterned symbol can be
bound and referred to just like any other symbol, but since it is
created by for
, we know that it cannot already appear in the
user's program. Since it is not interned, there is no way the user can
put it into the program later. It will never appear anywhere except
where put by for
. Here is a definition of for
that works
this way:
(defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." (let ((tempvar (make-symbol "max"))) `(let ((,var ,init) (,tempvar ,final)) (while (<= ,var ,tempvar) ,@body (inc ,var)))))
This creates an uninterned symbol named max
and puts it in the
expansion instead of the usual interned symbol max
that appears
in expressions ordinarily.
Another problem can happen if the macro definition itself
evaluates any of the macro argument expressions, such as by calling
eval
(see section Eval). If the argument is supposed to refer to the
user's variables, you may have trouble if the user happens to use a
variable with the same name as one of the macro arguments. Inside the
macro body, the macro argument binding is the most local binding of this
variable, so any references inside the form being evaluated do refer to
it. Here is an example:
(defmacro foo (a) (list 'setq (eval a) t)) => foo (setq x 'b) (foo x) ==> (setq b t) => t ; andb
has been set. ;; but (setq a 'c) (foo a) ==> (setq a t) => t ; but this seta
, notc
.
It makes a difference whether the user's variable is named a
or
x
, because a
conflicts with the macro argument variable
a
.
Another problem with calling eval
in a macro definition is that
it probably won't do what you intend in a compiled program. The
byte-compiler runs macro definitions while compiling the program, when
the program's own computations (which you might have wished to access
with eval
) don't occur and its local variable bindings don't
exist.
To avoid these problems, don't evaluate an argument expression while computing the macro expansion. Instead, substitute the expression into the macro expansion, so that its value will be computed as part of executing the expansion. This is how the other examples in this chapter work.
Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded.
Therefore, you should avoid side effects in computation of the macro expansion, unless you really know what you are doing.
One special kind of side effect can't be avoided: constructing Lisp objects. Almost all macro expansions include constructed lists; that is the whole point of most macros. This is usually safe; there is just one case where you must be careful: when the object you construct is part of a quoted constant in the macro expansion.
If the macro is expanded just once, in compilation, then the object is constructed just once, during compilation. But in interpreted execution, the macro is expanded each time the macro call runs, and this means a new object is constructed each time.
In most clean Lisp code, this difference won't matter. It can matter only if you perform side-effects on the objects constructed by the macro definition. Thus, to avoid trouble, avoid side effects on objects constructed by macro definitions. Here is an example of how such side effects can get you into trouble:
(defmacro empty-object () (list 'quote (cons nil nil))) (defun initialize (condition) (let ((object (empty-object))) (if condition (setcar object condition)) object))
If initialize
is interpreted, a new list (nil)
is
constructed each time initialize
is called. Thus, no side effect
survives between calls. If initialize
is compiled, then the
macro empty-object
is expanded during compilation, producing a
single "constant" (nil)
that is reused and altered each time
initialize
is called.
One way to avoid pathological cases like this is to think of
empty-object
as a funny kind of constant, not as a memory
allocation construct. You wouldn't use setcar
on a constant such
as '(nil)
, so naturally you won't use it on (empty-object)
either.
This chapter describes how to declare user options for customization, and also customization groups for classifying them. We use the term customization item to include both kinds of customization definitions--as well as face definitions (see section Defining Faces).
All kinds of customization declarations (for variables and groups, and for faces) accept keyword arguments for specifying various information. This section describes some keywords that apply to all kinds.
All of these keywords, except :tag
, can be used more than once
in a given item. Each use of the keyword has an independent effect.
The keyword :tag
is an exception because any given item can only
display one name.
:tag name
:group group
:group
in a defgroup
, it makes the new group a subgroup of
group.
If you use this keyword more than once, you can put a single item into
more than one group. Displaying any of those groups will show this
item. Be careful not to overdo this!
:link link-data
(custom-manual info-node)
"(emacs)Top"
. The link appears as
`[manual]' in the customization buffer.
(info-link info-node)
custom-manual
except that the link appears
in the customization buffer with the Info node name.
(url-link url)
:tag name
after the first element of the link-data;
for example, (info-link :tag "foo" "(emacs)Top")
makes a link to
the Emacs manual which appears in the buffer as `foo'.
An item can have more than one external link; however, most items have
none at all.
:load file
load-library
, and only if the file is
not already loaded.
:require feature
require
.
The most common reason to use :require
is when a variable enables
a feature such as a minor mode, and just setting the variable won't have
any effect unless the code which implements the mode is loaded.
Each Emacs Lisp package should have one main customization group which contains all the options, faces and other groups in the package. If the package has a small number of options and faces, use just one group and put everything in it. When there are more than twelve or so options and faces, then you should structure them into subgroups, and put the subgroups under the package's main customization group. It is OK to put some of the options and faces in the package's main group alongside the subgroups.
The package's main or only group should be a member of one or more of
the standard customization groups. (To display the full list of them,
use M-x customize.) Choose one or more of them (but not too
many), and add your group to each of them using the :group
keyword.
The way to declare new customization groups is with defgroup
.
The argument members is a list specifying an initial set of
customization items to be members of the group. However, most often
members is nil
, and you specify the group's members by
using the :group
keyword when defining those members.
If you want to specify group members through members, each element
should have the form (name widget)
. Here name
is a symbol, and widget is a widget type for editing that symbol.
Useful widgets are custom-variable
for a variable,
custom-face
for a face, and custom-group
for a group.
In addition to the common keywords (see section Common Keywords for All Kinds of Items), you can
use this keyword in defgroup
:
:prefix prefix
The prefix-discarding feature is currently turned off, which means
that :prefix
currently has no effect. We did this because we
found that discarding the specified prefixes often led to confusing
names for options. This happened because the people who wrote the
defgroup
definitions for various groups added :prefix
keywords whenever they make logical sense--that is, whenever the
variables in the library have a common prefix.
In order to obtain good results with :prefix
, it would be
necessary to check the specific effects of discarding a particular
prefix, given the specific items in a group and their names and
documentation. If the resulting text is not clear, then :prefix
should not be used in that case.
It should be possible to recheck all the customization groups, delete
the :prefix
specifications which give unclear results, and then
turn this feature back on, if someone would like to do the work.
Use defcustom
to declare user-editable variables.
If option is void, defcustom
initializes it to
default. default should be an expression to compute the
value; be careful in writing it, because it can be evaluated on more
than one occasion.
defcustom
accepts the following additional keywords:
:type type
:options list
hook
. In that
case, the elements of list should be functions that are useful as
elements of the hook value. The user is not restricted to using only
these functions, but they are offered as convenient alternatives.
:version version
(defcustom foo-max 34 "*Maximum number of foo's allowed." :type 'integer :group 'foo :version "20.3")
:set setfunction
set-default
.
:get getfunction
default-value
.
:initialize function
defcustom
is evaluated. It should take two arguments, the
symbol and value. Here are some predefined functions meant for use in
this way:
custom-initialize-set
:set
function to initialize the variable, but
do not reinitialize it if it is already non-void. This is the default
:initialize
function.
custom-initialize-default
custom-initialize-set
, but use the function
set-default
to set the variable, instead of the variable's
:set
function. This is the usual choice for a variable whose
:set
function enables or disables a minor mode; with this choice,
defining the variable will not call the minor mode function, but
customizing the variable will do so.
custom-initialize-reset
:set
function to initialize the variable. If the
variable is already non-void, reset it by calling the :set
function using the current value (returned by the :get
method).
custom-initialize-changed
:set
function to initialize the variable, if it is
already set or has been customized; otherwise, just use
set-default
.
The :require
option is useful for an option that turns on the
operation of a certain feature. Assuming that the package is coded to
check the value of the option, you still need to arrange for the package
to be loaded. You can do that with :require
. See section Common Keywords for All Kinds of Items. Here is an example, from the library `paren.el':
(defcustom show-paren-mode nil "Toggle Show Paren mode@enddots{}" :set (lambda (symbol value) (show-paren-mode (or value 0))) :initialize 'custom-initialize-default :type 'boolean :group 'paren-showing :require 'paren)
Internally, defcustom
uses the symbol property
standard-value
to record the expression for the default value,
and saved-value
to record the value saved by the user with the
customization buffer. The saved-value
property is actually a
list whose car is an expression which evaluates to the value.
When you define a user option with defcustom
, you must specify
its customization type. That is a Lisp object which describes (1)
which values are legitimate and (2) how to display the value in the
customization buffer for editing.
You specify the customization type in defcustom
with the
:type
keyword. The argument of :type
is evaluated; since
types that vary at run time are rarely useful, normally you use a quoted
constant. For example:
(defcustom diff-command "diff" "*The command to use to run diff." :type '(string) :group 'diff)
In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (see section Type Keywords).
Some of the type symbols do not use any arguments; those are called
simple types. For a simple type, if you do not use any
keyword-value pairs, you can omit the parentheses around the type
symbol. For example just string
as a customization type is
equivalent to (string)
.
This section describes all the simple customization types.
sexp
sexp
as a fall-back for any option, if you don't want to
take the time to work out a more specific type to use.
integer
number
string
regexp
string
except that the string must be a valid regular
expression.
character
file
(file :must-match t)
directory
hook
:options
keyword in a hook variable's
defcustom
to specify a list of functions recommended for use in
the hook; see section Defining Customization Variables.
symbol
function
variable
face
boolean
nil
or t
. Note that by
using choice
and const
together (see the next section),
you can specify that the value must be nil
or t
, but also
specify the text to describe each value in a way that fits the specific
meaning of the alternative.
When none of the simple types is appropriate, you can use composite types, which build new types from other types. Here are several ways of doing that:
(restricted-sexp :match-alternatives criteria)
nil
or non-nil
according to
the argument. Using a predicate in the list says that objects for which
the predicate returns non-nil
are acceptable.
'object
. This sort of element
in the list says that object itself is an acceptable value.
(restricted-sexp :match-alternatives (integerp 't 'nil))allows integers,
t
and nil
as legitimate values.
The customization buffer shows all legitimate values using their read
syntax, and the user edits them textually.
(cons car-type cdr-type)
(cons string
symbol)
is a customization type which matches values such as
("foo" . foo)
.
In the customization buffer, the CAR and the CDR are
displayed and edited separately, each according to the type
that you specify for it.
(list element-types...)
(list integer string function)
describes a list of
three elements; the first element must be an integer, the second a
string, and the third a function.
In the customization buffer, each element is displayed and edited
separately, according to the type specified for it.
(vector element-types...)
list
except that the value must be a vector instead of a
list. The elements work the same as in list
.
(choice alternative-types...)
(choice integer string)
allows either an
integer or a string.
In the customization buffer, the user selects one of the alternatives
using a menu, and can then edit the value in the usual way for that
alternative.
Normally the strings in this menu are determined automatically from the
choices; however, you can specify different strings for the menu by
including the :tag
keyword in the alternatives. For example, if
an integer stands for a number of spaces, while a string is text to use
verbatim, you might write the customization type this way,
(choice (integer :tag "Number of spaces") (string :tag "Literal text"))so that the menu offers `Number of spaces' and `Literal Text'. In any alternative for which
nil
is not a valid value, other than
a const
, you should specify a valid default for that alternative
using the :value
keyword. See section Type Keywords.
(const value)
const
is inside of choice
. For example,
(choice integer (const nil))
allows either an integer or
nil
.
:tag
is often used with const
, inside of choice
.
For example,
(choice (const :tag "Yes" t) (const :tag "No" nil) (const :tag "Ask" foo))describes a variable for which
t
means yes, nil
means no,
and foo
means "ask."
(other value)
other
is as the last element of choice
.
For example,
(choice (const :tag "Yes" t) (const :tag "No" nil) (other :tag "Ask" foo))describes a variable for which
t
means yes, nil
means no,
and anything else means "ask." If the user chooses `Ask' from
the menu of alternatives, that specifies the value foo
; but any
other value (not t
, nil
or foo
) displays as
`Ask', just like foo
.
(function-item function)
const
, but used for values which are functions. This
displays the documentation string as well as the function name.
The documentation string is either the one you specify with
:doc
, or function's own documentation string.
(variable-item variable)
const
, but used for values which are variable names. This
displays the documentation string as well as the variable name. The
documentation string is either the one you specify with :doc
, or
variable's own documentation string.
(set elements...)
(repeat element-type)
The :inline
feature lets you splice a variable number of
elements into the middle of a list or vector. You use it in a
set
, choice
or repeat
type which appears among the
element-types of a list
or vector
.
Normally, each of the element-types in a list
or vector
describes one and only one element of the list or vector. Thus, if an
element-type is a repeat
, that specifies a list of unspecified
length which appears as one element.
But when the element-type uses :inline
, the value it matches is
merged directly into the containing sequence. For example, if it
matches a list with three elements, those become three elements of the
overall sequence. This is analogous to using `,@' in the backquote
construct.
For example, to specify a list whose first element must be t
and whose remaining arguments should be zero or more of foo
and
bar
, use this customization type:
(list (const t) (set :inline t foo bar))
This matches values such as (t)
, (t foo)
, (t bar)
and (t foo bar)
.
When the element-type is a choice
, you use :inline
not
in the choice
itself, but in (some of) the alternatives of the
choice
. For example, to match a list which must start with a
file name, followed either by the symbol t
or two strings, use
this customization type:
(list file (choice (const t) (list :inline t string string)))
If the user chooses the first alternative in the choice, then the
overall list has two elements and the second element is t
. If
the user chooses the second alternative, then the overall list has three
elements and the second and third must be strings.
You can specify keyword-argument pairs in a customization type after the type name symbol. Here are the keywords you can use, and their meanings:
:value default
choice
; it specifies the default value to use, at first, if and
when the user selects this alternative with the menu in the
customization buffer.
Of course, if the actual value of the option fits this alternative, it
will appear showing the actual value, not default.
If nil
is not a valid value for the alternative, then it is
essential to specify a valid default with :value
.
:format format-string
:action
attribute specifies what the button will do if the user invokes it;
its value is a function which takes two arguments--the widget which
the button appears in, and the event.
There is no way to specify two different buttons with different
actions.
:sample-face
.
:tag
keyword.
:action action
:button-face face
:button-prefix prefix
:button-suffix suffix
nil
:tag tag
:doc doc
:format
, and use `%d' or `%h'
in that value.
The usual reason to specify a documentation string for a type is to
provide more information about the meanings of alternatives inside a
:choice
type or the parts of some other composite type.
:help-echo motion-doc
widget-forward
or
widget-backward
, it will display the string motion-doc
in the echo area.
:match function
nil
if
the value is acceptable.
Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file.
The load functions evaluate all the expressions in a file just
as the eval-current-buffer
function evaluates all the
expressions in a buffer. The difference is that the load functions
read and evaluate the text in the file as found on disk, not the text
in an Emacs buffer.
The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a top-level form. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions.
A file containing Lisp code is often called a library. Thus, the "Rmail library" is a file containing code for Rmail mode. Similarly, a "Lisp library directory" is a directory of files containing Lisp code.
Emacs Lisp has several interfaces for loading. For example,
autoload
creates a placeholder object for a function defined in a
file; trying to call the autoloading function loads the file to get the
function's real definition (see section Autoload). require
loads a
file if it isn't already loaded (see section Features). Ultimately,
all these facilities call the load
function to do the work.
To find the file, load
first looks for a file named
`filename.elc', that is, for a file whose name is
filename with `.elc' appended. If such a file exists, it is
loaded. If there is no file by that name, then load
looks for a
file named `filename.el'. If that file exists, it is loaded.
Finally, if neither of those names is found, load
looks for a
file named filename with nothing appended, and loads it if it
exists. (The load
function is not clever about looking at
filename. In the perverse case of a file named `foo.el.el',
evaluation of (load "foo.el")
will indeed find it.)
If the optional argument nosuffix is non-nil
, then the
suffixes `.elc' and `.el' are not tried. In this case, you
must specify the precise file name you want. By specifying the precise
file name and using t
for nosuffix, you can prevent
perverse file names such as `foo.el.el' from being tried.
If the optional argument must-suffix is non-nil
, then
load
insists that the file name used must end in either
`.el' or `.elc', unless it contains an explicit directory
name. If filename does not contain an explicit directory name,
and does not end in a suffix, then load
insists on adding one.
If filename is a relative file name, such as `foo' or
`baz/foo.bar', load
searches for the file using the variable
load-path
. It appends filename to each of the directories
listed in load-path
, and loads the first file it finds whose name
matches. The current default directory is tried only if it is specified
in load-path
, where nil
stands for the default directory.
load
tries all three possible suffixes in the first directory in
load-path
, then all three suffixes in the second directory, and
so on. See section Library Search.
If you get a warning that `foo.elc' is older than `foo.el', it means you should consider recompiling `foo.el'. See section Byte Compilation.
When loading a source file (not compiled), load
performs
character set translation just as Emacs would do when visiting the file.
See section Coding Systems.
Messages like `Loading foo...' and `Loading foo...done' appear
in the echo area during loading unless nomessage is
non-nil
.
Any unhandled errors while loading a file terminate loading. If the
load was done for the sake of autoload
, any function definitions
made during the loading are undone.
If load
can't find the file to load, then normally it signals the
error file-error
(with `Cannot open load file
filename'). But if missing-ok is non-nil
, then
load
just returns nil
.
You can use the variable load-read-function
to specify a function
for load
to use instead of read
for reading expressions.
See below.
load
returns t
if the file loads successfully.
load-path
is not used, and suffixes are not appended. Use this
command if you wish to specify precisely the file name to load.
load
, except in how it reads its argument interactively.
nil
if Emacs is in the process of loading a
file, and it is nil
otherwise.
load
and eval-region
to use instead of read
.
The function should accept one argument, just as read
does.
Normally, the variable's value is nil
, which means those
functions should use read
.
Note: Instead of using this variable, it is cleaner to use
another, newer feature: to pass the function as the read-function
argument to eval-region
. See section Eval.
For information about how load
is used in building Emacs, see
section Building Emacs.
When Emacs loads a Lisp library, it searches for the library
in a list of directories specified by the variable load-path
.
load
. Each element is a string (which must be
a directory name) or nil
(which stands for the current working
directory).
The value of load-path
is initialized from the environment
variable EMACSLOADPATH
, if that exists; otherwise its default
value is specified in `emacs/src/paths.h' when Emacs is built.
Then the list is expanded by adding subdirectories of the directories
in the list.
The syntax of EMACSLOADPATH
is the same as used for PATH
;
`:' (or `;', according to the operating system) separates
directory names, and `.' is used for the current default directory.
Here is an example of how to set your EMACSLOADPATH
variable from
a csh
`.login' file:
setenv EMACSLOADPATH .:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp
Here is how to set it using sh
:
export EMACSLOADPATH EMACSLOADPATH=.:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp
Here is an example of code you can place in a `.emacs' file to add
several directories to the front of your default load-path
:
(setq load-path (append (list nil "/user/bil/emacs" "/usr/local/lisplib" "~/emacs") load-path))
In this example, the path searches the current working directory first, followed then by the `/user/bil/emacs' directory, the `/usr/local/lisplib' directory, and the `~/emacs' directory, which are then followed by the standard directories for Lisp code.
Dumping Emacs uses a special value of load-path
. If the value of
load-path
at the end of dumping is unchanged (that is, still the
same special value), the dumped Emacs switches to the ordinary
load-path
value when it starts up, as described above. But if
load-path
has any other value at the end of dumping, that value
is used for execution of the dumped Emacs also.
Therefore, if you want to change load-path
temporarily for
loading a few libraries in `site-init.el' or `site-load.el',
you should bind load-path
locally with let
around the
calls to load
.
The default value of load-path
, when running an Emacs which has
been installed on the system, includes two special directories (and
their subdirectories as well):
"/usr/local/share/emacs/version/site-lisp"
and
"/usr/local/share/emacs/site-lisp"
The first one is for locally installed packages for a particular Emacs version; the second is for locally installed packages meant for use with all installed Emacs versions.
There are several reasons why a Lisp package that works well in one Emacs version can cause trouble in another. Sometimes packages need updating for incompatible changes in Emacs; sometimes they depend on undocumented internal Emacs data that can change without notice; sometimes a newer Emacs version incorporates a version of the package, and should be used only with that version.
Emacs finds these directories' subdirectories and adds them to
load-path
when it starts up. Both immediate subdirectories and
subdirectories multiple levels down are added to load-path
.
Not all subdirectories are included, though. Subdirectories whose names do not start with a letter or digit are excluded. Subdirectories named `RCS' are excluded. Also, a subdirectory which contains a file named `.nosearch' is excluded. You can use these methods to prevent certain subdirectories of the `site-lisp' directories from being searched.
If you run Emacs from the directory where it was built--that is, an
executable that has not been formally installed--then load-path
normally contains two additional directories. These are the lisp
and site-lisp
subdirectories of the main build directory. (Both
are represented as absolute file names.)
load
does, and the
argument nosuffix has the same meaning as in load
: don't
add suffixes `.elc' or `.el' to the specified name
library.
If the path is non-nil
, that list of directories is used
instead of load-path
.
When locate-library
is called from a program, it returns the file
name as a string. When the user runs locate-library
interactively, the argument interactive-call is t
, and this
tells locate-library
to display the file name in the echo area.
When Emacs Lisp programs contain string constants with non-ASCII characters, these can be represented within Emacs either as unibyte strings or as multibyte strings (see section Text Representations). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. See section Coding Systems.
To make the results more predictable, Emacs always performs decoding into the multibyte representation when loading Lisp files, even if it was started with the `--unibyte' option. This means that string constants with non-ASCII characters translate into multibyte strings. The only exception is when a particular file specifies no decoding.
The reason Emacs is designed this way is so that Lisp programs give
predictable results, regardless of how Emacs was started. In addition,
this enables programs that depend on using multibyte text to work even
in a unibyte Emacs. Of course, such programs should be designed to
notice whether the user prefers unibyte or multibyte text, by checking
default-enable-multibyte-characters
, and convert representations
appropriately.
In most Emacs Lisp programs, the fact that non-ASCII strings are multibyte strings should not be noticeable, since inserting them in unibyte buffers converts them to unibyte automatically. However, if this does make a difference, you can force a particular Lisp file to be interpreted as unibyte by writing `-*-unibyte: t;-*-' in a comment on the file's first line. With that designator, the file will be unconditionally be interpreted as unibyte, even in an ordinary multibyte Emacs session.
The autoload facility allows you to make a function or macro known in Lisp, but put off loading the file that defines it. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along.
There are two ways to set up an autoloaded function: by calling
autoload
, and by writing a special "magic" comment in the
source before the real definition. autoload
is the low-level
primitive for autoloading; any Lisp program can call autoload
at
any time. Magic comments are the most convenient way to make a function
autoload, for packages installed along with Emacs. These comments do
nothing on their own, but they serve as a guide for the command
update-file-autoloads
, which constructs calls to autoload
and arranges to execute them when Emacs is built.
If filename does not contain either a directory name, or the
suffix .el
or .elc
, then autoload
insists on adding
one of these suffixes, and it will not load from a file whose name is
just filename with no added suffix.
The argument docstring is the documentation string for the
function. Normally, this should be identical to the documentation string
in the function definition itself. Specifying the documentation string
in the call to autoload
makes it possible to look at the
documentation without loading the function's real definition.
If interactive is non-nil
, that says function can be
called interactively. This lets completion in M-x work without
loading function's real definition. The complete interactive
specification is not given here; it's not needed unless the user
actually calls function, and when that happens, it's time to load
the real definition.
You can autoload macros and keymaps as well as ordinary functions.
Specify type as macro
if function is really a macro.
Specify type as keymap
if function is really a
keymap. Various parts of Emacs need to know this information without
loading the real definition.
An autoloaded keymap loads automatically during key lookup when a prefix
key's binding is the symbol function. Autoloading does not occur
for other kinds of access to the keymap. In particular, it does not
happen when a Lisp program gets the keymap from the value of a variable
and calls define-key
; not even if the variable name is the same
symbol function.
If function already has a non-void function definition that is not
an autoload object, autoload
does nothing and returns nil
.
If the function cell of function is void, or is already an autoload
object, then it is defined as an autoload object like this:
(autoload filename docstring interactive type)
For example,
(symbol-function 'run-prolog) => (autoload "prolog" 169681 t nil)
In this case, "prolog"
is the name of the file to load, 169681
refers to the documentation string in the
`emacs/etc/DOC-version' file (see section Documentation Basics),
t
means the function is interactive, and nil
that it is
not a macro or a keymap.
The autoloaded file usually contains other definitions and may require
or provide one or more features. If the file is not completely loaded
(due to an error in the evaluation of its contents), any function
definitions or provide
calls that occurred during the load are
undone. This is to ensure that the next attempt to call any function
autoloading from this file will try again to load the file. If not for
this, then some of the functions in the file might be defined by the
aborted load, but fail to work properly for the lack of certain
subroutines not loaded successfully because they come later in the file.
If the autoloaded file fails to define the desired Lisp function or
macro, then an error is signaled with data "Autoloading failed to
define function function-name"
.
A magic autoload comment consists of `;;;###autoload', on a line
by itself, just before the real definition of the function in its
autoloadable source file. The command M-x update-file-autoloads
writes a corresponding autoload
call into `loaddefs.el'.
Building Emacs loads `loaddefs.el' and thus calls autoload
.
M-x update-directory-autoloads is even more powerful; it updates
autoloads for all files in the current directory.
The same magic comment can copy any kind of form into `loaddefs.el'. If the form following the magic comment is not a function definition, it is copied verbatim. You can also use a magic comment to execute a form at build time without executing it when the file itself is loaded. To do this, write the form on the same line as the magic comment. Since it is in a comment, it does nothing when you load the source file; but M-x update-file-autoloads copies it to `loaddefs.el', where it is executed while building Emacs.
The following example shows how doctor
is prepared for
autoloading with a magic comment:
;;;###autoload (defun doctor () "Switch to *doctor* buffer and start giving psychotherapy." (interactive) (switch-to-buffer "*doctor*") (doctor-mode))
Here's what that produces in `loaddefs.el':
(autoload 'doctor "doctor" "\ Switch to *doctor* buffer and start giving psychotherapy." t)
The backslash and newline immediately following the double-quote are a
convention used only in the preloaded Lisp files such as
`loaddefs.el'; they tell make-docfile
to put the
documentation string in the `etc/DOC' file. See section Building Emacs.
You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.
When you load or reload files, bear in mind that the load
and
load-library
functions automatically load a byte-compiled file
rather than a non-compiled file of similar name. If you rewrite a file
that you intend to save and reinstall, you need to byte-compile the new
version; otherwise Emacs will load the older, byte-compiled file instead
of your newer, non-compiled file! If that happens, the message
displayed when loading the file includes, `(compiled; note, source is
newer)', to remind you to recompile it.
When writing the forms in a Lisp library file, keep in mind that the
file might be loaded more than once. For example, think about whether
each variable should be reinitialized when you reload the library;
defvar
does not change the value if the variable is already
initialized. (See section Defining Global Variables.)
The simplest way to add an element to an alist is like this:
(setq minor-mode-alist (cons '(leif-mode " Leif") minor-mode-alist))
But this would add multiple elements if the library is reloaded. To avoid the problem, write this:
(or (assq 'leif-mode minor-mode-alist) (setq minor-mode-alist (cons '(leif-mode " Leif") minor-mode-alist)))
To add an element to a list just once, you can also use add-to-list
(see section How to Alter a Variable Value).
Occasionally you will want to test explicitly whether a library has already been loaded. Here's one way to test, in a library, whether it has been loaded before:
(defvar foo-was-loaded nil) (unless foo-was-loaded execute-first-time-only (setq foo-was-loaded t))
If the library uses provide
to provide a named feature, you can
use featurep
earlier in the file to test whether the
provide
call has been executed before.
provide
and require
are an alternative to
autoload
for loading files automatically. They work in terms of
named features. Autoloading is triggered by calling a specific
function, but a feature is loaded the first time another program asks
for it by name.
A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should provide the feature. Another program that uses them may ensure they are defined by requiring the feature. This loads the file of definitions if it hasn't been loaded already.
To require the presence of a feature, call require
with the
feature name as argument. require
looks in the global variable
features
to see whether the desired feature has been provided
already. If not, it loads the feature from the appropriate file. This
file should call provide
at the top level to add the feature to
features
; if it fails to do so, require
signals an error.
For example, in `emacs/lisp/prolog.el',
the definition for run-prolog
includes the following code:
(defun run-prolog () "Run an inferior Prolog process, with I/O via buffer *prolog*." (interactive) (require 'comint) (switch-to-buffer (make-comint "prolog" prolog-program-name)) (inferior-prolog-mode))
The expression (require 'comint)
loads the file `comint.el'
if it has not yet been loaded. This ensures that make-comint
is
defined. Features are normally named after the files that provide them,
so that require
need not be given the file name.
The `comint.el' file contains the following top-level expression:
(provide 'comint)
This adds comint
to the global features
list, so that
(require 'comint)
will henceforth know that nothing needs to be
done.
When require
is used at top level in a file, it takes effect
when you byte-compile that file (see section Byte Compilation) as well as
when you load it. This is in case the required package contains macros
that the byte compiler must know about.
Although top-level calls to require
are evaluated during
byte compilation, provide
calls are not. Therefore, you can
ensure that a file of definitions is loaded before it is byte-compiled
by including a provide
followed by a require
for the same
feature, as in the following example.
(provide 'my-feature) ; Ignored by byte compiler,
; evaluated by load
.
(require 'my-feature) ; Evaluated by byte compiler.
The compiler ignores the provide
, then processes the
require
by loading the file in question. Loading the file does
execute the provide
call, so the subsequent require
call
does nothing when the file is loaded.
The direct effect of calling provide
is to add feature to
the front of the list features
if it is not already in the list.
The argument feature must be a symbol. provide
returns
feature.
features => (bar bish) (provide 'foo) => foo features => (foo bar bish)
When a file is loaded to satisfy an autoload, and it stops due to an
error in the evaluating its contents, any function definitions or
provide
calls that occurred during the load are undone.
See section Autoload.
(featurep feature)
; see below). The
argument feature must be a symbol.
If the feature is not present, then require
loads filename
with load
. If filename is not supplied, then the name of
the symbol feature is used as the base file name to load.
However, in this case, require
insists on finding feature
with an added suffix; a file whose name is just feature won't be
used.
If loading the file fails to provide feature, require
signals an error, `Required feature feature was not
provided'.
t
if feature has been provided in the
current Emacs session (i.e., if feature is a member of
features
.)
provide
. The order of the elements in the
features
list is not significant.
You can discard the functions and variables loaded by a library to
reclaim memory for other Lisp objects. To do this, use the function
unload-feature
:
defun
, defalias
, defsubst
,
defmacro
, defconst
, defvar
, and defcustom
.
It then restores any autoloads formerly associated with those symbols.
(Loading saves these in the autoload
property of the symbol.)
Before restoring the previous definitions, unload-feature
runs
remove-hook
to remove functions in the library from certain
hooks. These hooks include variables whose names end in `hook' or
`-hooks', plus those listed in loadhist-special-hooks
. This
is to prevent Emacs from ceasing to function because important hooks
refer to functions that are no longer defined.
If these measures are not sufficient to prevent malfunction, a library
can define an explicit unload hook. If feature-unload-hook
is defined, it is run as a normal hook before restoring the previous
definitions, instead of the usual hook-removing actions. The
unload hook ought to undo all the global state changes made by the
library that might cease to work once the library is unloaded.
Ordinarily, unload-feature
refuses to unload a library on which
other loaded libraries depend. (A library a depends on library
b if a contains a require
for b.) If the
optional argument force is non-nil
, dependencies are
ignored and you can unload any library.
The unload-feature
function is written in Lisp; its actions are
based on the variable load-history
.
Each element is a list and describes one library. The CAR of the list is the name of the library, as a string. The rest of the list is composed of these kinds of objects:
(require . feature)
indicating
features that were required.
(provide . feature)
indicating
features that were provided.
The value of load-history
may have one element whose CAR is
nil
. This element describes definitions made with
eval-buffer
on a buffer that is not visiting a file.
The command eval-region
updates load-history
, but does so
by adding the symbols defined to the element for the file being visited,
rather than replacing that element. See section Eval.
Preloaded libraries don't contribute to load-history
.
You can ask for code to be executed if and when a particular library is
loaded, by calling eval-after-load
.
The library name library must exactly match the argument of
load
. To get the proper results when an installed library is
found by searching load-path
, you should not include any
directory names in library.
An error in form does not undo the load, but does prevent execution of the rest of form.
In general, well-designed Lisp programs should not use this feature.
The clean and modular ways to interact with a Lisp library are (1)
examine and set the library's variables (those which are meant for
outside use), and (2) call the library's functions. If you wish to
do (1), you can do it immediately--there is no need to wait for when
the library is loaded. To do (2), you must load the library (preferably
with require
).
But it is OK to use eval-after-load
in your personal
customizations if you don't feel they must meet the design standards for
programs meant for wider use.
(filename forms...)
The function load
checks after-load-alist
in order to
implement eval-after-load
.
Emacs Lisp has a compiler that translates functions written in Lisp into a special representation called byte-code that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the byte-code interpreter.
Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code.
Compiling a Lisp file with the Emacs byte compiler always reads the file as multibyte text, even if Emacs was started with `--unibyte', unless the file specifies otherwise. This is so that compilation gives results compatible with running the same file without compilation. See section Loading Non-ASCII Characters.
In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. A major incompatible change was introduced in Emacs version 19.29, and files compiled with versions since that one will definitely not run in earlier versions unless you specify a special option. See section Documentation Strings and Compilation. In addition, the modifier bits in keyboard characters were renumbered in Emacs 19.29; as a result, files compiled in versions before 19.29 will not work in subsequent versions if they contain character constants with modifier bits.
See section Debugging Problems in Compilation, for how to investigate errors occurring in byte compilation.
A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example:
(defun silly-loop (n) "Return time before and after N iterations of a loop." (let ((t1 (current-time-string))) (while (> (setq n (1- n)) 0)) (list t1 (current-time-string)))) => silly-loop (silly-loop 100000) => ("Fri Mar 18 17:25:57 1994" "Fri Mar 18 17:26:28 1994") ; 31 seconds (byte-compile 'silly-loop) => [Compiled code not shown] (silly-loop 100000) => ("Fri Mar 18 17:26:52 1994" "Fri Mar 18 17:26:58 1994") ; 6 seconds
In this example, the interpreted code required 31 seconds to run, whereas the byte-compiled code required 6 seconds. These results are representative, but actual results will vary greatly.
You can byte-compile an individual function or macro definition with
the byte-compile
function. You can compile a whole file with
byte-compile-file
, or several files with
byte-recompile-directory
or batch-byte-compile
.
The byte compiler produces error messages and warnings about each file in a buffer called `*Compile-Log*'. These report things in your program that suggest a problem but are not necessarily erroneous.
Be careful when writing macro calls in files that you may someday byte-compile. Macro calls are expanded when they are compiled, so the macros must already be defined for proper compilation. For more details, see section Macros and Byte Compilation.
Normally, compiling a file does not evaluate the file's contents or
load the file. But it does execute any require
calls at top
level in the file. One way to ensure that necessary macro definitions
are available during compilation is to require the file that defines
them (see section Features). To avoid loading the macro definition files
when someone runs the compiled program, write
eval-when-compile
around the require
calls (see section Evaluation During Compilation).
byte-compile
returns the new, compiled definition of
symbol.
If symbol's definition is a byte-code function object,
byte-compile
does nothing and returns nil
. Lisp records
only one function definition for any symbol, and if that is already
compiled, non-compiled code is not available anywhere. So there is no
way to "compile the same definition again."
(defun factorial (integer) "Compute factorial of INTEGER." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) => factorial (byte-compile 'factorial) => #[(integer) "^H\301U\203^H^@\301\207\302^H\303^HS!\"\207" [integer 1 * factorial] 4 "Compute factorial of INTEGER."]
The result is a byte-code function object. The string it contains is the actual byte-code; each character in it is an instruction or an operand of an instruction. The vector contains all the constants, variable names and function names used by the function, except for certain primitives that are coded as special instructions.
Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read.
This command returns t
. When called interactively, it prompts
for the file name.
% ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el (byte-compile-file "~/emacs/push.el") => t % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el -rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc
When a `.el' file has no corresponding `.elc' file, flag
says what to do. If it is nil
, these files are ignored. If it
is non-nil
, the user is asked whether to compile each such file.
The returned value of this command is unpredictable.
byte-compile-file
on files specified on the
command line. This function must be used only in a batch execution of
Emacs, as it kills Emacs on completion. An error in one file does not
prevent processing of subsequent files, but no output file will be
generated for it, and the Emacs process will terminate with a nonzero
status code.
% emacs -batch -f batch-byte-compile *.el
byte-code
. Don't call
this function yourself--only the byte compiler knows how to generate
valid calls to this function.
In Emacs version 18, byte-code was always executed by way of a call to
the function byte-code
. Nowadays, byte-code is usually executed
as part of a byte-code function object, and only rarely through an
explicit call to byte-code
.
Functions and variables loaded from a byte-compiled file access their documentation strings dynamically from the file whenever needed. This saves space within Emacs, and makes loading faster because the documentation strings themselves need not be processed while loading the file. Actual access to the documentation strings becomes slower as a result, but this normally is not enough to bother users.
Dynamic access to documentation strings does have drawbacks:
If your site installs Emacs following the usual procedures, these problems will never normally occur. Installing a new version uses a new directory with a different name; as long as the old version remains installed, its files will remain unmodified in the places where they are expected to be.
However, if you have built Emacs yourself and use it from the directory where you built it, you will experience this problem occasionally if you edit and recompile Lisp files. When it happens, you can cure the problem by reloading the file after recompiling it.
Byte-compiled files made with recent versions of Emacs (since 19.29)
will not load into older versions because the older versions don't
support this feature. You can turn off this feature at compile time by
setting byte-compile-dynamic-docstrings
to nil
; then you
can compile files that will load into older Emacs versions. You can do
this globally, or for one source file by specifying a file-local binding
for the variable. One way to do that is by adding this string to the
file's first line:
-*-byte-compile-dynamic-docstrings: nil;-*-
nil
, the byte compiler generates compiled files
that are set up for dynamic loading of documentation strings.
The dynamic documentation string feature writes compiled files that use a special Lisp reader construct, `#@count'. This construct skips the next count characters. It also uses the `#$' construct, which stands for "the name of this file, as a string." It is usually best not to use these constructs in Lisp source files, since they are not designed to be clear to humans reading the file.
When you compile a file, you can optionally enable the dynamic function loading feature (also known as lazy loading). With dynamic function loading, loading the file doesn't fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder.
The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate user-callable functions, if using one of them does not imply you will probably also use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides.
The dynamic loading feature has certain disadvantages:
These problems will never happen in normal circumstances with installed Emacs files. But they are quite likely to happen with Lisp files that you are changing. The easiest way to prevent these problems is to reload the new compiled file immediately after each recompilation.
The byte compiler uses the dynamic function loading feature if the
variable byte-compile-dynamic
is non-nil
at compilation
time. Do not set this variable globally, since dynamic loading is
desirable only for certain files. Instead, enable the feature for
specific source files with file-local variable bindings. For example,
you could do it by writing this text in the source file's first line:
-*-byte-compile-dynamic: t;-*-
nil
, the byte compiler generates compiled files
that are set up for dynamic function loading.
These features permit you to write code to be evaluated during compilation of a program.
You can get a similar result by putting body in a separate file
and referring to that file with require
. That method is
preferable when body is large.
Common Lisp Note: At top level, this is analogous to the Common
Lisp idiom (eval-when (compile eval) ...)
. Elsewhere, the
Common Lisp `#.' reader macro (but not when interpreting) is closer
to what eval-when-compile
does.
Byte-compiled functions have a special data type: they are byte-code function objects.
Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. The printed representation for a byte-code function object is like that for a vector, with an additional `#' before the opening `['.
A byte-code function object must have at least four elements; there is no maximum number, but only the first six elements have any normal use. They are:
nil
. The value may
be a number or a list, in case the documentation string is stored in a
file. Use the function documentation
to get the real
documentation string (see section Access to Documentation Strings).
nil
for a function that isn't interactive.
Here's an example of a byte-code function object, in printed
representation. It is the definition of the command
backward-sexp
.
#[(&optional arg) "^H\204^F^@\301^P\302^H[!\207" [arg 1 forward-sexp] 2 254435 "p"]
The primitive way to create a byte-code object is with
make-byte-code
:
You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope).
You can access the elements of a byte-code object using aref
;
you can also use vconcat
to create a vector with the same
elements.
People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into humanly readable form.
The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function.
In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack.
standard-output
. The
argument object can be a function name or a lambda expression.
As a special exception, if this function is used interactively, it outputs to a buffer named `*Disassemble*'.
Here are two examples of using the disassemble
function. We
have added explanatory comments to help you relate the byte-code to the
Lisp source; these do not appear in the output of disassemble
.
These examples show unoptimized byte-code. Nowadays byte-code is
usually optimized, but we did not want to rewrite these examples, since
they still serve their purpose.
(defun factorial (integer) "Compute factorial of an integer." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) => factorial (factorial 4) => 24 (disassemble 'factorial) -| byte-code for factorial: doc: Compute factorial of an integer. args: (integer) 0 constant 1 ; Push 1 onto stack. 1 varref integer ; Get value ofinteger
; from the environment ; and push the value ; onto the stack. 2 eqlsign ; Pop top two values off stack, ; compare them, ; and push result onto stack. 3 goto-if-nil 10 ; Pop and test top of stack; ; ifnil
, go to 10, ; else continue. 6 constant 1 ; Push 1 onto top of stack. 7 goto 17 ; Go to 17 (in this case, 1 will be ; returned by the function). 10 constant * ; Push symbol*
onto stack. 11 varref integer ; Push value ofinteger
onto stack. 12 constant factorial ; Pushfactorial
onto stack. 13 varref integer ; Push value ofinteger
onto stack. 14 sub1 ; Popinteger
, decrement value, ; push new value onto stack. ; Stack now contains: ; - decremented value ofinteger
; -factorial
; - value ofinteger
; -*
15 call 1 ; Call functionfactorial
using ; the first (i.e., the top) element ; of the stack as the argument; ; push returned value onto stack. ; Stack now contains: ; - result of recursive ; call tofactorial
; - value ofinteger
; -*
16 call 2 ; Using the first two ; (i.e., the top two) ; elements of the stack ; as arguments, ; call the function*
, ; pushing the result onto the stack. 17 return ; Return the top element ; of the stack. => nil
The silly-loop
function is somewhat more complex:
(defun silly-loop (n) "Return time before and after N iterations of a loop." (let ((t1 (current-time-string))) (while (> (setq n (1- n)) 0)) (list t1 (current-time-string)))) => silly-loop (disassemble 'silly-loop) -| byte-code for silly-loop: doc: Return time before and after N iterations of a loop. args: (n) 0 constant current-time-string ; Push ;current-time-string
; onto top of stack. 1 call 0 ; Callcurrent-time-string
; with no argument, ; pushing result onto stack. 2 varbind t1 ; Pop stack and bindt1
; to popped value. 3 varref n ; Get value ofn
from ; the environment and push ; the value onto the stack. 4 sub1 ; Subtract 1 from top of stack. 5 dup ; Duplicate the top of the stack; ; i.e., copy the top of ; the stack and push the ; copy onto the stack. 6 varset n ; Pop the top of the stack, ; and bindn
to the value. ; In effect, the sequencedup varset
; copies the top of the stack ; into the value ofn
; without popping it. 7 constant 0 ; Push 0 onto stack. 8 gtr ; Pop top two values off stack, ; test if n is greater than 0 ; and push result onto stack. 9 goto-if-nil-else-pop 17 ; Goto 17 ifn
<= 0 ; (this exits the while loop). ; else pop top of stack ; and continue 12 constant nil ; Pushnil
onto stack ; (this is the body of the loop). 13 discard ; Discard result of the body ; of the loop (a while loop ; is always evaluated for ; its side effects). 14 goto 3 ; Jump back to beginning ; of while loop. 17 discard ; Discard result of while loop ; by popping top of stack. ; This result is the valuenil
that ; was not popped by the goto at 9. 18 varref t1 ; Push value oft1
onto stack. 19 constant current-time-string ; Push ;current-time-string
; onto top of stack. 20 call 0 ; Callcurrent-time-string
again. 21 list2 ; Pop top two elements off stack, ; create a list of them, ; and push list onto stack. 22 unbind 1 ; Unbindt1
in local environment. 23 return ; Return value of the top of stack. => nil
The advice feature lets you add to the existing definition of a function, by advising the function. This is a clean method for a library to customize functions defined by other parts of Emacs--cleaner than redefining the whole function.
Each function can have multiple pieces of advice, separately defined. Each defined piece of advice can be enabled or disabled explicitly. The enabled pieces of advice for any given function actually take effect when you activate advice for that function, or when that function is subsequently defined or redefined.
Usage Note: Advice is useful for altering the behavior of existing calls to an existing function. If you want the new behavior for new calls, or for key bindings, it is cleaner to define a new function (or a new command) which uses the existing function.
The command next-line
moves point down vertically one or more
lines; it is the standard binding of C-n. When used on the last
line of the buffer, this command inserts a newline to create a line to
move to (if next-line-add-newlines
is non-nil
).
Suppose you wanted to add a similar feature to previous-line
,
which would insert a new line at the beginning of the buffer for the
command to move to. How could you do this?
You could do it by redefining the whole function, but that is not modular. The advice feature provides a cleaner alternative: you can effectively add your code to the existing function definition, without actually changing or even seeing that definition. Here is how to do this:
(defadvice previous-line (before next-line-at-end (arg)) "Insert an empty line when moving up from the top line." (if (and next-line-add-newlines (= arg 1) (save-excursion (beginning-of-line) (bobp))) (progn (beginning-of-line) (newline))))
This expression defines a piece of advice for the function
previous-line
. This piece of advice is named
next-line-at-end
, and the symbol before
says that it is
before-advice which should run before the regular definition of
previous-line
. (arg)
specifies how the advice code can
refer to the function's arguments.
When this piece of advice runs, it creates an additional line, in the situation where that is appropriate, but does not move point to that line. This is the correct way to write the advice, because the normal definition will run afterward and will move back to the newly inserted line.
Defining the advice doesn't immediately change the function
previous-line
. That happens when you activate the advice,
like this:
(ad-activate 'previous-line)
This is what actually begins to use the advice that has been defined so
far for the function previous-line
. Henceforth, whenever that
function is run, whether invoked by the user with C-p or
M-x, or called from Lisp, it runs the advice first, and its
regular definition second.
This example illustrates before-advice, which is one class of advice: it runs before the function's base definition. There are two other advice classes: after-advice, which runs after the base definition, and around-advice, which lets you specify an expression to wrap around the invocation of the base definition.
To define a piece of advice, use the macro defadvice
. A call
to defadvice
has the following syntax, which is based on the
syntax of defun
and defmacro
, but adds more:
(defadvice function (class name [position] [arglist] flags...) [documentation-string] [interactive-form] body-forms...)
Here, function is the name of the function (or macro or special form) to be advised. From now on, we will write just "function" when describing the entity being advised, but this always includes macros and special forms.
class specifies the class of the advice--one of before
,
after
, or around
. Before-advice runs before the function
itself; after-advice runs after the function itself; around-advice is
wrapped around the execution of the function itself. After-advice and
around-advice can override the return value by setting
ad-return-value
.
The argument name is the name of the advice, a non-nil
symbol. The advice name uniquely identifies one piece of advice, within all
the pieces of advice in a particular class for a particular
function. The name allows you to refer to the piece of
advice--to redefine it, or to enable or disable it.
In place of the argument list in an ordinary definition, an advice definition calls for several different pieces of information.
The optional position specifies where, in the current list of
advice of the specified class, this new advice should be placed.
It should be either first
, last
or a number that specifies
a zero-based position (first
is equivalent to 0). If no position
is specified, the default is first
. Position values outside the
range of existing positions in this class are mapped to the beginning or
the end of the range, whichever is closer. The position value is
ignored when redefining an existing piece of advice.
The optional arglist can be used to define the argument list for the sake of advice. This becomes the argument list of the combined definition that is generated in order to run the advice (see section The Combined Definition). Therefore, the advice expressions can use the argument variables in this list to access argument values.
This argument list must be compatible with the argument list of the original function, so that it can handle the ways the function is actually called. If more than one piece of advice specifies an argument list, then the first one (the one with the smallest position) found in the list of all classes of advice is used.
The remaining elements, flags, are symbols that specify further information about how to use this piece of advice. Here are the valid symbols and their meanings:
activate
protect
unwind-protect
form, so that it will execute even if the
previous code gets an error or uses throw
. See section Cleaning Up from Nonlocal Exits.
compile
activate
is also specified.
See section The Combined Definition.
disable
preactivate
defadvice
is
compiled or macroexpanded. This generates a compiled advised definition
according to the current advice state, which will be used during
activation if appropriate.
This is useful only if this defadvice
is byte-compiled.
The optional documentation-string serves to document this piece of
advice. When advice is active for function, the documentation for
function (as returned by documentation
) combines the
documentation strings of all the advice for function with the
documentation string of its original function definition.
The optional interactive-form form can be supplied to change the interactive behavior of the original function. If more than one piece of advice has an interactive-form, then the first one (the one with the smallest position) found among all the advice takes precedence.
The possibly empty list of body-forms specifies the body of the advice. The body of an advice can access or change the arguments, the return value, the binding environment, and perform any other kind of side effect.
Warning: When you advise a macro, keep in mind that macros are expanded when a program is compiled, not when a compiled program is run. All subroutines used by the advice need to be available when the byte compiler expands the macro.
Around-advice lets you "wrap" a Lisp expression "around" the
original function definition. You specify where the original function
definition should go by means of the special symbol ad-do-it
.
Where this symbol occurs inside the around-advice body, it is replaced
with a progn
containing the forms of the surrounded code. Here
is an example:
(defadvice foo (around foo-around) "Ignore case in `foo'." (let ((case-fold-search t)) ad-do-it))
Its effect is to make sure that case is ignored in
searches when the original definition of foo
is run.
If the around-advice does not use ad-do-it
, then it does not run
the original function definition. This provides a way to override the
original definition completely. (It also overrides lower-positioned
pieces of around-advice).
The macro defadvice
resembles defun
in that the code for
the advice, and all other information about it, are explicitly stated in
the source code. You can also create advice whose details are computed,
using the function ad-add-advice
.
ad-add-advice
adds advice as a piece of advice to
function in class class. The argument advice has
this form:
(name protected enabled definition)
Here protected and enabled are flags, and definition
is the expression that says what the advice should do. If enabled
is nil
, this piece of advice is initially disabled
(see section Enabling and Disabling Advice).
If function already has one or more pieces of advice in the
specified class, then position specifies where in the list
to put the new piece of advice. The value of position can either
be first
, last
, or a number (counting from 0 at the
beginning of the list). Numbers outside the range are mapped to the
closest extreme position.
If function already has a piece of advice with the same name, then the position argument is ignored and the old advice is replaced with the new one.
By default, advice does not take effect when you define it--only when
you activate advice for the function that was advised. You can
request the activation of advice for a function when you define the
advice, by specifying the activate
flag in the defadvice
.
But normally you activate the advice for a function by calling the
function ad-activate
or one of the other activation commands
listed below.
Separating the activation of advice from the act of defining it permits you to add several pieces of advice to one function efficiently, without redefining the function over and over as each advice is added. More importantly, it permits defining advice for a function before that function is actually defined.
When a function's advice is first activated, the function's original definition is saved, and all enabled pieces of advice for that function are combined with the original definition to make a new definition. (Pieces of advice that are currently disabled are not used; see section Enabling and Disabling Advice.) This definition is installed, and optionally byte-compiled as well, depending on conditions described below.
In all of the commands to activate advice, if compile is t
,
the command also compiles the combined definition which implements the
advice.
To activate advice for a function whose advice is already active is not a no-op. It is a useful operation which puts into effect any changes in that function's advice since the previous activation of advice for that function.
Reactivating a function's advice is useful for putting into effect all the changes that have been made in its advice (including enabling and disabling specific pieces of advice; see section Enabling and Disabling Advice) since the last time it was activated.
If the advised definition was constructed during "preactivation"
(see section Preactivation), then that definition must already be compiled,
because it was constructed during byte-compilation of the file that
contained the defadvice
with the preactivate
flag.
Each piece of advice has a flag that says whether it is enabled or
not. By enabling or disabling a piece of advice, you can turn it on
and off without having to undefine and redefine it. For example, here is
how to disable a particular piece of advice named my-advice
for
the function foo
:
(ad-disable-advice 'foo 'before 'my-advice)
This function by itself only changes the enable flag for a piece of
advice. To make the change take effect in the advised definition, you
must activate the advice for foo
again:
(ad-activate 'foo)
You can also disable many pieces of advice at once, for various functions, using a regular expression. As always, the changes take real effect only when you next reactivate advice for the functions in question.
Constructing a combined definition to execute advice is moderately expensive. When a library advises many functions, this can make loading the library slow. In that case, you can use preactivation to construct suitable combined definitions in advance.
To use preactivation, specify the preactivate
flag when you
define the advice with defadvice
. This defadvice
call
creates a combined definition which embodies this piece of advice
(whether enabled or not) plus any other currently enabled advice for the
same function, and the function's own definition. If the
defadvice
is compiled, that compiles the combined definition
also.
When the function's advice is subsequently activated, if the enabled advice for the function matches what was used to make this combined definition, then the existing combined definition is used, thus avoiding the need to construct one. Thus, preactivation never causes wrong results--but it may fail to do any good, if the enabled advice at the time of activation doesn't match what was used for preactivation.
Here are some symptoms that can indicate that a preactivation did not work properly, because of a mismatch.
byte-compile
is included in the value of features
even
though you did not ever explicitly use the byte-compiler.
Compiled preactivated advice works properly even if the function itself is not defined until later; however, the function needs to be defined when you compile the preactivated advice.
There is no elegant way to find out why preactivated advice is not being
used. What you can do is to trace the function
ad-cache-id-verification-code
(with the function
trace-function-background
) before the advised function's advice
is activated. After activation, check the value returned by
ad-cache-id-verification-code
for that function: verified
means that the preactivated advice was used, while other values give
some information about why they were considered inappropriate.
Warning: There is one known case that can make preactivation fail, in that a preconstructed combined definition is used even though it fails to match the current state of advice. This can happen when two packages define different pieces of advice with the same name, in the same class, for the same function. But you should avoid that anyway.
The simplest way to access the arguments of an advised function in the body of a piece of advice is to use the same names that the function definition uses. To do this, you need to know the names of the argument variables of the original function.
While this simple method is sufficient in many cases, it has a disadvantage: it is not robust, because it hard-codes the argument names into the advice. If the definition of the original function changes, the advice might break.
Another method is to specify an argument list in the advice itself. This avoids the need to know the original function definition's argument names, but it has a limitation: all the advice on any particular function must use the same argument list, because the argument list actually used for all the advice comes from the first piece of advice for that function.
A more robust method is to use macros that are translated into the proper access forms at activation time, i.e., when constructing the advised definition. Access macros access actual arguments by position regardless of how these actual arguments get distributed onto the argument variables of a function. This is robust because in Emacs Lisp the meaning of an argument is strictly determined by its position in the argument list.
Now an example. Suppose the function foo
is defined as
(defun foo (x y &optional z &rest r) ...)
and is then called with
(foo 0 1 2 3 4 5 6)
which means that x is 0, y is 1, z is 2 and r is
(3 4 5 6)
within the body of foo
. Here is what
ad-get-arg
and ad-get-args
return in this case:
(ad-get-arg 0) => 0 (ad-get-arg 1) => 1 (ad-get-arg 2) => 2 (ad-get-arg 3) => 3 (ad-get-args 2) => (2 3 4 5 6) (ad-get-args 4) => (4 5 6)
Setting arguments also makes sense in this example:
(ad-set-arg 5 "five")
has the effect of changing the sixth argument to "five"
. If this
happens in advice executed before the body of foo
is run, then
r will be (3 4 "five" 6)
within that body.
Here is an example of setting a tail of the argument list:
(ad-set-args 0 '(5 4 3 2 1 0))
If this happens in advice executed before the body of foo
is run,
then within that body, x will be 5, y will be 4, z
will be 3, and r will be (2 1 0)
inside the body of
foo
.
These argument constructs are not really implemented as Lisp macros. Instead they are implemented specially by the advice mechanism.
When the advice facility constructs the combined definition, it needs
to know the argument list of the original function. This is not always
possible for primitive functions. When advice cannot determine the
argument list, it uses (&rest ad-subr-args)
, which always works
but is inefficient because it constructs a list of the argument values.
You can use ad-define-subr-args
to declare the proper argument
names for a primitive function:
For example,
(ad-define-subr-args 'fset '(sym newdef))
specifies the argument list for the function fset
.
Suppose that a function has n pieces of before-advice, m pieces of around-advice and k pieces of after-advice. Assuming no piece of advice is protected, the combined definition produced to implement the advice for a function looks like this:
(lambda arglist [ [advised-docstring] [(interactive ...)] ] (let (ad-return-value) before-0-body-form... .... before-n-1-body-form... around-0-body-form... around-1-body-form... .... around-m-1-body-form... (setq ad-return-value apply original definition to arglist) other-around-m-1-body-form... .... other-around-1-body-form... other-around-0-body-form... after-0-body-form... .... after-k-1-body-form... ad-return-value))
Macros are redefined as macros, which means adding macro
to
the beginning of the combined definition.
The interactive form is present if the original function or some piece
of advice specifies one. When an interactive primitive function is
advised, a special method is used: to call the primitive with
call-interactively
so that it will read its own arguments.
In this case, the advice cannot access the arguments.
The body forms of the various advice in each class are assembled according to their specified order. The forms of around-advice l are included in one of the forms of around-advice l - 1.
The innermost part of the around advice onion is
apply original definition to arglist
whose form depends on the type of the original function. The variable
ad-return-value
is set to whatever this returns. The variable is
visible to all pieces of advice, which can access and modify it before
it is actually returned from the advised function.
The semantic structure of advised functions that contain protected
pieces of advice is the same. The only difference is that
unwind-protect
forms ensure that the protected advice gets
executed even if some previous piece of advice had an error or a
non-local exit. If any around-advice is protected, then the whole
around-advice onion is protected as a result.
There are three ways to investigate a problem in an Emacs Lisp program, depending on what you are doing with the program when the problem appears.
Another useful debugging tool is the dribble file. When a dribble file is open, Emacs copies all keyboard input characters to that file. Afterward, you can examine the file to find out what input was used. See section Terminal Input.
For debugging problems in terminal descriptions, the
open-termscript
function can be useful. See section Terminal Output.
The ordinary Lisp debugger provides the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a break), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of Emacs are available; you can even run programs that will enter the debugger recursively. See section Recursive Editing.
The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error.
However, entry to the debugger is not a normal consequence of an
error. Many commands frequently cause Lisp errors when invoked
inappropriately (such as C-f at the end of the buffer), and during
ordinary editing it would be very inconvenient to enter the debugger
each time this happens. So if you want errors to enter the debugger, set
the variable debug-on-error
to non-nil
. (The command
toggle-debug-on-error
provides an easy way to do this.)
debug-on-error
is t
, all
kinds of errors call the debugger (except those listed in
debug-ignored-errors
). If it is nil
, none call the
debugger.
The value can also be a list of error conditions that should call the
debugger. For example, if you set it to the list
(void-variable)
, then only errors about a variable that has no
value invoke the debugger.
When this variable is non-nil
, Emacs does not create an error
handler around process filter functions and sentinels. Therefore,
errors in these functions also invoke the debugger. See section Processes.
debug-on-error
.
The normal value of this variable lists several errors that happen often
during editing but rarely result from bugs in Lisp programs. However,
"rarely" is not "never"; if your program fails with an error that
matches this list, you will need to change this list in order to debug
the error. The easiest way is usually to set
debug-ignored-errors
to nil
.
condition-case
never run the
debugger, even if debug-on-error
is non-nil
. In other
words, condition-case
gets a chance to handle the error before
the debugger gets a chance.
If you set debug-on-signal
to a non-nil
value, then the
debugger gets the first chance at every error; an error will invoke the
debugger regardless of any condition-case
, if it fits the
criteria specified by the values of debug-on-error
and
debug-ignored-errors
.
Warning: This variable is strong medicine! Various parts of
Emacs handle errors in the normal course of affairs, and you may not
even realize that errors happen there. If you set
debug-on-signal
to a non-nil
value, those errors will
enter the debugger.
Warning: debug-on-signal
has no effect when
debug-on-error
is nil
.
To debug an error that happens during loading of the `.emacs'
file, use the option `--debug-init', which binds
debug-on-error
to t
while loading `.emacs', and
bypasses the condition-case
which normally catches errors in the
init file.
If your `.emacs' file sets debug-on-error
, the effect may
not last past the end of loading `.emacs'. (This is an undesirable
byproduct of the code that implements the `--debug-init' command
line option.) The best way to make `.emacs' set
debug-on-error
permanently is with after-init-hook
, like
this:
(add-hook 'after-init-hook '(lambda () (setq debug-on-error t)))
When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with C-g, which causes quit.
Ordinary quitting gives no information about why the program was
looping. To get more information, you can set the variable
debug-on-quit
to non-nil
. Quitting with C-g is not
considered an error, and debug-on-error
has no effect on the
handling of C-g. Likewise, debug-on-quit
has no effect on
errors.
Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you will probably get enough information to solve the problem.
quit
is signaled and not handled. If debug-on-quit
is non-nil
,
then the debugger is called whenever you quit (that is, type C-g).
If debug-on-quit
is nil
, then the debugger is not called
when you quit. See section Quitting.
To investigate a problem that happens in the middle of a program, one useful technique is to enter the debugger whenever a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller.
(debug 'debug)
into
the function definition as the first form.
Any function defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. If the function is a command, it will enter the debugger when called from Lisp and when called interactively (after the reading of the arguments). You can't debug primitive functions (i.e., those written in C) this way.
When debug-on-entry
is called interactively, it prompts for
function-name in the minibuffer. If the function is already set
up to invoke the debugger on entry, debug-on-entry
does nothing.
debug-on-entry
always returns function-name.
Note: if you redefine a function after using
debug-on-entry
on it, the code to enter the debugger is discarded
by the redefinition. In effect, redefining the function cancels
the break-on-entry feature for that function.
(defun fact (n) (if (zerop n) 1 (* n (fact (1- n))))) => fact (debug-on-entry 'fact) => fact (fact 3) ------ Buffer: *Backtrace* ------ Entering: * fact(3) eval-region(4870 4878 t) byte-code("...") eval-last-sexp(nil) (let ...) eval-insert-last-sexp(nil) * call-interactively(eval-insert-last-sexp) ------ Buffer: *Backtrace* ------ (symbol-function 'fact) => (lambda (n) (debug (quote debug)) (if (zerop n) 1 (* n (fact (1- n)))))
debug-on-entry
on
function-name. When called interactively, it prompts for
function-name in the minibuffer. If function-name is
nil
or the empty string, it cancels break-on-entry for all
functions.
Calling cancel-debug-on-entry
does nothing to a function which is
not currently set up to break on entry. It always returns
function-name.
You can cause the debugger to be called at a certain point in your
program by writing the expression (debug)
at that point. To do
this, visit the source file, insert the text `(debug)' at the
proper place, and type C-M-x. Warning: if you do this
for temporary debugging purposes, be sure to undo this insertion before
you save the file!
The place where you insert `(debug)' must be a place where an
additional form can be evaluated and its value ignored. (If the value
of (debug)
isn't ignored, it will alter the execution of the
program!) The most common suitable places are inside a progn
or
an implicit progn
(see section Sequencing).
When the debugger is entered, it displays the previously selected buffer in one window and a buffer named `*Backtrace*' in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error).
The backtrace buffer is read-only and uses a special major mode, Debugger mode, in which letters are defined as debugger commands. The usual Emacs editing commands are available; thus, you can switch windows to examine the buffer that was being edited at the time of the error, switch buffers, visit files, or do any other sort of editing. However, the debugger is a recursive editing level (see section Recursive Editing) and it is wise to go back to the backtrace buffer and exit the debugger (with the q command) when you are finished with it. Exiting the debugger gets out of the recursive edit and kills the backtrace buffer.
The backtrace buffer shows you the functions that are executing and their argument values. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The frame whose line point is on is considered the current frame. Some of the debugger commands operate on the current frame.
The debugger itself must be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted.
Inside the debugger (in Debugger mode), these special commands are available in addition to the usual cursor motion commands. (Keep in mind that all the usual facilities of Emacs, such as switching windows or buffers, are still available.)
The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source for the function and type C-M-x on its definition.)
Here is a list of Debugger mode commands:
debug
and use its return value. Otherwise, r has the same
effect as c, and the specified return value does not matter.
You can't use r when the debugger was entered due to an error.
Here we describe in full detail the function debug
that is used
to invoke the debugger.
The Debugger mode c and r commands exit the recursive edit;
then debug
switches back to the previous buffer and returns to
whatever called debug
. This is the only way the function
debug
can return to its caller.
The use of the debugger-args is that debug
displays the
rest of its arguments at the top of the `*Backtrace*' buffer, so
that the user can see them. Except as described below, this is the
only way these arguments are used.
However, certain values for first argument to debug
have a
special significance. (Normally, these values are used only by the
internals of Emacs, and not by programmers calling debug
.) Here
is a table of these special values:
lambda
lambda
means debug
was called because
of entry to a function when debug-on-next-call
was
non-nil
. The debugger displays `Entering:' as a line of
text at the top of the buffer.
debug
debug
as first argument indicates a call to debug
because
of entry to a function that was set to debug on entry. The debugger
displays `Entering:', just as in the lambda
case. It also
marks the stack frame for that function so that it will invoke the
debugger when exited.
t
t
, this indicates a call to
debug
due to evaluation of a list form when
debug-on-next-call
is non-nil
. The debugger displays the
following as the top line in the buffer:
Beginning evaluation of function call form:
exit
exit
, it indicates the exit of a stack
frame previously marked to invoke the debugger on exit. The second
argument given to debug
in this case is the value being returned
from the frame. The debugger displays `Return value:' in the top
line of the buffer, followed by the value being returned.
error
error
, the debugger indicates that
it is being entered because an error or quit
was signaled and not
handled, by displaying `Signaling:' followed by the error signaled
and any arguments to signal
. For example,
(let ((debug-on-error t)) (/ 1 0)) ------ Buffer: *Backtrace* ------ Signaling: (arith-error) /(1 0) ... ------ Buffer: *Backtrace* ------If an error was signaled, presumably the variable
debug-on-error
is non-nil
. If quit
was signaled,
then presumably the variable debug-on-quit
is non-nil
.
nil
nil
as the first of the debugger-args when you want
to enter the debugger explicitly. The rest of the debugger-args
are printed on the top line of the buffer. You can use this feature to
display messages--for example, to remind yourself of the conditions
under which debug
is called.
This section describes functions and variables used internally by the debugger.
debug
.
The first argument that Lisp hands to the function indicates why it
was called. The convention for arguments is detailed in the description
of debug
.
debug
to fill up the
`*Backtrace*' buffer. It is written in C, since it must have access
to the stack to determine which function calls are active. The return
value is always nil
.
In the following example, a Lisp expression calls backtrace
explicitly. This prints the backtrace to the stream
standard-output
: in this case, to the buffer
`backtrace-output'. Each line of the backtrace represents one
function call. The line shows the values of the function's arguments if
they are all known. If they are still being computed, the line says so.
The arguments of special forms are elided.
(with-output-to-temp-buffer "backtrace-output" (let ((var 1)) (save-excursion (setq var (eval '(progn (1+ var) (list 'testing (backtrace)))))))) => nil ----------- Buffer: backtrace-output ------------ backtrace() (list ...computing arguments...) (progn ...) eval((progn (1+ var) (list (quote testing) (backtrace)))) (setq ...) (save-excursion ...) (let ...) (with-output-to-temp-buffer ...) eval-region(1973 2142 #<buffer *scratch*>) byte-code("... for eval-print-last-sexp ...") eval-print-last-sexp(nil) * call-interactively(eval-print-last-sexp) ----------- Buffer: backtrace-output ------------
The character `*' indicates a frame whose debug-on-exit flag is set.
nil
, it says to call the debugger before
the next eval
, apply
or funcall
. Entering the
debugger sets debug-on-next-call
to nil
.
The d command in the debugger works by setting this variable.
nil
, this will cause the debugger to be entered when that
frame later exits. Even a nonlocal exit through that frame will enter
the debugger.
This function is used only by the debugger.
nil
. The debugger can set this variable to leave
information for future debugger invocations during the same command
invocation.
The advantage, for the debugger, of using this variable rather than an ordinary global variable is that the data will never carry over to a subsequent command invocation.
backtrace-frame
is intended for use in Lisp
debuggers. It returns information about what computation is happening
in the stack frame frame-number levels down.
If that frame has not evaluated the arguments yet (or is a special
form), the value is (nil function arg-forms...)
.
If that frame has evaluated its arguments and called its function
already, the value is (t function
arg-values...)
.
In the return value, function is whatever was supplied as the
CAR of the evaluated list, or a lambda
expression in the
case of a macro call. If the function has a &rest
argument, that
is represented as the tail of the list arg-values.
If frame-number is out of range, backtrace-frame
returns
nil
.
Edebug is a source-level debugger for Emacs Lisp programs with which you can:
The first three sections below should tell you enough about Edebug to enable you to use it.
To debug a Lisp program with Edebug, you must first instrument
the Lisp code that you want to debug. A simple way to do this is to
first move point into the definition of a function or macro and then do
C-u C-M-x (eval-defun
with a prefix argument). See
section Instrumenting for Edebug, for alternative ways to instrument code.
Once a function is instrumented, any call to the function activates Edebug. Activating Edebug may stop execution and let you step through the function, or it may update the display and continue execution while checking for debugging commands, depending on which Edebug execution mode you have selected. The default execution mode is step, which does stop execution. See section Edebug Execution Modes.
Within Edebug, you normally view an Emacs buffer showing the source of the Lisp code you are debugging. This is referred to as the source code buffer. This buffer is temporarily read-only.
An arrow at the left margin indicates the line where the function is executing. Point initially shows where within the line the function is executing, but this ceases to be true if you move point yourself.
If you instrument the definition of fac
(shown below) and then
execute (fac 3)
, here is what you normally see. Point is at the
open-parenthesis before if
.
(defun fac (n) =>-!-(if (< 0 n) (* n (fac (1- n))) 1))
The places within a function where Edebug can stop execution are called
stop points. These occur both before and after each subexpression
that is a list, and also after each variable reference.
Here we show with periods the stop points found in the function
fac
:
(defun fac (n) .(if .(< 0 n.). .(* n. .(fac (1- n.).).). 1).)
The special commands of Edebug are available in the source code buffer
in addition to the commands of Emacs Lisp mode. For example, you can
type the Edebug command SPC to execute until the next stop point.
If you type SPC once after entry to fac
, here is the
display you will see:
(defun fac (n) =>(if -!-(< 0 n) (* n (fac (1- n))) 1))
When Edebug stops execution after an expression, it displays the expression's value in the echo area.
Other frequently used commands are b to set a breakpoint at a stop point, g to execute until a breakpoint is reached, and q to exit Edebug and return to the top-level command loop. Type ? to display a list of all Edebug commands.
In order to use Edebug to debug Lisp code, you must first instrument the code. Instrumenting code inserts additional code into it, to invoke Edebug at the proper places.
Once you have loaded Edebug, the command C-M-x
(eval-defun
) is redefined so that when invoked with a prefix
argument on a definition, it instruments the definition before
evaluating it. (The source code itself is not modified.) If the
variable edebug-all-defs
is non-nil
, that inverts the
meaning of the prefix argument: then C-M-x instruments the
definition unless it has a prefix argument. The default value of
edebug-all-defs
is nil
. The command M-x
edebug-all-defs toggles the value of the variable
edebug-all-defs
.
If edebug-all-defs
is non-nil
, then the commands
eval-region
, eval-current-buffer
, and eval-buffer
also instrument any definitions they evaluate. Similarly,
edebug-all-forms
controls whether eval-region
should
instrument any form, even non-defining forms. This doesn't apply
to loading or evaluations in the minibuffer. The command M-x
edebug-all-forms toggles this option.
Another command, M-x edebug-eval-top-level-form, is available to
instrument any top-level form regardless of the values of
edebug-all-defs
and edebug-all-forms
.
While Edebug is active, the command I
(edebug-instrument-callee
) instruments the definition of the
function or macro called by the list form after point, if is not already
instrumented. This is possible only if Edebug knows where to find the
source for that function; after loading Edebug, eval-region
records the position of every definition it evaluates, even if not
instrumenting it. See also the i command (see section Jumping), which
steps into the call after instrumenting the function.
Edebug knows how to instrument all the standard special forms,
interactive
forms with an expression argument, anonymous lambda
expressions, and other defining forms. Edebug cannot know what a
user-defined macro will do with the arguments of a macro call, so you
must tell it; see section Instrumenting Macro Calls, for details.
When Edebug is about to instrument code for the first time in a
session, it runs the hook edebug-setup-hook
, then sets it to
nil
. You can use this to arrange to load Edebug specifications
(see section Instrumenting Macro Calls) associated with a package you are
using, but actually load them only if you use Edebug.
To remove instrumentation from a definition, simply re-evaluate its
definition in a way that does not instrument. There are two ways of
evaluating forms that never instrument them: from a file with
load
, and from the minibuffer with eval-expression
(M-:).
If Edebug detects a syntax error while instrumenting, it leaves point
at the erroneous code and signals an invalid-read-syntax
error.
See section Evaluation, for other evaluation functions available inside of Edebug.
Edebug supports several execution modes for running the program you are debugging. We call these alternatives Edebug execution modes; do not confuse them with major or minor modes. The current Edebug execution mode determines how far Edebug continues execution before stopping--whether it stops at each stop point, or continues to the next breakpoint, for example--and how much Edebug displays the progress of the evaluation before it stops.
Normally, you specify the Edebug execution mode by typing a command to continue the program in a certain mode. Here is a table of these commands. All except for S resume execution of the program, at least for a certain distance.
edebug-stop
).
edebug-step-mode
).
edebug-next-mode
). Also see edebug-forward-sexp
in
section Miscellaneous Edebug Commands.
edebug-trace-mode
).
edebug-Trace-fast-mode
).
edebug-go-mode
). See section Breakpoints.
edebug-continue-mode
).
edebug-Continue-fast-mode
).
edebug-Go-nonstop-mode
). You
can still stop the program by typing S, or any editing command.
In general, the execution modes earlier in the above list run the program more slowly or stop sooner than the modes later in the list.
While executing or tracing, you can interrupt the execution by typing any Edebug command. Edebug stops the program at the next stop point and then executes the command you typed. For example, typing t during execution switches to trace mode at the next stop point. You can use S to stop execution without doing anything else.
If your function happens to read input, a character you type intending to interrupt execution may be read by the function instead. You can avoid such unintended results by paying attention to when your program wants input.
Keyboard macros containing the commands in this section do not
completely work: exiting from Edebug, to resume the program, loses track
of the keyboard macro. This is not easy to fix. Also, defining or
executing a keyboard macro outside of Edebug does not affect commands
inside Edebug. This is usually an advantage. But see the
edebug-continue-kbd-macro
option (see section Edebug Options).
When you enter a new Edebug level, the initial execution mode comes from
the value of the variable edebug-initial-mode
. By default, this
specifies step mode. Note that you may reenter the same Edebug level
several times if, for example, an instrumented function is called
several times from one command.
The commands described in this section execute until they reach a specified location. All except i make a temporary breakpoint to establish the place to stop, then switch to go mode. Any other breakpoint reached before the intended stop point will also stop execution. See section Breakpoints, for the details on breakpoints.
These commands may fail to work as expected in case of nonlocal exit, because a nonlocal exit can bypass the temporary breakpoint where you expected the program to stop.
edebug-goto-here
).
edebug-forward-sexp
).
The h command proceeds to the stop point near the current location of point, using a temporary breakpoint. See section Breakpoints, for more information about breakpoints.
The f command runs the program forward over one expression. More precisely, it sets a temporary breakpoint at the position that C-M-f would reach, then executes in go mode so that the program will stop at breakpoints.
With a prefix argument n, the temporary breakpoint is placed n sexps beyond point. If the containing list ends before n more elements, then the place to stop is after the containing expression.
Be careful that the position C-M-f finds is a place that the
program will really get to; this may not be true in a
cond
, for example.
The f command does forward-sexp
starting at point, rather
than at the stop point, for flexibility. If you want to execute one
expression from the current stop point, type w first, to
move point there, and then type f.
The o command continues "out of" an expression. It places a temporary breakpoint at the end of the sexp containing point. If the containing sexp is a function definition itself, o continues until just before the last sexp in the definition. If that is where you are now, it returns from the function and then stops. In other words, this command does not exit the currently executing function unless you are positioned after the last sexp.
The i command steps into the function or macro called by the list form after point, and stops at its first stop point. Note that the form need not be the one about to be evaluated. But if the form is a function call about to be evaluated, remember to use this command before any of the arguments are evaluated, since otherwise it will be too late.
The i command instruments the function or macro it's supposed to step into, if it isn't instrumented already. This is convenient, but keep in mind that the function or macro remains instrumented unless you explicitly arrange to deinstrument it.
Some miscellaneous Edebug commands are described here.
edebug-help
).
abort-recursive-edit
).
top-level
). This
exits all recursive editing levels, including all levels of Edebug
activity. However, instrumented code protected with
unwind-protect
or condition-case
forms may resume
debugging.
top-level-nonstop
).
edebug-previous-result
).
edebug-backtrace
).
You cannot use debugger commands in the backtrace buffer in Edebug as
you would in the standard debugger.
The backtrace buffer is killed automatically when you continue
execution.
From the Edebug recursive edit, you may invoke commands that activate Edebug again recursively. Any time Edebug is active, you can quit to the top level with q or abort one recursive edit level with C-]. You can display a backtrace of all the pending evaluations with d.
Edebug's step mode stops execution at the next stop point reached. There are three other ways to stop Edebug execution once it has started: breakpoints, the global break condition, and source breakpoints.
While using Edebug, you can specify breakpoints in the program you are testing: points where execution should stop. You can set a breakpoint at any stop point, as defined in section Using Edebug. For setting and unsetting breakpoints, the stop point that is affected is the first one at or after point in the source code buffer. Here are the Edebug commands for breakpoints:
edebug-set-breakpoint
). If you use a prefix argument, the
breakpoint is temporary (it turns off the first time it stops the
program).
edebug-unset-breakpoint
).
nil
value
(edebug-set-conditional-breakpoint
). With a prefix argument, the
breakpoint is temporary.
edebug-next-breakpoint
).
While in Edebug, you can set a breakpoint with b and unset one with u. First move point to the Edebug stop point of your choice, then type b or u to set or unset a breakpoint there. Unsetting a breakpoint where none has been set has no effect.
Re-evaluating or reinstrumenting a definition forgets all its breakpoints.
A conditional breakpoint tests a condition each time the program
gets there. Any errors that occur as a result of evaluating the
condition are ignored, as if the result were nil
. To set a
conditional breakpoint, use x, and specify the condition
expression in the minibuffer. Setting a conditional breakpoint at a
stop point that has a previously established conditional breakpoint puts
the previous condition expression in the minibuffer so you can edit it.
You can make a conditional or unconditional breakpoint temporary by using a prefix argument with the command to set the breakpoint. When a temporary breakpoint stops the program, it is automatically unset.
Edebug always stops or pauses at a breakpoint except when the Edebug mode is Go-nonstop. In that mode, it ignores breakpoints entirely.
To find out where your breakpoints are, use the B command, which moves point to the next breakpoint following point, within the same function, or to the first breakpoint if there are no following breakpoints. This command does not continue execution--it just moves point in the buffer.
A global break condition stops execution when a specified
condition is satisfied, no matter where that may occur. Edebug
evaluates the global break condition at every stop point. If it
evaluates to a non-nil
value, then execution stops or pauses
depending on the execution mode, as if a breakpoint had been hit. If
evaluating the condition gets an error, execution does not stop.
The condition expression is stored in
edebug-global-break-condition
. You can specify a new expression
using the X command (edebug-set-global-break-condition
).
The global break condition is the simplest way to find where in your
code some event occurs, but it makes code run much more slowly. So you
should reset the condition to nil
when not using it.
All breakpoints in a definition are forgotten each time you
reinstrument it. To make a breakpoint that won't be forgotten, you can
write a source breakpoint, which is simply a call to the function
edebug
in your source code. You can, of course, make such a call
conditional. For example, in the fac
function, insert the first
line as shown below to stop when the argument reaches zero:
(defun fac (n) (if (= n 0) (edebug)) (if (< 0 n) (* n (fac (1- n))) 1))
When the fac
definition is instrumented and the function is
called, the call to edebug
acts as a breakpoint. Depending on
the execution mode, Edebug stops or pauses there.
If no instrumented code is being executed when edebug
is called,
that function calls debug
.
Emacs normally displays an error message when an error is signaled and
not handled with condition-case
. While Edebug is active and
executing instrumented code, it normally responds to all unhandled
errors. You can customize this with the options edebug-on-error
and edebug-on-quit
; see section Edebug Options.
When Edebug responds to an error, it shows the last stop point encountered before the error. This may be the location of a call to a function which was not instrumented, within which the error actually occurred. For an unbound variable error, the last known stop point might be quite distant from the offending variable reference. In that case you might want to display a full backtrace (see section Miscellaneous Edebug Commands).
If you change debug-on-error
or debug-on-quit
while
Edebug is active, these changes will be forgotten when Edebug becomes
inactive. Furthermore, during Edebug's recursive edit, these variables
are bound to the values they had outside of Edebug.
These Edebug commands let you view aspects of the buffer and window status as they were before entry to Edebug. The outside window configuration is the collection of windows and contents that were in effect outside of Edebug.
edebug-view-outside
).
edebug-bounce-point
). With a prefix argument n,
pause for n seconds instead.
edebug-where
).
If you use this command in a different window displaying the same
buffer, that window will be used instead to display the current
definition in the future.
edebug-toggle-save-windows
).
With a prefix argument, W
only toggles saving and restoring of
the selected window. To specify a window that is not displaying the
source code buffer, you must use C-x X W from the global keymap.
You can view the outside window configuration with v or just bounce to the point in the current buffer with p, even if it is not normally displayed. After moving point, you may wish to jump back to the stop point with w from a source code buffer.
Each time you use W to turn saving off, Edebug forgets the saved outside window configuration--so that even if you turn saving back on, the current window configuration remains unchanged when you next exit Edebug (by continuing the program). However, the automatic redisplay of `*edebug*' and `*edebug-trace*' may conflict with the buffers you wish to see unless you have enough windows open.
While within Edebug, you can evaluate expressions "as if" Edebug were not running. Edebug tries to be invisible to the expression's evaluation and printing. Evaluation of expressions that cause side effects will work as expected except for things that Edebug explicitly saves and restores. See section The Outside Context, for details on this process.
edebug-eval-expression
). That is, Edebug tries to minimize its
interference with the evaluation.
edebug-eval-last-sexp
).
Edebug supports evaluation of expressions containing references to
lexically bound symbols created by the following constructs in
`cl.el' (version 2.03 or later): lexical-let
,
macrolet
, and symbol-macrolet
.
You can use the evaluation list buffer, called `*edebug*', to evaluate expressions interactively. You can also set up the evaluation list of expressions to be evaluated automatically each time Edebug updates the display.
edebug-visit-eval-list
).
In the `*edebug*' buffer you can use the commands of Lisp Interaction mode (see section `Lisp Interaction' in The GNU Emacs Manual) as well as these special commands:
edebug-eval-print-last-sexp
).
edebug-eval-last-sexp
).
edebug-update-eval-list
).
edebug-delete-eval-item
).
edebug-where
).
You can evaluate expressions in the evaluation list window with C-j or C-x C-e, just as you would in `*scratch*'; but they are evaluated in the context outside of Edebug.
The expressions you enter interactively (and their results) are lost when you continue execution; but you can set up an evaluation list consisting of expressions to be evaluated each time execution stops.
To do this, write one or more evaluation list groups in the evaluation list buffer. An evaluation list group consists of one or more Lisp expressions. Groups are separated by comment lines.
The command C-c C-u (edebug-update-eval-list
) rebuilds the
evaluation list, scanning the buffer and using the first expression of
each group. (The idea is that the second expression of the group is the
value previously computed and displayed.)
Each entry to Edebug redisplays the evaluation list by inserting each expression in the buffer, followed by its current value. It also inserts comment lines so that each expression becomes its own group. Thus, if you type C-c C-u again without changing the buffer text, the evaluation list is effectively unchanged.
If an error occurs during an evaluation from the evaluation list, the error message is displayed in a string as if it were the result. Therefore, expressions that use variables not currently valid do not interrupt your debugging.
Here is an example of what the evaluation list window looks like after several expressions have been added to it:
(current-buffer) #<buffer *scratch*> ;--------------------------------------------------------------- (selected-window) #<window 16 on *scratch*> ;--------------------------------------------------------------- (point) 196 ;--------------------------------------------------------------- bad-var "Symbol's value as variable is void: bad-var" ;--------------------------------------------------------------- (recursion-depth) 0 ;--------------------------------------------------------------- this-command eval-last-sexp ;---------------------------------------------------------------
To delete a group, move point into it and type C-c C-d, or simply delete the text for the group and update the evaluation list with C-c C-u. To add a new expression to the evaluation list, insert the expression at a suitable place, and insert a new comment line. (You need not insert dashes in the comment line--its contents don't matter.) Then type C-c C-u.
After selecting `*edebug*', you can return to the source code buffer with C-c C-w. The `*edebug*' buffer is killed when you continue execution, and recreated next time it is needed.
If an expression in your program produces a value containing circular list structure, you may get an error when Edebug attempts to print it.
One way to cope with circular structure is to set print-length
or print-level
to truncate the printing. Edebug does this for
you; it binds print-length
and print-level
to 50 if they
were nil
. (Actually, the variables edebug-print-length
and edebug-print-level
specify the values to use within Edebug.)
See section Variables Affecting Output.
nil
, bind print-length
to this while printing
results in Edebug. The default value is 50
.
nil
, bind print-level
to this while printing
results in Edebug. The default value is 50
.
You can also print circular structures and structures that share elements more informatively by using the `cust-print' package.
To load `cust-print' and activate custom printing only for Edebug, simply use the command M-x edebug-install-custom-print. To restore the standard print functions, use M-x edebug-uninstall-custom-print.
Here is an example of code that creates a circular structure:
(setq a '(x y)) (setcar a a)
Custom printing prints this as `Result: #1=(#1# y)'. The `#1=' notation labels the structure that follows it with the label `1', and the `#1#' notation references the previously labeled structure. This notation is used for any shared elements of lists or vectors.
nil
, bind print-circle
to this while printing
results in Edebug. The default value is nil
.
Other programs can also use custom printing; see `cust-print.el' for details.
Edebug can record an execution trace, storing it in a buffer named
`*edebug-trace*'. This is a log of function calls and returns,
showing the function names and their arguments and values. To enable
trace recording, set edebug-trace
to a non-nil
value.
Making a trace buffer is not the same thing as using trace execution mode (see section Edebug Execution Modes).
When trace recording is enabled, each function entry and exit adds lines to the trace buffer. A function entry record looks like `::::{' followed by the function name and argument values. A function exit record looks like `::::}' followed by the function name and result of the function.
The number of `:'s in an entry shows its recursion depth. You can use the braces in the trace buffer to find the matching beginning or end of function calls.
You can customize trace recording for function entry and exit by
redefining the functions edebug-print-trace-before
and
edebug-print-trace-after
.
edebug-tracing
returns the value of the last form in body.
(apply 'format format-string format-args)
.
It also appends a newline to separate entries.
edebug-tracing
and edebug-trace
insert lines in the
trace buffer whenever they are called, even if Edebug is not active.
Adding text to the trace buffer also scrolls its window to show the last
lines inserted.
Edebug provides rudimentary coverage testing and display of execution frequency.
Coverage testing works by comparing the result of each expression with the previous result; each form in the program is considered "covered" if it has returned two different values since you began testing coverage in the current Emacs session. Thus, to do coverage testing on your program, execute it under various conditions and note whether it behaves correctly; Edebug will tell you when you have tried enough different conditions that each form has returned two different values.
Coverage testing makes execution slower, so it is only done if
edebug-test-coverage
is non-nil
. Frequency counting is
performed for all execution of an instrumented function, even if the
execution mode is Go-nonstop, and regardless of whether coverage testing
is enabled.
Use M-x edebug-display-freq-count to display both the coverage information and the frequency counts for a definition.
The frequency counts appear as comment lines after each line of code,
and you can undo all insertions with one undo
command. The
counts appear under the `(' before an expression or the `)'
after an expression, or on the last character of a variable. To
simplify the display, a count is not shown if it is equal to the
count of an earlier expression on the same line.
The character `=' following the count for an expression says that the expression has returned the same value each time it was evaluated. In other words, it is not yet "covered" for coverage testing purposes.
To clear the frequency count and coverage data for a definition,
simply reinstrument it with eval-defun
.
For example, after evaluating (fac 5)
with a source
breakpoint, and setting edebug-test-coverage
to t
, when
the breakpoint is reached, the frequency data looks like this:
(defun fac (n) (if (= n 0) (edebug)) ;#6 1 0 =5 (if (< 0 n) ;#5 = (* n (fac (1- n))) ;# 5 0 1)) ;# 0
The comment lines show that fac
was called 6 times. The
first if
statement returned 5 times with the same result each
time; the same is true of the condition on the second if
.
The recursive call of fac
did not return at all.
Edebug tries to be transparent to the program you are debugging, but it does not succeed completely. Edebug also tries to be transparent when you evaluate expressions with e or with the evaluation list buffer, by temporarily restoring the outside context. This section explains precisely what context Edebug restores, and how Edebug fails to be completely transparent.
Whenever Edebug is entered, it needs to save and restore certain data before even deciding whether to make trace information or stop the program.
max-lisp-eval-depth
and max-specpdl-size
are both
incremented once to reduce Edebug's impact on the stack. You could,
however, still run out of stack space when using Edebug.
executing-macro
is bound to
edebug-continue-kbd-macro
.
When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from "outside" Edebug (see section Window Configurations). When you exit Edebug (by continuing the program), it restores the previous window configuration.
Emacs redisplays only when it pauses. Usually, when you continue execution, the program comes back into Edebug at a breakpoint or after stepping without pausing or reading input in between. In such cases, Emacs never gets a chance to redisplay the "outside" configuration. What you see is the same window configuration as the last time Edebug was active, with no interruption.
Entry to Edebug for displaying something also saves and restores the following data, but some of these are deliberately not restored if an error or quit signal occurs.
edebug-save-windows
is non-nil
(see section Edebug Display Update).
The window configuration is not restored on error or quit, but the
outside selected window is reselected even on error or quit in
case a save-excursion
is active. If the value of
edebug-save-windows
is a list, only the listed windows are saved
and restored.
The window start and horizontal scrolling of the source code buffer are
not restored, however, so that the display remains coherent within Edebug.
edebug-save-displayed-buffer-points
is non-nil
.
overlay-arrow-position
and
overlay-arrow-string
are saved and restored. So you can safely
invoke Edebug from the recursive edit elsewhere in the same buffer.
cursor-in-echo-area
is locally bound to nil
so that
the cursor shows up in the window.
When Edebug is entered and actually reads commands from the user, it saves (and later restores) these additional data:
last-command
, this-command
, last-command-char
,
last-input-char
, last-input-event
,
last-command-event
, last-event-frame
,
last-nonmenu-event
, and track-mouse
. Commands used within
Edebug do not affect these variables outside of Edebug.
The key sequence returned by this-command-keys
is changed by
executing commands within Edebug and there is no way to reset
the key sequence from Lisp.
Edebug cannot save and restore the value of
unread-command-events
. Entering Edebug while this variable has a
nontrivial value can interfere with execution of the program you are
debugging.
command-history
. In rare cases this can alter execution.
standard-output
and standard-input
are bound to nil
by the recursive-edit
, but Edebug temporarily restores them during
evaluations.
defining-kbd-macro
is bound to
edebug-continue-kbd-macro
.
When Edebug instruments an expression that calls a Lisp macro, it needs additional information about the macro to do the job properly. This is because there is no a-priori way to tell which subexpressions of the macro call are forms to be evaluated. (Evaluation may occur explicitly in the macro body, or when the resulting expansion is evaluated, or any time later.)
Therefore, you must define an Edebug specification for each macro that
Edebug will encounter, to explain the format of calls to that macro. To
do this, use def-edebug-spec
.
The macro argument can actually be any symbol, not just a macro name.
Here is a simple example that defines the specification for the
for
example macro (see section Evaluating Macro Arguments Repeatedly), followed by an
alternative, equivalent specification.
(def-edebug-spec for (symbolp "from" form "to" form "do" &rest form)) (def-edebug-spec for (symbolp ['from form] ['to form] ['do body]))
Here is a table of the possibilities for specification and how each directs processing of arguments.
t
0
A specification list is required for an Edebug specification if
some arguments of a macro call are evaluated while others are not. Some
elements in a specification list match one or more arguments, but others
modify the processing of all following elements. The latter, called
specification keywords, are symbols beginning with `&' (such
as &optional
).
A specification list may contain sublists which match arguments that are themselves lists, or it may contain vectors used for grouping. Sublists and groups thus subdivide the specification list into a hierarchy of levels. Specification keywords apply only to the remainder of the sublist or group they are contained in.
When a specification list involves alternatives or repetition, matching it against an actual macro call may require backtracking. See section Backtracking in Specifications, for more details.
Edebug specifications provide the power of regular expression matching, plus some context-free grammar constructs: the matching of sublists with balanced parentheses, recursive processing of forms, and recursion via indirect specifications.
Here's a table of the possible elements of a specification list, with their meanings:
sexp
form
place
setf
construct.
body
&rest form
. See &rest
below.
function-form
quote
rather than
function
, since it instruments the body of the lambda expression
either way.
lambda-expr
&optional
[&optional specs...]
. To specify that several
elements must all match or none, use &optional
[specs...]
. See the defun
example below.
&rest
[&rest specs...]
.
To specify several elements that must all match on every repetition, use
&rest [specs...]
.
&or
&or
specification fails.
Each list element following &or
is a single alternative. To
group two or more list elements as a single alternative, enclose them in
[...]
.
¬
&or
, but if any of them match, the specification fails. If none
of them match, nothing is matched, but the ¬
specification
succeeds.
&define
&define
keyword should be the first element in
a list specification.
nil
gate
let
example
below.
other-symbol
def-edebug-spec
just as for macros. See the defun
example below.
Otherwise, the symbol should be a predicate. The predicate is called
with the argument and the specification fails if the predicate returns
nil
. In either case, that argument is not instrumented.
Some suitable predicates include symbolp
, integerp
,
stringp
, vectorp
, and atom
.
[elements...]
"string"
'symbol
, where the name
of symbol is the string, but the string form is preferred.
(vector elements...)
(elements...)
(spec . [(more
specs...)])
) whose elements match the non-dotted list arguments.
This is useful in recursive specifications such as in the backquote
example below. Also see the description of a nil
specification
above for terminating such recursion.
Note that a sublist specification written as (specs . nil)
is equivalent to (specs)
, and (specs .
(sublist-elements...))
is equivalent to (specs
sublist-elements...)
.
Here is a list of additional specifications that may appear only after
&define
. See the defun
example below.
name
:name
:name
should be a symbol; it is used as an additional
name component for the definition. You can use this to add a unique,
static component to the name of the definition. It may be used more
than once.
arg
lambda-list
def-body
body
, described above, but a definition body must be instrumented
with a different Edebug call that looks up information associated with
the definition. Use def-body
for the highest level list of forms
within the definition.
def-form
def-body
, except use this to match a single form rather than
a list of forms. As a special case, def-form
also means that
tracing information is not output when the form is executed. See the
interactive
example below.
If a specification fails to match at some point, this does not
necessarily mean a syntax error will be signaled; instead,
backtracking will take place until all alternatives have been
exhausted. Eventually every element of the argument list must be
matched by some element in the specification, and every required element
in the specification must match some argument.
When a syntax error is detected, it might not be reported until much
later after higher-level alternatives have been exhausted, and with the
point positioned further from the real error. But if backtracking is
disabled when an error occurs, it can be reported immediately. Note
that backtracking is also reenabled automatically in several situations;
it is reenabled when a new alternative is established by
&optional
, &rest
, or &or
, or at the start of
processing a sublist, group, or indirect specification. The effect of
enabling or disabling backtracking is limited to the remainder of the
level currently being processed and lower levels.
Backtracking is disabled while matching any of the
form specifications (that is, form
, body
, def-form
, and
def-body
). These specifications will match any form so any error
must be in the form itself rather than at a higher level.
Backtracking is also disabled after successfully matching a quoted
symbol or string specification, since this usually indicates a
recognized construct. But if you have a set of alternative constructs that
all begin with the same symbol, you can usually work around this
constraint by factoring the symbol out of the alternatives, e.g.,
["foo" &or [first case] [second case] ...]
.
Most needs are satisfied by these two ways that bactracking is
automatically disabled, but occasionally it is useful to explicitly
disable backtracking by using the gate
specification. This is
useful when you know that no higher alternatives could apply. See the
example of the let
specification.
It may be easier to understand Edebug specifications by studying the examples provided here.
A let
special form has a sequence of bindings and a body. Each
of the bindings is either a symbol or a sublist with a symbol and
optional expression. In the specification below, notice the gate
inside of the sublist to prevent backtracking once a sublist is found.
(def-edebug-spec let ((&rest &or symbolp (gate symbolp &optional form)) body))
Edebug uses the following specifications for defun
and
defmacro
and the associated argument list and interactive
specifications. It is necessary to handle interactive forms specially
since an expression argument it is actually evaluated outside of the
function body.
(def-edebug-spec defmacro defun) ; Indirect ref todefun
spec. (def-edebug-spec defun (&define name lambda-list [&optional stringp] ; Match the doc string, if present. [&optional ("interactive" interactive)] def-body)) (def-edebug-spec lambda-list (([&rest arg] [&optional ["&optional" arg &rest arg]] &optional ["&rest" arg] ))) (def-edebug-spec interactive (&optional &or stringp def-form)) ; Notice:def-form
The specification for backquote below illustrates how to match
dotted lists and use nil
to terminate recursion. It also
illustrates how components of a vector may be matched. (The actual
specification defined by Edebug does not support dotted lists because
doing so causes very deep recursion that could fail.)
(def-edebug-spec ` (backquote-form)) ; Alias just for clarity. (def-edebug-spec backquote-form (&or ([&or "," ",@"] &or ("quote" backquote-form) form) (backquote-form . [&or nil backquote-form]) (vector &rest backquote-form) sexp))
These options affect the behavior of Edebug:
edebug-setup-hook
is reset to nil
. You could use this to
load up Edebug specifications associated with a package you are using
but only when you also use Edebug.
See section Instrumenting for Edebug.
nil
, normal evaluation of defining forms such as
defun
and defmacro
instruments them for Edebug. This
applies to eval-defun
, eval-region
, eval-buffer
,
and eval-current-buffer
.
Use the command M-x edebug-all-defs to toggle the value of this option. See section Instrumenting for Edebug.
nil
, the commands eval-defun
,
eval-region
, eval-buffer
, and eval-current-buffer
instrument all forms, even those that don't define anything.
This doesn't apply to loading or evaluations in the minibuffer.
Use the command M-x edebug-all-forms to toggle the value of this option. See section Instrumenting for Edebug.
nil
, Edebug saves and restores the window
configuration. That takes some time, so if your program does not care
what happens to the window configurations, it is better to set this
variable to nil
.
If the value is a list, only the listed windows are saved and restored.
You can use the W command in Edebug to change this variable interactively. See section Edebug Display Update.
nil
, Edebug saves and restores point in all
displayed buffers.
Saving and restoring point in other buffers is necessary if you are debugging code that changes the point of a buffer which is displayed in a non-selected window. If Edebug or the user then selects the window, point in that buffer will move to the window's value of point.
Saving and restoring point in all buffers is expensive, since it requires selecting each window twice, so enable this only if you need it. See section Edebug Display Update.
nil
, it specifies the initial execution
mode for Edebug when it is first activated. Possible values are
step
, next
, go
, Go-nonstop
, trace
,
Trace-fast
, continue
, and Continue-fast
.
The default value is step
.
See section Edebug Execution Modes.
nil
means display a trace of function entry and exit.
Tracing output is displayed in a buffer named `*edebug-trace*', one
function entry or exit per line, indented by the recursion level.
The default value is nil
.
Also see edebug-tracing
, in section Trace Buffer.
nil
, Edebug tests coverage of all expressions debugged.
See section Coverage Testing.
nil
, continue defining or executing any keyboard macro
that is executing outside of Edebug. Use this with caution since it is not
debugged.
See section Edebug Execution Modes.
debug-on-error
to this value, if
debug-on-error
was previously nil
. See section Trapping Errors.
debug-on-quit
to this value, if
debug-on-quit
was previously nil
. See section Trapping Errors.
If you change the values of edebug-on-error
or
edebug-on-quit
while Edebug is active, their values won't be used
until the next time Edebug is invoked via a new command.
nil
, an expression to test for at every stop point.
If the result is non-nil, then break. Errors are ignored.
See section Global Break Condition.
The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error "End of file during parsing" in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, "Invalid read syntax: ")"" indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change?
If the problem is not simply an imbalance of parentheses, a useful technique is to try C-M-e at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun.
However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases. (In addition, just moving point through the code with Show Paren mode enabled might find the mismatch.)
The first step is to find the defun that is unbalanced. If there is
an excess open parenthesis, the way to do this is to insert a
close parenthesis at the end of the file and type C-M-b
(backward-sexp
). This will move you to the beginning of the
defun that is unbalanced. (Then type C-SPC C-_ C-u
C-SPC to set the mark there, undo the insertion of the
close parenthesis, and finally return to the mark.)
The next step is to determine precisely what is wrong. There is no way to be sure of this except by studying the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with C-M-q and see what moves. But don't do this yet! Keep reading, first.
Before you do this, make sure the defun has enough close parentheses. Otherwise, C-M-q will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don't use C-M-e to move there, since that too will fail to work until the defun is balanced.
Now you can go to the beginning of the defun and type C-M-q. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses.
After you think you have fixed the problem, use C-M-q again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.
To deal with an excess close parenthesis, first insert an open parenthesis at the beginning of the file, back up over it, and type C-M-f to find the end of the unbalanced defun. (Then type C-SPC C-_ C-u C-SPC to set the mark there, undo the insertion of the open parenthesis, and finally return to the mark.)
Then find the actual matching close parenthesis by typing C-M-f at the beginning of that defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity.
If you don't see a problem at that point, the next thing to do is to type C-M-q at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses.
After you think you have fixed the problem, use C-M-q again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.
When an error happens during byte compilation, it is normally due to invalid syntax in the program you are compiling. The compiler prints a suitable error message in the `*Compile-Log*' buffer, and then stops. The message may state a function name in which the error was found, or it may not. Either way, here is how to find out where in the file the error occurred.
What you should do is switch to the buffer ` *Compiler Input*'. (Note that the buffer name starts with a space, so it does not show up in M-x list-buffers.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read.
If the error was due to invalid Lisp syntax, point shows exactly where the invalid syntax was detected. The cause of the error is not necessarily near by! Use the techniques in the previous section to find the error.
If the error was detected while compiling a form that had been read successfully, then point is located at the end of the form. In this case, this technique can't localize the error precisely, but can still show you which function to check.
Printing and reading are the operations of converting Lisp objects to textual form and vice versa. They use the printed representations and read syntax described in section Lisp Data Types.
This chapter describes the Lisp functions for reading and printing. It also describes streams, which specify where to get the text (if reading) or where to put it (if printing).
Reading a Lisp object means parsing a Lisp expression in textual
form and producing a corresponding Lisp object. This is how Lisp
programs get into Lisp from files of Lisp code. We call the text the
read syntax of the object. For example, the text `(a . 5)'
is the read syntax for a cons cell whose CAR is a
and whose
CDR is the number 5.
Printing a Lisp object means producing text that represents that object--converting the object to its printed representation (see section Printed Representation and Read Syntax). Printing the cons cell described above produces the text `(a . 5)'.
Reading and printing are more or less inverse operations: printing the
object that results from reading a given piece of text often produces
the same text, and reading the text that results from printing an object
usually produces a similar-looking object. For example, printing the
symbol foo
produces the text `foo', and reading that text
returns the symbol foo
. Printing a list whose elements are
a
and b
produces the text `(a b)', and reading that
text produces a list (but not the same list) with elements a
and b
.
However, these two operations are not precisely inverses. There are three kinds of exceptions:
Most of the Lisp functions for reading text take an input stream as an argument. The input stream specifies where or how to get the characters of the text to be read. Here are the possible types of input stream:
t
t
used as a stream means that the input is read from the
minibuffer. In fact, the minibuffer is invoked once and the text
given by the user is made into a string that is then used as the
input stream.
nil
nil
supplied as an input stream means to use the value of
standard-input
instead; that value is the default input
stream, and must be a non-nil
input stream.
Here is an example of reading from a stream that is a buffer, showing where point is located before and after:
---------- Buffer: foo ---------- This-!- is the contents of foo. ---------- Buffer: foo ---------- (read (get-buffer "foo")) => is (read (get-buffer "foo")) => the ---------- Buffer: foo ---------- This is the-!- contents of foo. ---------- Buffer: foo ----------
Note that the first read skips a space. Reading skips any amount of whitespace preceding the significant text.
Here is an example of reading from a stream that is a marker,
initially positioned at the beginning of the buffer shown. The value
read is the symbol This
.
---------- Buffer: foo ---------- This is the contents of foo. ---------- Buffer: foo ---------- (setq m (set-marker (make-marker) 1 (get-buffer "foo"))) => #<marker at 1 in foo> (read m) => This m => #<marker at 5 in foo> ;; Before the first space.
Here we read from the contents of a string:
(read "(When in) the course") => (When in)
The following example reads from the minibuffer. The
prompt is: `Lisp expression: '. (That is always the prompt
used when you read from the stream t
.) The user's input is shown
following the prompt.
(read t) => 23 ---------- Buffer: Minibuffer ---------- Lisp expression: 23 RET ---------- Buffer: Minibuffer ----------
Finally, here is an example of a stream that is a function, named
useless-stream
. Before we use the stream, we initialize the
variable useless-list
to a list of characters. Then each call to
the function useless-stream
obtains the next character in the list
or unreads a character by adding it to the front of the list.
(setq useless-list (append "XY()" nil)) => (88 89 40 41) (defun useless-stream (&optional unread) (if unread (setq useless-list (cons unread useless-list)) (prog1 (car useless-list) (setq useless-list (cdr useless-list))))) => useless-stream
Now we read using the stream thus constructed:
(read 'useless-stream) => XY useless-list => (40 41)
Note that the open and close parentheses remain in the list. The Lisp
reader encountered the open parenthesis, decided that it ended the
input, and unread it. Another attempt to read from the stream at this
point would read `()' and return nil
.
load
. Don't use this function
yourself.
This section describes the Lisp functions and variables that pertain to reading.
In the functions below, stream stands for an input stream (see
the previous section). If stream is nil
or omitted, it
defaults to the value of standard-input
.
An end-of-file
error is signaled if reading encounters an
unterminated list, vector, or string.
If start is supplied, then reading begins at index start in the string (where the first character is at index 0). If you specify end, then reading is forced to stop just before that index, as if the rest of the string were not there.
For example:
(read-from-string "(setq x 55) (setq y 5)") => ((setq x 55) . 11) (read-from-string "\"A short string\"") => ("A short string" . 16) ;; Read starting at the first character. (read-from-string "(list 112)" 0) => ((list 112) . 10) ;; Read starting at the second character. (read-from-string "(list 112)" 1) => (list . 5) ;; Read starting at the seventh character, ;; and stopping at the ninth. (read-from-string "(list 112)" 6 8) => (11 . 8)
read
uses when the stream argument is nil
.
An output stream specifies what to do with the characters produced by printing. Most print functions accept an output stream as an optional argument. Here are the possible types of output stream:
t
nil
nil
specified as an output stream means to use the value of
standard-output
instead; that value is the default output
stream, and must not be nil
.
Many of the valid output streams are also valid as input streams. The difference between input and output streams is therefore more a matter of how you use a Lisp object, than of different types of object.
Here is an example of a buffer used as an output stream. Point is initially located as shown immediately before the `h' in `the'. At the end, point is located directly before that same `h'.
---------- Buffer: foo ---------- This is t-!-he contents of foo. ---------- Buffer: foo ---------- (print "This is the output" (get-buffer "foo")) => "This is the output" ---------- Buffer: foo ---------- This is t "This is the output" -!-he contents of foo. ---------- Buffer: foo ----------
Now we show a use of a marker as an output stream. Initially, the
marker is in buffer foo
, between the `t' and the `h' in
the word `the'. At the end, the marker has advanced over the
inserted text so that it remains positioned before the same `h'.
Note that the location of point, shown in the usual fashion, has no
effect.
---------- Buffer: foo ---------- This is the -!-output ---------- Buffer: foo ---------- (setq m (copy-marker 10)) => #<marker at 10 in foo> (print "More output for foo." m) => "More output for foo." ---------- Buffer: foo ---------- This is t "More output for foo." he -!-output ---------- Buffer: foo ---------- m => #<marker at 34 in foo>
The following example shows output to the echo area:
(print "Echo Area output" t) => "Echo Area output" ---------- Echo Area ---------- "Echo Area output" ---------- Echo Area ----------
Finally, we show the use of a function as an output stream. The
function eat-output
takes each character that it is given and
conses it onto the front of the list last-output
(see section Building Cons Cells and Lists). At the end, the list contains all the characters output, but
in reverse order.
(setq last-output nil) => nil (defun eat-output (c) (setq last-output (cons c last-output))) => eat-output (print "This is the output" 'eat-output) => "This is the output" last-output => (10 34 116 117 112 116 117 111 32 101 104 116 32 115 105 32 115 105 104 84 34 10)
Now we can put the output in the proper order by reversing the list:
(concat (nreverse last-output)) => " \"This is the output\" "
Calling concat
converts the list to a string so you can see its
contents more clearly.
This section describes the Lisp functions for printing Lisp objects--converting objects into their printed representation.
Some of the Emacs printing functions add quoting characters to the output when necessary so that it can be read properly. The quoting characters used are `"' and `\'; they distinguish strings from symbols, and prevent punctuation characters in strings and symbols from being taken as delimiters when reading. See section Printed Representation and Read Syntax, for full details. You specify quoting or no quoting by the choice of printing function.
If the text is to be read back into Lisp, then you should print with quoting characters to avoid ambiguity. Likewise, if the purpose is to describe a Lisp object clearly for a Lisp programmer. However, if the purpose of the output is to look nice for humans, then it is usually better to print without quoting.
Lisp objects can refer to themselves. Printing a self-referential object in the normal way would require an infinite amount of text, and the attempt could cause infinite recursion. Emacs detects such recursion and prints `#level' instead of recursively printing an object already being printed. For example, here `#0' indicates a recursive reference to the object at level 0 of the current print operation:
(setq foo (list nil)) => (nil) (setcar foo foo) => (#0)
In the functions below, stream stands for an output stream.
(See the previous section for a description of output streams.) If
stream is nil
or omitted, it defaults to the value of
standard-output
.
print
function is a convenient way of printing. It outputs
the printed representation of object to stream, printing in
addition one newline before object and another after it. Quoting
characters are used. print
returns object. For example:
(progn (print 'The\ cat\ in) (print "the hat") (print " came back")) -| -| The\ cat\ in -| -| "the hat" -| -| " came back" -| => " came back"
print
does, but it does use quoting characters just like
print
. It returns object.
(progn (prin1 'The\ cat\ in) (prin1 "the hat") (prin1 " came back")) -| The\ cat\ in"the hat"" came back" => " came back"
This function is intended to produce output that is readable by people,
not by read
, so it doesn't insert quoting characters and doesn't
put double-quotes around the contents of strings. It does not add any
spacing between calls.
(progn (princ 'The\ cat) (princ " in the \"hat\"")) -| The cat in the "hat" => " in the \"hat\""
prin1
would have printed for the same argument.
(prin1-to-string 'foo) => "foo" (prin1-to-string (mark-marker)) => "#<marker at 2773 in strings.texi>"
If noescape is non-nil
, that inhibits use of quoting
characters in the output. (This argument is supported in Emacs versions
19 and later.)
(prin1-to-string "foo") => "\"foo\"" (prin1-to-string "foo" t) => "foo"
See format
, in section Conversion of Characters and Strings, for other ways to obtain
the printed representation of a Lisp object as a string.
standard-output
set
up to feed output into a string. Then it returns that string.
For example, if the current buffer name is `foo',
(with-output-to-string (princ "The buffer is ") (princ (buffer-name)))
returns "The buffer is foo"
.
nil
.
nil
, then newline characters in strings
are printed as `\n' and formfeeds are printed as `\f'.
Normally these characters are printed as actual newlines and formfeeds.
This variable affects the print functions prin1
and print
that print with quoting. It does not affect princ
. Here is an
example using prin1
:
(prin1 "a\nb") -| "a -| b" => "a b" (let ((print-escape-newlines t)) (prin1 "a\nb")) -| "a\nb" => "a b"
In the second expression, the local binding of
print-escape-newlines
is in effect during the call to
prin1
, but not during the printing of the result.
nil
, then unibyte non-ASCII
characters in strings are unconditionally printed as backslash sequences
by the print functions prin1
and print
that print with
quoting.
Those functions also use backslash sequences for unibyte non-ASCII characters, regardless of the value of this variable, when the output stream is a multibyte buffer or a marker pointing into one.
nil
, then multibyte non-ASCII
characters in strings are unconditionally printed as backslash sequences
by the print functions prin1
and print
that print with
quoting.
Those functions also use backslash sequences for multibyte non-ASCII characters, regardless of the value of this variable, when the output stream is a unibyte buffer or a marker pointing into one.
If the value is nil
(the default), then there is no limit.
(setq print-length 2) => 2 (print '(1 2 3 4 5)) -| (1 2 ...) => (1 2 ...)
nil
(which is the default) means no limit.
A minibuffer is a special buffer that Emacs commands use to read arguments more complicated than the single numeric prefix argument. These arguments include file names, buffer names, and command names (as in M-x). The minibuffer is displayed on the bottom line of the frame, in the same place as the echo area, but only while it is in use for reading an argument.
In most ways, a minibuffer is a normal Emacs buffer. Most operations within a buffer, such as editing commands, work normally in a minibuffer. However, many operations for managing buffers do not apply to minibuffers. The name of a minibuffer always has the form ` *Minibuf-number', and it cannot be changed. Minibuffers are displayed only in special windows used only for minibuffers; these windows always appear at the bottom of a frame. (Sometimes frames have no minibuffer window, and sometimes a special kind of frame contains nothing but a minibuffer window; see section Minibuffers and Frames.)
The minibuffer's window is normally a single line. You can resize it temporarily with the window sizing commands; it reverts to its normal size when the minibuffer is exited. You can resize it permanently by using the window sizing commands in the frame's other window, when the minibuffer is not active. If the frame contains just a minibuffer, you can change the minibuffer's size by changing the frame's size.
If a command uses a minibuffer while there is an active minibuffer,
this is called a recursive minibuffer. The first minibuffer is
named ` *Minibuf-0*'. Recursive minibuffers are named by
incrementing the number at the end of the name. (The names begin with a
space so that they won't show up in normal buffer lists.) Of several
recursive minibuffers, the innermost (or most recently entered) is the
active minibuffer. We usually call this "the" minibuffer. You can
permit or forbid recursive minibuffers by setting the variable
enable-recursive-minibuffers
or by putting properties of that
name on command symbols (see section Minibuffer Miscellany).
Like other buffers, a minibuffer may use any of several local keymaps (see section Keymaps); these contain various exit commands and in some cases completion commands (see section Completion).
minibuffer-local-map
is for ordinary input (no completion).
minibuffer-local-ns-map
is similar, except that SPC exits
just like RET. This is used mainly for Mocklisp compatibility.
minibuffer-local-completion-map
is for permissive completion.
minibuffer-local-must-match-map
is for strict completion and
for cautious completion.
Most often, the minibuffer is used to read text as a string. It can
also be used to read a Lisp object in textual form. The most basic
primitive for minibuffer input is read-from-minibuffer
; it can do
either one.
In most cases, you should not call minibuffer input functions in the
middle of a Lisp function. Instead, do all minibuffer input as part of
reading the arguments for a command, in the interactive
specification. See section Defining Commands.
nil
, then it uses
read
to convert the text into a Lisp object (see section Input Functions).
The first thing this function does is to activate a minibuffer and display it with prompt-string as the prompt. This value must be a string. Then the user can edit text in the minibuffer.
When the user types a command to exit the minibuffer,
read-from-minibuffer
constructs the return value from the text in
the minibuffer. Normally it returns a string containing that text.
However, if read is non-nil
, read-from-minibuffer
reads the text and returns the resulting Lisp object, unevaluated.
(See section Input Functions, for information about reading.)
The argument default specifies a default value to make available
through the history commands. It should be a string, or nil
. If
read is non-nil
, then default is also used as the
input to read
, if the user enters empty input. However, in the
usual case (where read is nil
), read-from-minibuffer
does not return default when the user enters empty input; it
returns an empty string, ""
. In this respect, it is different
from all the other minibuffer input functions in this chapter.
If keymap is non-nil
, that keymap is the local keymap to
use in the minibuffer. If keymap is omitted or nil
, the
value of minibuffer-local-map
is used as the keymap. Specifying
a keymap is the most important way to customize the minibuffer for
various applications such as completion.
The argument hist specifies which history list variable to use
for saving the input and for history commands used in the minibuffer.
It defaults to minibuffer-history
. See section Minibuffer History.
If the variable minibuffer-allow-text-properties
is
non-nil
, then the string which is returned includes whatever text
properties were present in the minibuffer. Otherwise all the text
properties are stripped when the value is returned.
If the argument inherit-input-method is non-nil
, then the
minibuffer inherits the current input method (see section Input Methods) and
the setting of enable-multibyte-characters
(see section Text Representations) from whichever buffer was current before entering the
minibuffer.
If initial-contents is a string, read-from-minibuffer
inserts it into the minibuffer, leaving point at the end, before the
user starts to edit the text. The minibuffer appears with this text as
its initial contents.
Alternatively, initial-contents can be a cons cell of the form
(string . position)
. This means to insert
string in the minibuffer but put point position characters
from the beginning, rather than at the end.
Usage note: The initial-contents argument and the
default argument are two alternative features for more or less the
same job. It does not make sense to use both features in a single call
to read-from-minibuffer
. In general, we recommend using
default, since this permits the user to insert the default value
when it is wanted, but does not burden the user with deleting it from
the minibuffer on other occasions.
read-from-minibuffer
. The keymap used is
minibuffer-local-map
.
The optional argument history, if non-nil, specifies a history list and optionally the initial position in the list. The optional argument default specifies a default value to return if the user enters null input; it should be a string. The optional argument inherit-input-method specifies whether to inherit the current buffer's input method.
This function is a simplified interface to the
read-from-minibuffer
function:
(read-string prompt initial history default inherit) == (let ((value (read-from-minibuffer prompt initial nil nil history default inherit))) (if (equal value "") default value))
nil
, then read-from-minibuffer
strips
all text properties from the minibuffer input before returning it.
Since all minibuffer input uses read-from-minibuffer
, this
variable applies to all minibuffer input.
Note that the completion functions discard text properties unconditionally, regardless of the value of this variable.
exit-minibuffer
exit-minibuffer
abort-recursive-edit
next-history-element
previous-history-element
next-matching-history-element
previous-matching-history-element
read-from-minibuffer
.
This is a simplified interface to the read-from-minibuffer
function, and passes the value of the minibuffer-local-ns-map
keymap as the keymap argument for that function. Since the keymap
minibuffer-local-ns-map
does not rebind C-q, it is
possible to put a space into the string, by quoting it.
(read-no-blanks-input prompt initial) == (read-from-minibuffer prompt initial minibuffer-local-ns-map)
read-no-blanks-input
. By default, it makes the
following bindings, in addition to those of minibuffer-local-map
:
exit-minibuffer
exit-minibuffer
self-insert-and-exit
This section describes functions for reading Lisp objects with the minibuffer.
read-from-minibuffer
.
This is a simplified interface to the
read-from-minibuffer
function:
(read-minibuffer prompt initial) == (read-from-minibuffer prompt initial nil t)
Here is an example in which we supply the string "(testing)"
as
initial input:
(read-minibuffer "Enter an expression: " (format "%s" '(testing))) ;; Here is how the minibuffer is displayed: ---------- Buffer: Minibuffer ---------- Enter an expression: (testing)-!- ---------- Buffer: Minibuffer ----------
The user can type RET immediately to use the initial input as a default, or can edit the input.
read-from-minibuffer
.
This function simply evaluates the result of a call to
read-minibuffer
:
(eval-minibuffer prompt initial) == (eval (read-minibuffer prompt initial))
eval-minibuffer
is that here the initial form is not
optional and it is treated as a Lisp object to be converted to printed
representation rather than as a string of text. It is printed with
prin1
, so if it is a string, double-quote characters (`"')
appear in the initial text. See section Output Functions.
The first thing edit-and-eval-command
does is to activate the
minibuffer with prompt as the prompt. Then it inserts the printed
representation of form in the minibuffer, and lets the user edit it.
When the user exits the minibuffer, the edited text is read with
read
and then evaluated. The resulting value becomes the value
of edit-and-eval-command
.
In the following example, we offer the user an expression with initial text which is a valid form already:
(edit-and-eval-command "Please edit: " '(forward-word 1)) ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------- Please edit: (forward-word 1)-!- ---------- Buffer: Minibuffer ----------
Typing RET right away would exit the minibuffer and evaluate the
expression, thus moving point forward one word.
edit-and-eval-command
returns nil
in this example.
A minibuffer history list records previous minibuffer inputs so the user can reuse them conveniently. A history list is actually a symbol, not a list; it is a variable whose value is a list of strings (previous inputs), most recent first.
There are many separate history lists, used for different kinds of inputs. It's the Lisp programmer's job to specify the right history list for each use of the minibuffer.
The basic minibuffer input functions read-from-minibuffer
and
completing-read
both accept an optional argument named hist
which is how you specify the history list. Here are the possible
values:
If you don't specify hist, then the default history list
minibuffer-history
is used. For other standard history lists,
see below. You can also create your own history list variable; just
initialize it to nil
before the first use.
Both read-from-minibuffer
and completing-read
add new
elements to the history list automatically, and provide commands to
allow the user to reuse items on the list. The only thing your program
needs to do to use a history list is to initialize it and to pass its
name to the input functions when you wish. But it is safe to modify the
list by hand when the minibuffer input functions are not using it.
Here are some of the standard minibuffer history list variables:
query-replace
(and similar
arguments to other commands).
Completion is a feature that fills in the rest of a name
starting from an abbreviation for it. Completion works by comparing the
user's input against a list of valid names and determining how much of
the name is determined uniquely by what the user has typed. For
example, when you type C-x b (switch-to-buffer
) and then
type the first few letters of the name of the buffer to which you wish
to switch, and then type TAB (minibuffer-complete
), Emacs
extends the name as far as it can.
Standard Emacs commands offer completion for names of symbols, files, buffers, and processes; with the functions in this section, you can implement completion for other kinds of names.
The try-completion
function is the basic primitive for
completion: it returns the longest determined completion of a given
initial string, with a given set of strings to match against.
The function completing-read
provides a higher-level interface
for completion. A call to completing-read
specifies how to
determine the list of valid names. The function then activates the
minibuffer with a local keymap that binds a few keys to commands useful
for completion. Other functions provide convenient simple interfaces
for reading certain kinds of names with completion.
The two functions try-completion
and all-completions
have nothing in themselves to do with minibuffers. We describe them in
this chapter so as to keep them near the higher-level completion
features that do use the minibuffer.
Completion compares string against each of the permissible
completions specified by collection; if the beginning of the
permissible completion equals string, it matches. If no permissible
completions match, try-completion
returns nil
. If only
one permissible completion matches, and the match is exact, then
try-completion
returns t
. Otherwise, the value is the
longest initial sequence common to all the permissible completions that
match.
If collection is an alist (see section Association Lists), the CARs of the alist elements form the set of permissible completions.
If collection is an obarray (see section Creating and Interning Symbols), the names
of all symbols in the obarray form the set of permissible completions. The
global variable obarray
holds an obarray containing the names of
all interned Lisp symbols.
Note that the only valid way to make a new obarray is to create it
empty and then add symbols to it one by one using intern
.
Also, you cannot intern a given symbol in more than one obarray.
If the argument predicate is non-nil
, then it must be a
function of one argument. It is used to test each possible match, and
the match is accepted only if predicate returns non-nil
.
The argument given to predicate is either a cons cell from the alist
(the CAR of which is a string) or else it is a symbol (not a
symbol name) from the obarray.
You can also use a symbol that is a function as collection. Then
the function is solely responsible for performing completion;
try-completion
returns whatever this function returns. The
function is called with three arguments: string, predicate
and nil
. (The reason for the third argument is so that the same
function can be used in all-completions
and do the appropriate
thing in either case.) See section Programmed Completion.
In the first of the following examples, the string `foo' is
matched by three of the alist CARs. All of the matches begin with
the characters `fooba', so that is the result. In the second
example, there is only one possible match, and it is exact, so the value
is t
.
(try-completion "foo" '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))) => "fooba" (try-completion "foo" '(("barfoo" 2) ("foo" 3))) => t
In the following example, numerous symbols begin with the characters `forw', and all of them begin with the word `forward'. In most of the symbols, this is followed with a `-', but not in all, so no more than `forward' can be completed.
(try-completion "forw" obarray) => "forward"
Finally, in the following example, only two of the three possible
matches pass the predicate test
(the string `foobaz' is
too short). Both of those begin with the string `foobar'.
(defun test (s) (> (length (car s)) 6)) => test (try-completion "foo" '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) 'test) => "foobar"
try-completion
.
If collection is a function, it is called with three arguments:
string, predicate and t
; then all-completions
returns whatever the function returns. See section Programmed Completion.
If nospace is non-nil
, completions that start with a space
are ignored unless string also starts with a space.
Here is an example, using the function test
shown in the
example for try-completion
:
(defun test (s) (> (length (car s)) 6)) => test (all-completions "foo" '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) 'test) => ("foobar1" "foobar2")
nil
, Emacs does not consider case significant in completion.
This section describes the basic interface for reading from the minibuffer with completion.
The actual completion is done by passing collection and
predicate to the function try-completion
. This happens in
certain commands bound in the local keymaps used for completion.
If require-match is nil
, the exit commands work regardless
of the input in the minibuffer. If require-match is t
, the
usual minibuffer exit commands won't exit unless the input completes to
an element of collection. If require-match is neither
nil
nor t
, then the exit commands won't exit unless the
input already in the buffer matches an element of collection.
However, empty input is always permitted, regardless of the value of
require-match; in that case, completing-read
returns
default. The value of default (if non-nil
) is also
available to the user through the history commands.
The user can exit with null input by typing RET with an empty
minibuffer. Then completing-read
returns ""
. This is how
the user requests whatever default the command uses for the value being
read. The user can return using RET in this way regardless of the
value of require-match, and regardless of whether the empty string
is included in collection.
The function completing-read
works by calling
read-minibuffer
. It uses minibuffer-local-completion-map
as the keymap if require-match is nil
, and uses
minibuffer-local-must-match-map
if require-match is
non-nil
. See section Minibuffer Commands That Do Completion.
The argument hist specifies which history list variable to use for
saving the input and for minibuffer history commands. It defaults to
minibuffer-history
. See section Minibuffer History.
If initial is non-nil
, completing-read
inserts it
into the minibuffer as part of the input. Then it allows the user to
edit the input, providing several commands to attempt completion.
In most cases, we recommend using default, and not initial.
If the argument inherit-input-method is non-nil
, then the
minibuffer inherits the current input method (see section Input Methods) and the setting of enable-multibyte-characters
(see section Text Representations) from whichever buffer was current before
entering the minibuffer.
Completion ignores case when comparing the input against the possible
matches, if the built-in variable completion-ignore-case
is
non-nil
. See section Basic Completion Functions.
Here's an example of using completing-read
:
(completing-read "Complete a foo: " '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) nil t "fo") ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------- Complete a foo: fo-!- ---------- Buffer: Minibuffer ----------
If the user then types DEL DEL b RET,
completing-read
returns barfoo
.
The completing-read
function binds three variables to pass
information to the commands that actually do completion. These
variables are minibuffer-completion-table
,
minibuffer-completion-predicate
and
minibuffer-completion-confirm
. For more information about them,
see section Minibuffer Commands That Do Completion.
This section describes the keymaps, commands and user options used in the minibuffer to do completion.
completing-read
uses this value as the local keymap when an
exact match of one of the completions is not required. By default, this
keymap makes the following bindings:
minibuffer-completion-help
minibuffer-complete-word
minibuffer-complete
with other characters bound as in minibuffer-local-map
(see section Reading Text Strings with the Minibuffer).
completing-read
uses this value as the local keymap when an
exact match of one of the completions is required. Therefore, no keys
are bound to exit-minibuffer
, the command that exits the
minibuffer unconditionally. By default, this keymap makes the following
bindings:
minibuffer-completion-help
minibuffer-complete-word
minibuffer-complete
minibuffer-complete-and-exit
minibuffer-complete-and-exit
with other characters bound as in minibuffer-local-map
.
completing-read
passes to try-completion
. It is used by
minibuffer completion commands such as minibuffer-complete-word
.
completing-read
passes to try-completion
. The variable is also used by the other
minibuffer completion functions.
minibuffer-complete-word
does not add any characters beyond the
first character that is not a word constituent. See section Syntax Tables.
minibuffer-completion-confirm
is nil
. If confirmation
is required, it is given by repeating this command
immediately--the command is programmed to work without confirmation
when run twice in succession.
nil
, Emacs asks for
confirmation of a completion before exiting the minibuffer. The
function minibuffer-complete-and-exit
checks the value of this
variable before it exits.
all-completions
using the value of the variable minibuffer-completion-table
as
the collection argument, and the value of
minibuffer-completion-predicate
as the predicate argument.
The list of completions is displayed as text in a buffer named
`*Completions*'.
standard-output
, usually a buffer. (See section Reading and Printing Lisp Objects, for more
information about streams.) The argument completions is normally
a list of completions just returned by all-completions
, but it
does not have to be. Each element may be a symbol or a string, either
of which is simply printed, or a list of two strings, which is printed
as if the strings were concatenated.
This function is called by minibuffer-completion-help
. The
most common way to use it is together with
with-output-to-temp-buffer
, like this:
(with-output-to-temp-buffer "*Completions*" (display-completion-list (all-completions (buffer-string) my-alist)))
nil
, the completion commands
automatically display a list of possible completions whenever nothing
can be completed because the next character is not uniquely determined.
This section describes the higher-level convenient functions for reading certain sorts of names with completion.
In most cases, you should not call these functions in the middle of a
Lisp function. When possible, do all minibuffer input as part of
reading the arguments for a command, in the interactive
specification. See section Defining Commands.
nil
,
it should be a string or a buffer. It is mentioned in the prompt, but
is not inserted in the minibuffer as initial input.
If existing is non-nil
, then the name specified must be
that of an existing buffer. The usual commands to exit the minibuffer
do not exit if the text is not valid, and RET does completion to
attempt to find a valid name. (However, default is not checked
for validity; it is returned, whatever it is, if the user exits with the
minibuffer empty.)
In the following example, the user enters `minibuffer.t', and
then types RET. The argument existing is t
, and the
only buffer name starting with the given input is
`minibuffer.texi', so that name is the value.
(read-buffer "Buffer name? " "foo" t) ;; After evaluation of the preceding expression, ;; the following prompt appears, ;; with an empty minibuffer: ---------- Buffer: Minibuffer ---------- Buffer name? (default foo) -!- ---------- Buffer: Minibuffer ---------- ;; The user types minibuffer.t RET. => "minibuffer.texi"
iswitchb-read-buffer
, all Emacs commands
that call read-buffer
to read a buffer name will actually use the
iswitchb
package to read it.
read-from-minibuffer
. Recall that a command is anything for
which commandp
returns t
, and a command name is a symbol
for which commandp
returns t
. See section Interactive Call.
The argument default specifies what to return if the user enters
null input. It can be a symbol or a string; if it is a string,
read-command
interns it before returning it. If default is
nil
, that means no default has been specified; then if the user
enters null input, the return value is nil
.
(read-command "Command name? ") ;; After evaluation of the preceding expression, ;; the following prompt appears with an empty minibuffer: ---------- Buffer: Minibuffer ---------- Command name? ---------- Buffer: Minibuffer ----------
If the user types forward-c RET, then this function returns
forward-char
.
The read-command
function is a simplified interface to
completing-read
. It uses the variable obarray
so as to
complete in the set of extant Lisp symbols, and it uses the
commandp
predicate so as to accept only command names:
(read-command prompt) == (intern (completing-read prompt obarray 'commandp t nil))
The argument default specifies what to return if the user enters
null input. It can be a symbol or a string; if it is a string,
read-variable
interns it before returning it. If default
is nil
, that means no default has been specified; then if the
user enters null input, the return value is nil
.
(read-variable "Variable name? ") ;; After evaluation of the preceding expression, ;; the following prompt appears, ;; with an empty minibuffer: ---------- Buffer: Minibuffer ---------- Variable name? -!- ---------- Buffer: Minibuffer ----------
If the user then types fill-p RET, read-variable
returns fill-prefix
.
This function is similar to read-command
, but uses the
predicate user-variable-p
instead of commandp
:
(read-variable prompt) == (intern (completing-read prompt obarray 'user-variable-p t nil))
See also the functions read-coding-system
and
read-non-nil-coding-system
, in section User-Chosen Coding Systems.
Here is another high-level completion function, designed for reading a file name. It provides special features including automatic insertion of the default directory.
nil
, then the function returns default if the user just
types RET. default is not checked for validity; it is
returned, whatever it is, if the user exits with the minibuffer empty.
If existing is non-nil
, then the user must specify the name
of an existing file; RET performs completion to make the name
valid if possible, and then refuses to exit if it is not valid. If the
value of existing is neither nil
nor t
, then
RET also requires confirmation after completion. If
existing is nil
, then the name of a nonexistent file is
acceptable.
The argument directory specifies the directory to use for
completion of relative file names. If insert-default-directory
is non-nil
, directory is also inserted in the minibuffer as
initial input. It defaults to the current buffer's value of
default-directory
.
If you specify initial, that is an initial file name to insert in
the buffer (after directory, if that is inserted). In this
case, point goes at the beginning of initial. The default for
initial is nil
---don't insert any file name. To see what
initial does, try the command C-x C-v. Note: we
recommend using default rather than initial in most cases.
Here is an example:
(read-file-name "The file is ") ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------- The file is /gp/gnu/elisp/-!- ---------- Buffer: Minibuffer ----------
Typing manual TAB results in the following:
---------- Buffer: Minibuffer ---------- The file is /gp/gnu/elisp/manual.texi-!- ---------- Buffer: Minibuffer ----------
If the user types RET, read-file-name
returns the file name
as the string "/gp/gnu/elisp/manual.texi"
.
read-file-name
. Its value controls
whether read-file-name
starts by placing the name of the default
directory in the minibuffer, plus the initial file name if any. If the
value of this variable is nil
, then read-file-name
does
not place any initial input in the minibuffer (unless you specify
initial input with the initial argument). In that case, the
default directory is still used for completion of relative file names,
but is not displayed.
For example:
;; Here the minibuffer starts out with the default directory. (let ((insert-default-directory t)) (read-file-name "The file is ")) ---------- Buffer: Minibuffer ---------- The file is ~lewis/manual/-!- ---------- Buffer: Minibuffer ---------- ;; Here the minibuffer is empty and only the prompt ;; appears on its line. (let ((insert-default-directory nil)) (read-file-name "The file is ")) ---------- Buffer: Minibuffer ---------- The file is -!- ---------- Buffer: Minibuffer ----------
Sometimes it is not possible to create an alist or an obarray containing all the intended possible completions. In such a case, you can supply your own function to compute the completion of a given string. This is called programmed completion.
To use this feature, pass a symbol with a function definition as the
collection argument to completing-read
. The function
completing-read
arranges to pass your completion function along
to try-completion
and all-completions
, which will then let
your function do all the work.
The completion function should accept three arguments:
nil
if
none. Your function should call the predicate for each possible match,
and ignore the possible match if the predicate returns nil
.
There are three flag values for three operations:
nil
specifies try-completion
. The completion function
should return the completion of the specified string, or t
if the
string is a unique and exact match already, or nil
if the string
matches no possibility.
If the string is an exact match for one possibility, but also matches
other longer possibilities, the function should return the string, not
t
.
t
specifies all-completions
. The completion function
should return a list of all possible completions of the specified
string.
lambda
specifies a test for an exact match. The completion
function should return t
if the specified string is an exact
match for some possibility; nil
otherwise.
It would be consistent and clean for completion functions to allow lambda expressions (lists that are functions) as well as function symbols as collection, but this is impossible. Lists as completion tables are already assigned another meaning--as alists. It would be unreliable to fail to handle an alist normally because it is also a possible function. So you must arrange for any function you wish to use for completion to be encapsulated in a symbol.
Emacs uses programmed completion when completing file names. See section File Name Completion.
This section describes functions used to ask the user a yes-or-no
question. The function y-or-n-p
can be answered with a single
character; it is useful for questions where an inadvertent wrong answer
will not have serious consequences. yes-or-no-p
is suitable for
more momentous questions, since it requires three or four characters to
answer.
If either of these functions is called in a command that was invoked
using the mouse--more precisely, if last-nonmenu-event
(see section Information from the Command Loop) is either nil
or a list--then it
uses a dialog box or pop-up menu to ask the question. Otherwise, it
uses keyboard input. You can force use of the mouse or use of keyboard
input by binding last-nonmenu-event
to a suitable value around
the call.
Strictly speaking, yes-or-no-p
uses the minibuffer and
y-or-n-p
does not; but it seems best to describe them together.
t
if the user types y, nil
if the
user types n. This function also accepts SPC to mean yes
and DEL to mean no. It accepts C-] to mean "quit", like
C-g, because the question might look like a minibuffer and for
that reason the user might try to use C-] to get out. The answer
is a single character, with no RET needed to terminate it. Upper
and lower case are equivalent.
"Asking the question" means printing prompt in the echo area, followed by the string `(y or n) '. If the input is not one of the expected answers (y, n, SPC, DEL, or something that quits), the function responds `Please answer y or n.', and repeats the request.
This function does not actually use the minibuffer, since it does not allow editing of the answer. It actually uses the echo area (see section The Echo Area), which uses the same screen space as the minibuffer. The cursor moves to the echo area while the question is being asked.
The answers and their meanings, even `y' and `n', are not
hardwired. The keymap query-replace-map
specifies them.
See section Search and Replace.
In the following example, the user first types q, which is invalid. At the next prompt the user types y.
(y-or-n-p "Do you need a lift? ") ;; After evaluation of the preceding expression, ;; the following prompt appears in the echo area: ---------- Echo area ---------- Do you need a lift? (y or n) ---------- Echo area ---------- ;; If the user then types q, the following appears: ---------- Echo area ---------- Please answer y or n. Do you need a lift? (y or n) ---------- Echo area ---------- ;; When the user types a valid answer, ;; it is displayed after the question: ---------- Echo area ---------- Do you need a lift? (y or n) y ---------- Echo area ----------
We show successive lines of echo area messages, but only one actually appears on the screen at a time.
y-or-n-p
, except that if the user fails to answer within
seconds seconds, this function stops waiting and returns
default-value. It works by setting up a timer; see section Timers for Delayed Execution.
The argument seconds may be an integer or a floating point number.
t
if the user enters `yes',
nil
if the user types `no'. The user must type RET to
finalize the response. Upper and lower case are equivalent.
yes-or-no-p
starts by displaying prompt in the echo area,
followed by `(yes or no) '. The user must type one of the
expected responses; otherwise, the function responds `Please answer
yes or no.', waits about two seconds and repeats the request.
yes-or-no-p
requires more work from the user than
y-or-n-p
and is appropriate for more crucial decisions.
Here is an example:
(yes-or-no-p "Do you really want to remove everything? ") ;; After evaluation of the preceding expression, ;; the following prompt appears, ;; with an empty minibuffer: ---------- Buffer: minibuffer ---------- Do you really want to remove everything? (yes or no) ---------- Buffer: minibuffer ----------
If the user first types y RET, which is invalid because this function demands the entire word `yes', it responds by displaying these prompts, with a brief pause between them:
---------- Buffer: minibuffer ---------- Please answer yes or no. Do you really want to remove everything? (yes or no) ---------- Buffer: minibuffer ----------
When you have a series of similar questions to ask, such as "Do you
want to save this buffer" for each buffer in turn, you should use
map-y-or-n-p
to ask the collection of questions, rather than
asking each question individually. This gives the user certain
convenient facilities such as the ability to answer the whole series at
once.
The value of list specifies the objects to ask questions about.
It should be either a list of objects or a generator function. If it is
a function, it should expect no arguments, and should return either the
next object to ask about, or nil
meaning stop asking questions.
The argument prompter specifies how to ask each question. If prompter is a string, the question text is computed like this:
(format prompter object)
where object is the next object to ask about (as obtained from list).
If not a string, prompter should be a function of one argument
(the next object to ask about) and should return the question text. If
the value is a string, that is the question to ask the user. The
function can also return t
meaning do act on this object (and
don't ask the user), or nil
meaning ignore this object (and don't
ask the user).
The argument actor says how to act on the answers that the user gives. It should be a function of one argument, and it is called with each object that the user says yes for. Its argument is always an object obtained from list.
If the argument help is given, it should be a list of this form:
(singular plural action)
where singular is a string containing a singular noun that describes the objects conceptually being acted on, plural is the corresponding plural noun, and action is a transitive verb describing what actor does.
If you don't specify help, the default is ("object"
"objects" "act on")
.
Each time a question is asked, the user may enter y, Y, or
SPC to act on that object; n, N, or DEL to skip
that object; ! to act on all following objects; ESC or
q to exit (skip all following objects); . (period) to act on
the current object and then exit; or C-h to get help. These are
the same answers that query-replace
accepts. The keymap
query-replace-map
defines their meaning for map-y-or-n-p
as well as for query-replace
; see section Search and Replace.
You can use action-alist to specify additional possible answers
and what they mean. It is an alist of elements of the form
(char function help)
, each of which defines one
additional answer. In this element, char is a character (the
answer); function is a function of one argument (an object from
list); help is a string.
When the user responds with char, map-y-or-n-p
calls
function. If it returns non-nil
, the object is considered
"acted upon", and map-y-or-n-p
advances to the next object in
list. If it returns nil
, the prompt is repeated for the
same object.
If map-y-or-n-p
is called in a command that was invoked using the
mouse--more precisely, if last-nonmenu-event
(see section Information from the Command Loop) is either nil
or a list--then it uses a dialog box
or pop-up menu to ask the question. In this case, it does not use
keyboard input or the echo area. You can force use of the mouse or use
of keyboard input by binding last-nonmenu-event
to a suitable
value around the call.
The return value of map-y-or-n-p
is the number of objects acted on.
To read a password to pass to another program, you can use the
function read-passwd
.
The optional argument confirm, if non-nil
, says to read the
password twice and insist it must be the same both times. If it isn't
the same, the user has to type it over and over until the last two
times match.
The optional argument default specifies the default password to
return if the user enters empty input. If default is nil
,
then read-passwd
returns the null string in that case.
This section describes some basic functions and variables related to minibuffers.
last-command-char
;
see section Information from the Command Loop).
nil
.
help-form
locally inside the minibuffer (see section Help Functions).
nil
if none is currently active.
nil
, that stands for the current frame. Note
that the minibuffer window used by a frame need not be part of that
frame--a frame that has no minibuffer of its own necessarily uses some
other frame's minibuffer window.
nil
if window is a minibuffer window.
It is not correct to determine whether a given window is a minibuffer by
comparing it with the result of (minibuffer-window)
, because
there can be more than one minibuffer window if there is more than one
frame.
nil
if window, assumed to be
a minibuffer window, is currently active.
nil
, it should be a window
object. When the function scroll-other-window
is called in the
minibuffer, it scrolls this window.
Finally, some functions and variables deal with recursive minibuffers (see section Recursive Editing):
nil
, you can invoke commands (such as
find-file
) that use minibuffers even while the minibuffer window
is active. Such invocation produces a recursive editing level for a new
minibuffer. The outer-level minibuffer is invisible while you are
editing the inner one.
If this variable is nil
, you cannot invoke minibuffer
commands when the minibuffer window is active, not even if you switch to
another window to do it.
If a command name has a property enable-recursive-minibuffers
that is non-nil
, then the command can use the minibuffer to read
arguments even if it is invoked from the minibuffer. The minibuffer
command next-matching-history-element
(normally M-s in the
minibuffer) uses this feature.
When you run Emacs, it enters the editor command loop almost immediately. This loop reads key sequences, executes their definitions, and displays the results. In this chapter, we describe how these things are done, and the subroutines that allow Lisp programs to do them.
The first thing the command loop must do is read a key sequence, which
is a sequence of events that translates into a command. It does this by
calling the function read-key-sequence
. Your Lisp code can also
call this function (see section Key Sequence Input). Lisp programs can also
do input at a lower level with read-event
(see section Reading One Event) or discard pending input with discard-input
(see section Miscellaneous Event Input Features).
The key sequence is translated into a command through the currently
active keymaps. See section Key Lookup, for information on how this is done.
The result should be a keyboard macro or an interactively callable
function. If the key is M-x, then it reads the name of another
command, which it then calls. This is done by the command
execute-extended-command
(see section Interactive Call).
To execute a command requires first reading the arguments for it.
This is done by calling command-execute
(see section Interactive Call). For commands written in Lisp, the interactive
specification says how to read the arguments. This may use the prefix
argument (see section Prefix Command Arguments) or may read with prompting
in the minibuffer (see section Minibuffers). For example, the command
find-file
has an interactive
specification which says to
read a file name using the minibuffer. The command's function body does
not use the minibuffer; if you call this command from Lisp code as a
function, you must supply the file name string as an ordinary Lisp
function argument.
If the command is a string or vector (i.e., a keyboard macro) then
execute-kbd-macro
is used to execute it. You can call this
function yourself (see section Keyboard Macros).
To terminate the execution of a running command, type C-g. This character causes quitting (see section Quitting).
this-command
contains the command that is about to
run, and last-command
describes the previous command.
See section Hooks.
this-command
describes the command that just ran, and
last-command
describes the command before that. See section Hooks.
Quitting is suppressed while running pre-command-hook
and
post-command-hook
. If an error happens while executing one of
these hooks, it terminates execution of the hook, and clears the hook
variable to nil
so as to prevent an infinite loop of errors.
A Lisp function becomes a command when its body contains, at top
level, a form that calls the special form interactive
. This
form does nothing when actually executed, but its presence serves as a
flag to indicate that interactive calling is permitted. Its argument
controls the reading of arguments for an interactive call.
interactive
This section describes how to write the interactive
form that
makes a Lisp function an interactively-callable command.
A command may be called from Lisp programs like any other function, but then the caller supplies the arguments and arg-descriptor has no effect.
The interactive
form has its effect because the command loop
(actually, its subroutine call-interactively
) scans through the
function definition looking for it, before calling the function. Once
the function is called, all its body forms including the
interactive
form are executed, but at this time
interactive
simply returns nil
without even evaluating its
argument.
There are three possibilities for the argument arg-descriptor:
nil
; then the command is called with no
arguments. This leads quickly to an error if the command requires one
or more arguments.
(interactive (list (region-beginning) (region-end) (read-string "Foo: " nil 'my-history)))Here's how to avoid the problem, by examining point and the mark only after reading the keyboard input:
(interactive (let ((string (read-string "Foo: " nil 'my-history))) (list (region-beginning) (region-end) string)))
(interactive "bFrobnicate buffer: ")The code letter `b' says to read the name of an existing buffer, with completion. The buffer name is the sole argument passed to the command. The rest of the string is a prompt. If there is a newline character in the string, it terminates the prompt. If the string does not end there, then the rest of the string should contain another code character and prompt, specifying another argument. You can specify any number of arguments in this way. The prompt string can use `%' to include previous argument values (starting with the first argument) in the prompt. This is done using
format
(see section Formatting Strings). For example, here is how
you could read the name of an existing buffer followed by a new name to
give to that buffer:
(interactive "bBuffer to rename: \nsRename buffer %s to: ")If the first character in the string is `*', then an error is signaled if the buffer is read-only. If the first character in the string is `@', and if the key sequence used to invoke the command includes any mouse events, then the window associated with the first of those events is selected before the command is run. You can use `*' and `@' together; the order does not matter. Actual reading of arguments is controlled by the rest of the prompt string (starting with the first character that is not `*' or `@').
interactive
The code character descriptions below contain a number of key words, defined here as follows:
completing-read
(see section Completion). ? displays a list of possible completions.
Here are the code character descriptions for use with interactive
:
fboundp
). Existing,
Completion, Prompt.
commandp
). Existing,
Completion, Prompt.
default-directory
(see section Operating System Environment).
Existing, Completion, Default, Prompt.
default-directory
. Existing, Completion, Default,
Prompt.
nil
as
the argument's value. No I/O.
describe-key
and
global-set-key
.
user-variable-p
). See section High-Level Completion Functions. Existing,
Completion, Prompt.
nil
. See section Coding Systems. Completion,
Existing, Prompt.
nil
as the
argument value. Completion, Existing, Prompt.
interactive
Here are some examples of interactive
:
(defun foo1 () ;foo1
takes no arguments, (interactive) ; just moves forward two words. (forward-word 2)) => foo1 (defun foo2 (n) ;foo2
takes one argument, (interactive "p") ; which is the numeric prefix. (forward-word (* 2 n))) => foo2 (defun foo3 (n) ;foo3
takes one argument, (interactive "nCount:") ; which is read with the Minibuffer. (forward-word (* 2 n))) => foo3 (defun three-b (b1 b2 b3) "Select three existing buffers. Put them into three windows, selecting the last one." (interactive "bBuffer1:\nbBuffer2:\nbBuffer3:") (delete-other-windows) (split-window (selected-window) 8) (switch-to-buffer b1) (other-window 1) (split-window (selected-window) 8) (switch-to-buffer b2) (other-window 1) (switch-to-buffer b3)) => three-b (three-b "*scratch*" "declarations.texi" "*mail*") => nil
After the command loop has translated a key sequence into a command it
invokes that command using the function command-execute
. If the
command is a function, command-execute
calls
call-interactively
, which reads the arguments and calls the
command. You can also call these functions yourself.
t
if object is suitable for calling interactively;
that is, if object is a command. Otherwise, returns nil
.
The interactively callable objects include strings and vectors (treated
as keyboard macros), lambda expressions that contain a top-level call to
interactive
, byte-code function objects made from such lambda
expressions, autoload objects that are declared as interactive
(non-nil
fourth argument to autoload
), and some of the
primitive functions.
A symbol satisfies commandp
if its function definition satisfies
commandp
.
Keys and keymaps are not commands. Rather, they are used to look up commands (see section Keymaps).
See documentation
in section Access to Documentation Strings, for a
realistic example of using commandp
.
If record-flag is non-nil
, then this command and its
arguments are unconditionally added to the list command-history
.
Otherwise, the command is added only if it uses the minibuffer to read
an argument. See section Command History.
The argument keys, if given, specifies the sequence of events to supply if the command inquires which events were used to invoke it.
commandp
predicate; i.e., it must be an interactively
callable function or a keyboard macro.
A string or vector as command is executed with
execute-kbd-macro
. A function is passed to
call-interactively
, along with the optional record-flag.
A symbol is handled by using its function definition in its place. A
symbol with an autoload
definition counts as a command if it was
declared to stand for an interactively callable function. Such a
definition is handled by loading the specified library and then
rechecking the definition of the symbol.
The argument keys, if given, specifies the sequence of events to supply if the command inquires which events were used to invoke it.
completing-read
(see section Completion). Then it uses
command-execute
to call the specified command. Whatever that
command returns becomes the value of execute-extended-command
.
If the command asks for a prefix argument, it receives the value
prefix-argument. If execute-extended-command
is called
interactively, the current raw prefix argument is used for
prefix-argument, and thus passed on to whatever command is run.
execute-extended-command
is the normal definition of M-x,
so it uses the string `M-x ' as a prompt. (It would be better
to take the prompt from the events used to invoke
execute-extended-command
, but that is painful to implement.) A
description of the value of the prefix argument, if any, also becomes
part of the prompt.
(execute-extended-command 1) ---------- Buffer: Minibuffer ---------- 1 M-x forward-word RET ---------- Buffer: Minibuffer ---------- => t
t
if the containing function (the one whose
code includes the call to interactive-p
) was called
interactively, with the function call-interactively
. (It makes
no difference whether call-interactively
was called from Lisp or
directly from the editor command loop.) If the containing function was
called by Lisp evaluation (or with apply
or funcall
), then
it was not called interactively.
The most common use of interactive-p
is for deciding whether to
print an informative message. As a special exception,
interactive-p
returns nil
whenever a keyboard macro is
being run. This is to suppress the informative messages and speed
execution of the macro.
For example:
(defun foo () (interactive) (when (interactive-p) (message "foo"))) => foo (defun bar () (interactive) (setq foobar (list (foo) (interactive-p)))) => bar ;; Type M-x foo. -| foo ;; Type M-x bar. ;; This does not print anything. foobar => (nil t)
The other way to do this sort of job is to make the command take an
argument print-message
which should be non-nil
in an
interactive call, and use the interactive
spec to make sure it is
non-nil
. Here's how:
(defun foo (&optional print-message) (interactive "p") (when print-message (message "foo")))
The numeric prefix argument, provided by `p', is never nil
.
The editor command loop sets several Lisp variables to keep status records for itself and for commands that are run.
The value is copied from this-command
when a command returns to
the command loop, except when the command has specified a prefix
argument for the following command.
This variable is always local to the current terminal and cannot be buffer-local. See section Multiple Displays.
last-command
,
but never altered by Lisp programs.
last-command
, it is normally a symbol
with a function definition.
The command loop sets this variable just before running a command, and
copies its value into last-command
when the command finishes
(unless the command specified a prefix argument for the following
command).
Some commands set this variable during their execution, as a flag for
whatever command runs next. In particular, the functions for killing text
set this-command
to kill-region
so that any kill commands
immediately following will know to append the killed text to the
previous kill.
If you do not want a particular command to be recognized as the previous
command in the case where it got an error, you must code that command to
prevent this. One way is to set this-command
to t
at the
beginning of the command, and set this-command
back to its proper
value at the end, like this:
(defun foo (args...) (interactive ...) (let ((old-this-command this-command)) (setq this-command t) ...do the work... (setq this-command old-this-command)))
We do not bind this-command
with let
because that would
restore the old value in case of error--a feature of let
which
in this case does precisely what we want to avoid.
(this-command-keys) ;; Now use C-u C-x C-e to evaluate that. => "^U^X^E"
this-command-keys
, except that it always returns
the events in a vector, so you do never need to deal with the complexities
of storing input events in a string (see section Putting Keyboard Events in Strings).
One use of this variable is for telling x-popup-menu
where to pop
up a menu. It is also used internally by y-or-n-p
(see section Yes-or-No Queries).
self-insert-command
, which uses it to decide which
character to insert.
last-command-event ;; Now use C-u C-x C-e to evaluate that. => 5
The value is 5 because that is the ASCII code for C-e.
The alias last-command-char
exists for compatibility with
Emacs version 18.
The Emacs command loop reads a sequence of input events that represent keyboard or mouse activity. The events for keyboard activity are characters or symbols; mouse events are always lists. This section describes the representation and meaning of input events in detail.
nil
if object is an input event
or event type.
Note that any symbol might be used as an event or an event type.
eventp
cannot distinguish whether a symbol is intended by Lisp
code to be used as an event. Instead, it distinguishes whether the
symbol has actually been used in an event that has been read as input in
the current Emacs session. If a symbol has not yet been so used,
eventp
returns nil
.
There are two kinds of input you can get from the keyboard: ordinary keys, and function keys. Ordinary keys correspond to characters; the events they generate are represented in Lisp as characters. The event type of a character event is the character itself (an integer); see section Classifying Events.
An input character event consists of a basic code between 0 and 524287, plus any or all of these modifier bits:
It is best to avoid mentioning specific bit numbers in your program.
To test the modifier bits of a character, use the function
event-modifiers
(see section Classifying Events). When making key
bindings, you can use the read syntax for characters with modifier bits
(`\C-', `\M-', and so on). For making key bindings with
define-key
, you can use lists such as (control hyper ?x)
to
specify the characters (see section Changing Key Bindings). The function
event-convert-list
converts such a list into an event type
(see section Classifying Events).
Most keyboards also have function keys---keys that have names or
symbols that are not characters. Function keys are represented in Emacs
Lisp as symbols; the symbol's name is the function key's label, in lower
case. For example, pressing a key labeled F1 places the symbol
f1
in the input stream.
The event type of a function key event is the event symbol itself. See section Classifying Events.
Here are a few special cases in the symbol-naming convention for function keys:
backspace
, tab
, newline
, return
, delete
tab
.
Most of the time, it's not useful to distinguish the two. So normally
function-key-map
(see section Translating Input Events) is set up to map
tab
into 9. Thus, a key binding for character code 9 (the
character C-i) also applies to tab
. Likewise for the other
symbols in this group. The function read-char
likewise converts
these events into characters.
In ASCII, BS is really C-h. But backspace
converts into the character code 127 (DEL), not into code 8
(BS). This is what most users prefer.
left
, up
, right
, down
kp-add
, kp-decimal
, kp-divide
, ...
kp-0
, kp-1
, ...
kp-f1
, kp-f2
, kp-f3
, kp-f4
kp-home
, kp-left
, kp-up
, kp-right
, kp-down
home
, left
, ...
kp-prior
, kp-next
, kp-end
, kp-begin
, kp-insert
, kp-delete
You can use the modifier keys ALT, CTRL, HYPER, META, SHIFT, and SUPER with function keys. The way to represent them is with prefixes in the symbol name:
Thus, the symbol for the key F3 with META held down is
M-f3
. When you use more than one prefix, we recommend you
write them in alphabetical order; but the order does not matter in
arguments to the key-binding lookup and modification functions.
Emacs supports four kinds of mouse events: click events, drag events, button-down events, and motion events. All mouse events are represented as lists. The CAR of the list is the event type; this says which mouse button was involved, and which modifier keys were used with it. The event type can also distinguish double or triple button presses (see section Repeat Events). The rest of the list elements give position and time information.
For key lookup, only the event type matters: two events of the same type
necessarily run the same command. The command can access the full
values of these events using the `e' interactive code.
See section Code Characters for interactive
.
A key sequence that starts with a mouse event is read using the keymaps of the buffer in the window that the mouse was in, not the current buffer. This does not imply that clicking in a window selects that window or its buffer--that is entirely under the control of the command binding of the key sequence.
When the user presses a mouse button and releases it at the same location, that generates a click event. Mouse click events have this form:
(event-type (window buffer-pos (x . y) timestamp) click-count)
Here is what the elements normally mean:
mouse-1
, mouse-2
, ..., where the
buttons are numbered left to right.
You can also use prefixes `A-', `C-', `H-', `M-',
`S-' and `s-' for modifiers alt, control, hyper, meta, shift
and super, just as you would with function keys.
This symbol also serves as the event type of the event. Key bindings
describe events by their types; thus, if there is a key binding for
mouse-1
, that binding would apply to all events whose
event-type is mouse-1
.
(0 . 0)
.
The meanings of buffer-pos, x and y are somewhat different when the event location is in a special part of the screen, such as the mode line or a scroll bar.
If the location is in a scroll bar, then buffer-pos is the symbol
vertical-scroll-bar
or horizontal-scroll-bar
, and the pair
(x . y)
is replaced with a pair (portion
. whole)
, where portion is the distance of the click from
the top or left end of the scroll bar, and whole is the length of
the entire scroll bar.
If the position is on a mode line or the vertical line separating
window from its neighbor to the right, then buffer-pos is
the symbol mode-line
or vertical-line
. For the mode line,
y does not have meaningful data. For the vertical line, x
does not have meaningful data.
In one special case, buffer-pos is a list containing a symbol (one of the symbols listed above) instead of just the symbol. This happens after the imaginary prefix keys for the event are inserted into the input stream. See section Key Sequence Input.
With Emacs, you can have a drag event without even changing your clothes. A drag event happens every time the user presses a mouse button and then moves the mouse to a different character position before releasing the button. Like all mouse events, drag events are represented in Lisp as lists. The lists record both the starting mouse position and the final position, like this:
(event-type (window1 buffer-pos1 (x1 . y1) timestamp1) (window2 buffer-pos2 (x2 . y2) timestamp2) click-count)
For a drag event, the name of the symbol event-type contains the
prefix `drag-'. For example, dragging the mouse with button 2 held
down generates a drag-mouse-2
event. The second and third
elements of the event give the starting and ending position of the drag.
Aside from that, the data have the same meanings as in a click event
(see section Click Events). You can access the second element of any mouse
event in the same way, with no need to distinguish drag events from
others.
The `drag-' prefix follows the modifier key prefixes such as `C-' and `M-'.
If read-key-sequence
receives a drag event that has no key
binding, and the corresponding click event does have a binding, it
changes the drag event into a click event at the drag's starting
position. This means that you don't have to distinguish between click
and drag events unless you want to.
Click and drag events happen when the user releases a mouse button. They cannot happen earlier, because there is no way to distinguish a click from a drag until the button is released.
If you want to take action as soon as a button is pressed, you need to handle button-down events.(3) These occur as soon as a button is pressed. They are represented by lists that look exactly like click events (see section Click Events), except that the event-type symbol name contains the prefix `down-'. The `down-' prefix follows modifier key prefixes such as `C-' and `M-'.
The function read-key-sequence
ignores any button-down events
that don't have command bindings; therefore, the Emacs command loop
ignores them too. This means that you need not worry about defining
button-down events unless you want them to do something. The usual
reason to define a button-down event is so that you can track mouse
motion (by reading motion events) until the button is released.
See section Motion Events.
If you press the same mouse button more than once in quick succession without moving the mouse, Emacs generates special repeat mouse events for the second and subsequent presses.
The most common repeat events are double-click events. Emacs generates a double-click event when you click a button twice; the event happens when you release the button (as is normal for all click events).
The event type of a double-click event contains the prefix
`double-'. Thus, a double click on the second mouse button with
meta held down comes to the Lisp program as
M-double-mouse-2
. If a double-click event has no binding, the
binding of the corresponding ordinary click event is used to execute
it. Thus, you need not pay attention to the double click feature
unless you really want to.
When the user performs a double click, Emacs generates first an ordinary click event, and then a double-click event. Therefore, you must design the command binding of the double click event to assume that the single-click command has already run. It must produce the desired results of a double click, starting from the results of a single click.
This is convenient, if the meaning of a double click somehow "builds on" the meaning of a single click--which is recommended user interface design practice for double clicks.
If you click a button, then press it down again and start moving the mouse with the button held down, then you get a double-drag event when you ultimately release the button. Its event type contains `double-drag' instead of just `drag'. If a double-drag event has no binding, Emacs looks for an alternate binding as if the event were an ordinary drag.
Before the double-click or double-drag event, Emacs generates a double-down event when the user presses the button down for the second time. Its event type contains `double-down' instead of just `down'. If a double-down event has no binding, Emacs looks for an alternate binding as if the event were an ordinary button-down event. If it finds no binding that way either, the double-down event is ignored.
To summarize, when you click a button and then press it again right away, Emacs generates a down event and a click event for the first click, a double-down event when you press the button again, and finally either a double-click or a double-drag event.
If you click a button twice and then press it again, all in quick succession, Emacs generates a triple-down event, followed by either a triple-click or a triple-drag. The event types of these events contain `triple' instead of `double'. If any triple event has no binding, Emacs uses the binding that it would use for the corresponding double event.
If you click a button three or more times and then press it again, the events for the presses beyond the third are all triple events. Emacs does not have separate event types for quadruple, quintuple, etc. events. However, you can look at the event list to find out precisely how many times the button was pressed.
double-click-time
. Setting double-click-time
to
nil
disables multi-click detection entirely. Setting it to
t
removes the time limit; Emacs then detects multi-clicks by
position only.
Emacs sometimes generates mouse motion events to describe motion of the mouse without any button activity. Mouse motion events are represented by lists that look like this:
(mouse-movement (window buffer-pos (x . y) timestamp))
The second element of the list describes the current position of the mouse, just as in a click event (see section Click Events).
The special form track-mouse
enables generation of motion events
within its body. Outside of track-mouse
forms, Emacs does not
generate events for mere motion of the mouse, and these events do not
appear. See section Mouse Tracking.
Window systems provide general ways for the user to control which window gets keyboard input. This choice of window is called the focus. When the user does something to switch between Emacs frames, that generates a focus event. The normal definition of a focus event, in the global keymap, is to select a new frame within Emacs, as the user would expect. See section Input Focus.
Focus events are represented in Lisp as lists that look like this:
(switch-frame new-frame)
where new-frame is the frame switched to.
Most X window managers are set up so that just moving the mouse into a window is enough to set the focus there. Emacs appears to do this, because it changes the cursor to solid in the new frame. However, there is no need for the Lisp program to know about the focus change until some other kind of input arrives. So Emacs generates a focus event only when the user actually types a keyboard key or presses a mouse button in the new frame; just moving the mouse between frames does not generate a focus event.
A focus event in the middle of a key sequence would garble the sequence. So Emacs never generates a focus event in the middle of a key sequence. If the user changes focus in the middle of a key sequence--that is, after a prefix key--then Emacs reorders the events so that the focus event comes either before or after the multi-event key sequence, and not within it.
A few other event types represent occurrences within the window system.
(delete-frame (frame))
delete-frame
event is to delete frame.
(iconify-frame (frame))
ignore
; since the
frame has already been iconified, Emacs has no work to do. The purpose
of this event type is so that you can keep track of such events if you
want to.
(make-frame-visible (frame))
ignore
; since the
frame has already been made visible, Emacs has no work to do.
(mouse-wheel position delta)
(drag-n-drop position files)
If one of these events arrives in the middle of a key sequence--that is, after a prefix key--then Emacs reorders the events so that this event comes either before or after the multi-event key sequence, not within it.
If the user presses and releases the left mouse button over the same location, that generates a sequence of events like this:
(down-mouse-1 (#<window 18 on NEWS> 2613 (0 . 38) -864320)) (mouse-1 (#<window 18 on NEWS> 2613 (0 . 38) -864180))
While holding the control key down, the user might hold down the second mouse button, and drag the mouse from one line to the next. That produces two events, as shown here:
(C-down-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219)) (C-drag-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219) (#<window 18 on NEWS> 3510 (0 . 28) -729648))
While holding down the meta and shift keys, the user might press the second mouse button on the window's mode line, and then drag the mouse into another window. That produces a pair of events like these:
(M-S-down-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844)) (M-S-drag-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844) (#<window 20 on carlton-sanskrit.tex> 161 (33 . 3) -453816))
Every event has an event type, which classifies the event for key binding purposes. For a keyboard event, the event type equals the event value; thus, the event type for a character is the character, and the event type for a function key symbol is the symbol itself. For events that are lists, the event type is the symbol in the CAR of the list. Thus, the event type is always a symbol or a character.
Two events of the same type are equivalent where key bindings are concerned; thus, they always run the same command. That does not necessarily mean they do the same things, however, as some commands look at the whole event to decide what to do. For example, some commands use the location of a mouse event to decide where in the buffer to act.
Sometimes broader classifications of events are useful. For example, you might want to ask whether an event involved the META key, regardless of which other key or mouse button was used.
The functions event-modifiers
and event-basic-type
are
provided to get such information conveniently.
shift
, control
,
meta
, alt
, hyper
and super
. In addition,
the modifiers list of a mouse event symbol always contains one of
click
, drag
, and down
.
The argument event may be an entire event object, or just an event type.
Here are some examples:
(event-modifiers ?a) => nil (event-modifiers ?\C-a) => (control) (event-modifiers ?\C-%) => (control) (event-modifiers ?\C-\S-a) => (control shift) (event-modifiers 'f5) => nil (event-modifiers 's-f5) => (super) (event-modifiers 'M-S-f5) => (meta shift) (event-modifiers 'mouse-1) => (click) (event-modifiers 'down-mouse-1) => (down)
The modifiers list for a click event explicitly contains click
,
but the event symbol name itself does not contain `click'.
(event-basic-type ?a) => 97 (event-basic-type ?A) => 97 (event-basic-type ?\C-a) => 97 (event-basic-type ?\C-\S-a) => 97 (event-basic-type 'f5) => f5 (event-basic-type 's-f5) => f5 (event-basic-type 'M-S-f5) => f5 (event-basic-type 'down-mouse-1) => mouse-1
nil
if object is a mouse movement
event.
(event-convert-list '(control ?a)) => 1 (event-convert-list '(control meta ?a)) => -134217727 (event-convert-list '(control super f1)) => C-s-f1
This section describes convenient functions for accessing the data in a mouse button or motion event.
These two functions return the starting or ending position of a mouse-button event, as a list of this form:
(window buffer-position (x . y) timestamp)
If event is a click or button-down event, this returns the location of the event. If event is a drag event, this returns the drag's starting position.
If event is a drag event, this returns the position where the user released the mouse button. If event is a click or button-down event, the value is actually the starting position, which is the only position such events have.
These five functions take a position list as described above, and return various parts of it.
(x . y)
.
(col . row)
. These are computed from the
x and y values actually found in position.
These functions are useful for decoding scroll bar events.
(portion . whole)
containing two integers whose ratio
is the fractional position.
(num . denom)
---typically a
value returned by scroll-bar-event-ratio
.
This function is handy for scaling a position on a scroll bar into a buffer position. Here's how to do that:
(+ (point-min) (scroll-bar-scale (posn-x-y (event-start event)) (- (point-max) (point-min))))
Recall that scroll bar events have two integers forming a ratio, in place of a pair of x and y coordinates.
In most of the places where strings are used, we conceptualize the string as containing text characters--the same kind of characters found in buffers or files. Occasionally Lisp programs use strings that conceptually contain keyboard characters; for example, they may be key sequences or keyboard macro definitions. However, storing keyboard characters in a string is a complex matter, for reasons of historical compatibility, and it is not always possible.
We recommend that new programs avoid dealing with these complexities by not storing keyboard events in strings. Here is how to do that:
lookup-key
and
define-key
. For example, you can use
read-key-sequence-vector
instead of read-key-sequence
, and
this-command-keys-vector
instead of this-command-keys
.
define-key
.
listify-key-sequence
(see section Miscellaneous Event Input Features)
first, to convert it to a list.
The complexities stem from the modifier bits that keyboard input characters can include. Aside from the Meta modifier, none of these modifier bits can be included in a string, and the Meta modifier is allowed only in special cases.
The earliest GNU Emacs versions represented meta characters as codes
in the range of 128 to 255. At that time, the basic character codes
ranged from 0 to 127, so all keyboard character codes did fit in a
string. Many Lisp programs used `\M-' in string constants to stand
for meta characters, especially in arguments to define-key
and
similar functions, and key sequences and sequences of events were always
represented as strings.
When we added support for larger basic character codes beyond 127, and additional modifier bits, we had to change the representation of meta characters. Now the flag that represents the Meta modifier in a character is and such numbers cannot be included in a string.
To support programs with `\M-' in string constants, there are special rules for including certain meta characters in a string. Here are the rules for interpreting a string as a sequence of input characters:
Functions such as read-key-sequence
that construct strings of
keyboard input characters follow these rules: they construct vectors
instead of strings, when the events won't fit in a string.
When you use the read syntax `\M-' in a string, it produces a code in the range of 128 to 255--the same code that you get if you modify the corresponding keyboard event to put it in the string. Thus, meta events in strings work consistently regardless of how they get into the strings.
However, most programs would do well to avoid these issues by following the recommendations at the beginning of this section.
The editor command loop reads key sequences using the function
read-key-sequence
, which uses read-event
. These and other
functions for event input are also available for use in Lisp programs.
See also momentary-string-display
in section Temporary Displays,
and sit-for
in section Waiting for Elapsed Time or Input. See section Terminal Input, for
functions and variables for controlling terminal input modes and
debugging terminal input. See section Translating Input Events, for features you
can use for translating or modifying input events while reading them.
For higher-level input facilities, see section Minibuffers.
The command loop reads input a key sequence at a time, by calling
read-key-sequence
. Lisp programs can also call this function;
for example, describe-key
uses it to read the key to describe.
If the events are all characters and all can fit in a string, then
read-key-sequence
returns a string (see section Putting Keyboard Events in Strings).
Otherwise, it returns a vector, since a vector can hold all kinds of
events--characters, symbols, and lists. The elements of the string or
vector are the events in the key sequence.
The argument prompt is either a string to be displayed in the echo
area as a prompt, or nil
, meaning not to display a prompt.
In the example below, the prompt `?' is displayed in the echo area, and the user types C-x C-f.
(read-key-sequence "?") ---------- Echo Area ---------- ?C-x C-f ---------- Echo Area ---------- => "^X^F"
The function read-key-sequence
suppresses quitting: C-g
typed while reading with this function works like any other character,
and does not set quit-flag
. See section Quitting.
read-key-sequence
except that it always
returns the key sequence as a vector, never as a string.
See section Putting Keyboard Events in Strings.
If an input character is an upper-case letter and has no key binding,
but its lower-case equivalent has one, then read-key-sequence
converts the character to lower case. Note that lookup-key
does
not perform case conversion in this way.
The function read-key-sequence
also transforms some mouse events.
It converts unbound drag events into click events, and discards unbound
button-down events entirely. It also reshuffles focus events and
miscellaneous window events so that they never appear in a key sequence
with any other events.
When mouse events occur in special parts of a window, such as a mode
line or a scroll bar, the event type shows nothing special--it is the
same symbol that would normally represent that combination of mouse
button and modifier keys. The information about the window part is kept
elsewhere in the event--in the coordinates. But
read-key-sequence
translates this information into imaginary
"prefix keys", all of which are symbols: mode-line
,
vertical-line
, horizontal-scroll-bar
and
vertical-scroll-bar
. You can define meanings for mouse clicks in
special window parts by defining key sequences using these imaginary
prefix keys.
For example, if you call read-key-sequence
and then click the
mouse on the window's mode line, you get two events, like this:
(read-key-sequence "Click on the mode line: ") => [mode-line (mouse-1 (#<window 6 on NEWS> mode-line (40 . 63) 5959987))]
The lowest level functions for command input are those that read a single event.
If prompt is non-nil
, it should be a string to display in
the echo area as a prompt. Otherwise, read-event
does not
display any message to indicate it is waiting for input; instead, it
prompts by echoing: it displays descriptions of the events that led to
or were read by the current command. See section The Echo Area.
If suppress-input-method is non-nil
, then the current input
method is disabled for reading this event. If you want to read an event
without input-method processing, always do it this way; don't try binding
input-method-function
(see below).
If cursor-in-echo-area
is non-nil
, then read-event
moves the cursor temporarily to the echo area, to the end of any message
displayed there. Otherwise read-event
does not move the cursor.
If read-event
gets an event that is defined as a help character, in
some cases read-event
processes the event directly without
returning. See section Help Functions. Certain other events, called
special events, are also processed directly within
read-event
(see section Special Events).
Here is what happens if you call read-event
and then press the
right-arrow function key:
(read-event) => right
In the first example, the user types the character 1 (ASCII
code 49). The second example shows a keyboard macro definition that
calls read-char
from the minibuffer using eval-expression
.
read-char
reads the keyboard macro's very next character, which
is 1. Then eval-expression
displays its return value in
the echo area.
(read-char) => 49 ;; We assume here you use M-: to evaluate this. (symbol-function 'foo) => "^[:(read-char)^M1" (execute-kbd-macro 'foo) -| 49 => nil
read-event
also invokes the current input method, if any. If
the value of input-method-function
is non-nil
, it should
be a function; when read-event
reads a printing character
(including SPC) with no modifier bits, it calls that function,
passing the event as an argument.
nil
, its value specifies the current input method
function.
Note: Don't bind this variable with let
. It is often
buffer-local, and if you bind it around reading input (which is exactly
when you would bind it), switching buffers asynchronously while
Emacs is waiting will cause the value to be restored in the wrong
buffer.
The input method function should return a list of events which should
be used as input. (If the list is nil
, that means there is no
input, so read-event
waits for another event.) These events are
processed before the events in unread-command-events
. Events
returned by the input method function are not passed to the input method
function again, even if they are printing characters with no modifier
bits.
If the input method function calls read-event
or
read-key-sequence
, it should bind input-method-function
to
nil
first, to prevent recursion.
The input method function is not called when reading the second and
subsequent event of a key sequence. Thus, these characters are not
subject to input method processing. It is usually a good idea for the
input method processing to test the values of
overriding-local-map
and overriding-terminal-local-map
; if
either of these variables is non-nil
, the input method should put
its argument into a list and return that list with no further
processing.
You can use the function read-quoted-char
to ask the user to
specify a character, and allow the user to specify a control or meta
character conveniently, either literally or as an octal character code.
The command quoted-insert
uses this function.
read-char
, except that if the first
character read is an octal digit (0-7), it reads any number of octal
digits (but stopping if a non-octal digit is found), and returns the
character represented by that numeric character code.
Quitting is suppressed when the first character is read, so that the user can enter a C-g. See section Quitting.
If prompt is supplied, it specifies a string for prompting the user. The prompt string is always displayed in the echo area, followed by a single `-'.
In the following example, the user types in the octal number 177 (which is 127 in decimal).
(read-quoted-char "What character") ---------- Echo Area ---------- What character-177 ---------- Echo Area ---------- => 127
This section describes how to "peek ahead" at events without using
them up, how to check for pending input, and how to discard pending
input. See also the function read-passwd
(see section Reading a Password).
The variable is needed because in some cases a function reads an event and then decides not to use it. Storing the event in this variable causes it to be processed normally, by the command loop or by the functions to read command input.
For example, the function that implements numeric prefix arguments reads any number of digits. When it finds a non-digit event, it must unread the event so that it can be read normally by the command loop. Likewise, incremental search uses this feature to unread events with no special meaning in a search, because these events should exit the search and then execute normally.
The reliable and easy way to extract events from a key sequence so as to
put them in unread-command-events
is to use
listify-key-sequence
(see section Putting Keyboard Events in Strings).
Normally you add events to the front of this list, so that the events most recently unread will be reread first.
unread-command-events
.
This variable is mostly obsolete now that you can use
unread-command-events
instead; it exists only to support programs
written for Emacs versions 18 and earlier.
t
if
there is available input, nil
otherwise. On rare occasions it
may return t
when no input is available.
In the example below, the Lisp program reads the character 1,
ASCII code 49. It becomes the value of last-input-event
,
while C-e (we assume C-x C-e command is used to evaluate
this expression) remains the value of last-command-event
.
(progn (print (read-char)) (print last-command-event) last-input-event) -| 49 -| 5 => 49
The alias last-input-char
exists for compatibility with
Emacs version 18.
nil
.
In the following example, the user may type a number of characters right
after starting the evaluation of the form. After the sleep-for
finishes sleeping, discard-input
discards any characters typed
during the sleep.
(progn (sleep-for 2) (discard-input)) => nil
Special events are handled at a very low level--as soon as they are
read. The read-event
function processes these events itself, and
never returns them.
Events that are handled in this way do not echo, they are never grouped
into key sequences, and they never appear in the value of
last-command-event
or (this-command-keys)
. They do not
discard a numeric argument, they cannot be unread with
unread-command-events
, they may not appear in a keyboard macro,
and they are not recorded in a keyboard macro while you are defining
one.
These events do, however, appear in last-input-event
immediately
after they are read, and this is the way for the event's definition to
find the actual event.
The events types iconify-frame
, make-frame-visible
and
delete-frame
are normally handled in this way. The keymap which
defines how to handle special events--and which events are special--is
in the variable special-event-map
(see section Active Keymaps).
The wait functions are designed to wait for a certain amount of time
to pass or until there is input. For example, you may wish to pause in
the middle of a computation to allow the user time to view the display.
sit-for
pauses and updates the screen, and returns immediately if
input comes in, while sleep-for
pauses without updating the
screen.
t
if sit-for
waited the full
time with no input arriving (see input-pending-p
in section Miscellaneous Event Input Features). Otherwise, the value is nil
.
The argument seconds need not be an integer. If it is a floating
point number, sit-for
waits for a fractional number of seconds.
Some systems support only a whole number of seconds; on these systems,
seconds is rounded down.
The optional argument millisec specifies an additional waiting period measured in milliseconds. This adds to the period specified by seconds. If the system doesn't support waiting fractions of a second, you get an error if you specify nonzero millisec.
Redisplay is always preempted if input arrives, and does not happen at
all if input is available before it starts. Thus, there is no way to
force screen updating if there is pending input; however, if there is no
input pending, you can force an update with no delay by using
(sit-for 0)
.
If nodisp is non-nil
, then sit-for
does not
redisplay, but it still returns as soon as input is available (or when
the timeout elapses).
Iconifying or deiconifying a frame makes sit-for
return, because
that generates an event. See section Miscellaneous Window System Events.
The usual purpose of sit-for
is to give the user time to read
text that you display.
nil
.
The argument seconds need not be an integer. If it is a floating
point number, sleep-for
waits for a fractional number of seconds.
Some systems support only a whole number of seconds; on these systems,
seconds is rounded down.
The optional argument millisec specifies an additional waiting period measured in milliseconds. This adds to the period specified by seconds. If the system doesn't support waiting fractions of a second, you get an error if you specify nonzero millisec.
Use sleep-for
when you wish to guarantee a delay.
See section Time of Day, for functions to get the current time.
Typing C-g while a Lisp function is running causes Emacs to quit whatever it is doing. This means that control returns to the innermost active command loop.
Typing C-g while the command loop is waiting for keyboard input
does not cause a quit; it acts as an ordinary input character. In the
simplest case, you cannot tell the difference, because C-g
normally runs the command keyboard-quit
, whose effect is to quit.
However, when C-g follows a prefix key, they combine to form an
undefined key. The effect is to cancel the prefix key as well as any
prefix argument.
In the minibuffer, C-g has a different definition: it aborts out of the minibuffer. This means, in effect, that it exits the minibuffer and then quits. (Simply quitting would return to the command loop within the minibuffer.) The reason why C-g does not quit directly when the command reader is reading input is so that its meaning can be redefined in the minibuffer in this way. C-g following a prefix key is not redefined in the minibuffer, and it has its normal effect of canceling the prefix key and prefix argument. This too would not be possible if C-g always quit directly.
When C-g does directly quit, it does so by setting the variable
quit-flag
to t
. Emacs checks this variable at appropriate
times and quits if it is not nil
. Setting quit-flag
non-nil
in any way thus causes a quit.
At the level of C code, quitting cannot happen just anywhere; only at the
special places that check quit-flag
. The reason for this is
that quitting at other places might leave an inconsistency in Emacs's
internal state. Because quitting is delayed until a safe place, quitting
cannot make Emacs crash.
Certain functions such as read-key-sequence
or
read-quoted-char
prevent quitting entirely even though they wait
for input. Instead of quitting, C-g serves as the requested
input. In the case of read-key-sequence
, this serves to bring
about the special behavior of C-g in the command loop. In the
case of read-quoted-char
, this is so that C-q can be used
to quote a C-g.
You can prevent quitting for a portion of a Lisp function by binding
the variable inhibit-quit
to a non-nil
value. Then,
although C-g still sets quit-flag
to t
as usual, the
usual result of this--a quit--is prevented. Eventually,
inhibit-quit
will become nil
again, such as when its
binding is unwound at the end of a let
form. At that time, if
quit-flag
is still non-nil
, the requested quit happens
immediately. This behavior is ideal when you wish to make sure that
quitting does not happen within a "critical section" of the program.
In some functions (such as read-quoted-char
), C-g is
handled in a special way that does not involve quitting. This is done
by reading the input with inhibit-quit
bound to t
, and
setting quit-flag
to nil
before inhibit-quit
becomes nil
again. This excerpt from the definition of
read-quoted-char
shows how this is done; it also shows that
normal quitting is permitted after the first character of input.
(defun read-quoted-char (&optional prompt)
"...documentation..."
(let ((message-log-max nil) done (first t) (code 0) char)
(while (not done)
(let ((inhibit-quit first)
...)
(and prompt (message "%s-" prompt))
(setq char (read-event))
(if inhibit-quit (setq quit-flag nil)))
...set the variable code
...)
code))
nil
, then Emacs quits immediately, unless
inhibit-quit
is non-nil
. Typing C-g ordinarily sets
quit-flag
non-nil
, regardless of inhibit-quit
.
quit-flag
is set to a value other than nil
. If inhibit-quit
is
non-nil
, then quit-flag
has no special effect.
quit
condition with (signal 'quit
nil)
. This is the same thing that quitting does. (See signal
in section Errors.)
You can specify a character other than C-g to use for quitting.
See the function set-input-mode
in section Terminal Input.
Most Emacs commands can use a prefix argument, a number
specified before the command itself. (Don't confuse prefix arguments
with prefix keys.) The prefix argument is at all times represented by a
value, which may be nil
, meaning there is currently no prefix
argument. Each command may use the prefix argument or ignore it.
There are two representations of the prefix argument: raw and numeric. The editor command loop uses the raw representation internally, and so do the Lisp variables that store the information, but commands can request either representation.
Here are the possible values of a raw prefix argument:
nil
, meaning there is no prefix argument. Its numeric value is
1, but numerous commands make a distinction between nil
and the
integer 1.
-
. This indicates that M-- or C-u - was
typed, without following digits. The equivalent numeric value is
-1, but some commands make a distinction between the integer
-1 and the symbol -
.
We illustrate these possibilities by calling the following function with various prefixes:
(defun display-prefix (arg) "Display the value of the raw prefix arg." (interactive "P") (message "%s" arg))
Here are the results of calling display-prefix
with various
raw prefix arguments:
M-x display-prefix -| nil C-u M-x display-prefix -| (4) C-u C-u M-x display-prefix -| (16) C-u 3 M-x display-prefix -| 3 M-3 M-x display-prefix -| 3 ; (Same asC-u 3
.) C-u - M-x display-prefix -| - M-- M-x display-prefix -| - ; (Same asC-u -
.) C-u - 7 M-x display-prefix -| -7 M-- 7 M-x display-prefix -| -7 ; (Same asC-u -7
.)
Emacs uses two variables to store the prefix argument:
prefix-arg
and current-prefix-arg
. Commands such as
universal-argument
that set up prefix arguments for other
commands store them in prefix-arg
. In contrast,
current-prefix-arg
conveys the prefix argument to the current
command, so setting it has no effect on the prefix arguments for future
commands.
Normally, commands specify which representation to use for the prefix
argument, either numeric or raw, in the interactive
declaration.
(See section Using interactive
.) Alternatively, functions may look at the
value of the prefix argument directly in the variable
current-prefix-arg
, but this is less clean.
nil
, the value 1 is returned; if it is -
, the
value -1 is returned; if it is a number, that number is returned;
if it is a list, the CAR of that list (which should be a number) is
returned.
(interactive "P")
.
universal-argument
that specify prefix arguments for the following command work by setting
this variable.
The following commands exist to set up prefix arguments for the following command. Do not call them for any other reason.
The Emacs command loop is entered automatically when Emacs starts up. This top-level invocation of the command loop never exits; it keeps running as long as Emacs does. Lisp programs can also invoke the command loop. Since this makes more than one activation of the command loop, we call it recursive editing. A recursive editing level has the effect of suspending whatever command invoked it and permitting the user to do arbitrary editing before resuming that command.
The commands available during recursive editing are the same ones available in the top-level editing loop and defined in the keymaps. Only a few special commands exit the recursive editing level; the others return to the recursive editing level when they finish. (The special commands for exiting are always available, but they do nothing when recursive editing is not in progress.)
All command loops, including recursive ones, set up all-purpose error handlers so that an error in a command run from the command loop will not exit the loop.
Minibuffer input is a special kind of recursive editing. It has a few special wrinkles, such as enabling display of the minibuffer and the minibuffer window, but fewer than you might suppose. Certain keys behave differently in the minibuffer, but that is only because of the minibuffer's local map; if you switch windows, you get the usual Emacs commands.
To invoke a recursive editing level, call the function
recursive-edit
. This function contains the command loop; it also
contains a call to catch
with tag exit
, which makes it
possible to exit the recursive editing level by throwing to exit
(see section Explicit Nonlocal Exits: catch
and throw
). If you throw a value other than t
,
then recursive-edit
returns normally to the function that called
it. The command C-M-c (exit-recursive-edit
) does this.
Throwing a t
value causes recursive-edit
to quit, so that
control returns to the command loop one level up. This is called
aborting, and is done by C-] (abort-recursive-edit
).
Most applications should not use recursive editing, except as part of using the minibuffer. Usually it is more convenient for the user if you change the major mode of the current buffer temporarily to a special major mode, which should have a command to go back to the previous mode. (The e command in Rmail uses this technique.) Or, if you wish to give the user different text to edit "recursively", create and select a new buffer in a special mode. In this mode, define a command to complete the processing and go back to the previous buffer. (The m command in Rmail does this.)
Recursive edits are useful in debugging. You can insert a call to
debug
into a function definition as a sort of breakpoint, so that
you can look around when the function gets there. debug
invokes
a recursive edit but also provides the other features of the debugger.
Recursive editing levels are also used when you type C-r in
query-replace
or use C-x q (kbd-macro-query
).
In the following example, the function simple-rec
first
advances point one word, then enters a recursive edit, printing out a
message in the echo area. The user can then do any editing desired, and
then type C-M-c to exit and continue executing simple-rec
.
(defun simple-rec () (forward-word 1) (message "Recursive edit in progress") (recursive-edit) (forward-word 1)) => simple-rec (simple-rec) => nil
(throw 'exit
nil)
.
quit
after exiting the recursive edit. Its definition is effectively
(throw 'exit t)
. See section Quitting.
Disabling a command marks the command as requiring user confirmation before it can be executed. Disabling is used for commands which might be confusing to beginning users, to prevent them from using the commands by accident.
The low-level mechanism for disabling a command is to put a
non-nil
disabled
property on the Lisp symbol for the
command. These properties are normally set up by the user's
`.emacs' file with Lisp expressions such as this:
(put 'upcase-region 'disabled t)
For a few commands, these properties are present by default and may be removed by the `.emacs' file.
If the value of the disabled
property is a string, the message
saying the command is disabled includes that string. For example:
(put 'delete-region 'disabled "Text deleted this way cannot be yanked back!\n")
See section `Disabling' in The GNU Emacs Manual, for the details on what happens when a disabled command is invoked interactively. Disabling a command has no effect on calling it as a function from Lisp programs.
this-command-keys
to determine what the user typed to run the
command, and thus find the command itself. See section Hooks.
By default, disabled-command-hook
contains a function that asks
the user whether to proceed.
The command loop keeps a history of the complex commands that have
been executed, to make it convenient to repeat these commands. A
complex command is one for which the interactive argument reading
uses the minibuffer. This includes any M-x command, any
M-: command, and any command whose interactive
specification reads an argument from the minibuffer. Explicit use of
the minibuffer during the execution of the command itself does not cause
the command to be considered complex.
history-length
), the oldest elements are deleted as new ones are
added.
command-history => ((switch-to-buffer "chistory.texi") (describe-key "^X^[") (visit-tags-table "~/emacs/src/") (find-tag "repeat-complex-command"))
This history list is actually a special case of minibuffer history (see section Minibuffer History), with one special twist: the elements are expressions rather than strings.
There are a number of commands devoted to the editing and recall of
previous commands. The commands repeat-complex-command
, and
list-command-history
are described in the user manual
(see section `Repetition' in The GNU Emacs Manual). Within the
minibuffer, the usual minibuffer history commands are available.
A keyboard macro is a canned sequence of input events that can be considered a command and made the definition of a key. The Lisp representation of a keyboard macro is a string or vector containing the events. Don't confuse keyboard macros with Lisp macros (see section Macros).
If kbdmacro is a symbol, then its function definition is used in place of kbdmacro. If that is another symbol, this process repeats. Eventually the result should be a string or vector. If the result is not a symbol, string, or vector, an error is signaled.
The argument count is a repeat count; kbdmacro is executed that
many times. If count is omitted or nil
, kbdmacro is
executed once. If it is 0, kbdmacro is executed over and over until it
encounters an error or a failing search.
See section Reading One Event, for an example of using execute-kbd-macro
.
nil
if no macro is
currently executing. A command can test this variable so as to behave
differently when run from an executing macro. Do not set this variable
yourself.
start-kbd-macro
and
end-kbd-macro
set this variable--do not set it yourself.
The variable is always local to the current terminal and cannot be buffer-local. See section Multiple Displays.
nil
.
The variable is always local to the current terminal and cannot be buffer-local. See section Multiple Displays.
The bindings between input events and commands are recorded in data structures called keymaps. Each binding in a keymap associates (or binds) an individual event type either to another keymap or to a command. When an event type is bound to a keymap, that keymap is used to look up the next input event; this continues until a command is found. The whole process is called key lookup.
A keymap is a table mapping event types to definitions (which can be any Lisp objects, though only certain types are meaningful for execution by the command loop). Given an event (or an event type) and a keymap, Emacs can get the event's definition. Events include characters, function keys, and mouse actions (see section Input Events).
A sequence of input events that form a unit is called a key sequence, or key for short. A sequence of one event is always a key sequence, and so are some multi-event sequences.
A keymap determines a binding or definition for any key sequence. If the key sequence is a single event, its binding is the definition of the event in the keymap. The binding of a key sequence of more than one event is found by an iterative process: the binding of the first event is found, and must be a keymap; then the second event's binding is found in that keymap, and so on until all the events in the key sequence are used up.
If the binding of a key sequence is a keymap, we call the key sequence
a prefix key. Otherwise, we call it a complete key (because
no more events can be added to it). If the binding is nil
,
we call the key undefined. Examples of prefix keys are C-c,
C-x, and C-x 4. Examples of defined complete keys are
X, RET, and C-x 4 C-f. Examples of undefined complete
keys are C-x C-g, and C-c 3. See section Prefix Keys, for more
details.
The rule for finding the binding of a key sequence assumes that the intermediate bindings (found for the events before the last) are all keymaps; if this is not so, the sequence of events does not form a unit--it is not really one key sequence. In other words, removing one or more events from the end of any valid key sequence must always yield a prefix key. For example, C-f C-n is not a key sequence; C-f is not a prefix key, so a longer sequence starting with C-f cannot be a key sequence.
The set of possible multi-event key sequences depends on the bindings for prefix keys; therefore, it can be different for different keymaps, and can change when bindings are changed. However, a one-event sequence is always a key sequence, because it does not depend on any prefix keys for its well-formedness.
At any time, several primary keymaps are active---that is, in use for finding key bindings. These are the global map, which is shared by all buffers; the local keymap, which is usually associated with a specific major mode; and zero or more minor mode keymaps, which belong to currently enabled minor modes. (Not all minor modes have keymaps.) The local keymap bindings shadow (i.e., take precedence over) the corresponding global bindings. The minor mode keymaps shadow both local and global keymaps. See section Active Keymaps, for details.
A keymap is a list whose CAR is the symbol keymap
. The
remaining elements of the list define the key bindings of the keymap.
Use the function keymapp
(see below) to test whether an object is
a keymap.
Several kinds of elements may appear in a keymap, after the symbol
keymap
that begins it:
(type . binding)
(t . binding)
vector
nil
for that
character. Such a binding of nil
overrides any default key
binding in the keymap, for ASCII characters. However, default
bindings are still meaningful for events other than ASCII
characters. A binding of nil
does not override
lower-precedence keymaps; thus, if the local map gives a binding of
nil
, Emacs uses the binding from the global map.
string
Keymaps do not directly record bindings for the meta characters.
Instead, meta characters are regarded for
purposes of key lookup as sequences of two characters, the first of
which is ESC (or whatever is currently the value of
meta-prefix-char
). Thus, the key M-a is really represented
as ESC a, and its global binding is found at the slot for
a in esc-map
(see section Prefix Keys).
Here as an example is the local keymap for Lisp mode, a sparse keymap. It defines bindings for DEL and TAB, plus C-c C-l, M-C-q, and M-C-x.
lisp-mode-map => (keymap ;; TAB (9 . lisp-indent-line) ;; DEL (127 . backward-delete-char-untabify) (3 keymap ;; C-c C-l (12 . run-lisp)) (27 keymap ;; M-C-q, treated as ESC C-q (17 . indent-sexp) ;; M-C-x, treated as ESC C-x (24 . lisp-send-defun)))
t
if object is a keymap, nil
otherwise. More precisely, this function tests for a list whose
CAR is keymap
.
(keymapp '(keymap)) => t (keymapp (current-global-map)) => t
Here we describe the functions for creating keymaps.
nil
, and does not bind any other kind of event.
(make-keymap) => (keymap [nil nil nil ... nil nil])
If you specify prompt, that becomes the overall prompt string for the keymap. The prompt string is useful for menu keymaps (see section Menu Keymaps).
make-keymap
.
(make-sparse-keymap) => (keymap)
(setq map (copy-keymap (current-local-map))) => (keymap ;; (This implements meta characters.) (27 keymap (83 . center-paragraph) (115 . center-line)) (9 . tab-to-tab-stop)) (eq map (current-local-map)) => nil (equal map (current-local-map)) => t
A keymap can inherit the bindings of another keymap, which we call the parent keymap. Such a keymap looks like this:
(keymap bindings... . parent-keymap)
The effect is that this keymap inherits all the bindings of parent-keymap, whatever they may be at the time a key is looked up, but can add to them or override them with bindings.
If you change the bindings in parent-keymap using define-key
or other key-binding functions, these changes are visible in the
inheriting keymap unless shadowed by bindings. The converse is
not true: if you use define-key
to change the inheriting keymap,
that affects bindings, but has no effect on parent-keymap.
The proper way to construct a keymap with a parent is to use
set-keymap-parent
; if you have code that directly constructs a
keymap with a parent, please convert the program to use
set-keymap-parent
instead.
keymap-parent
returns nil
.
nil
, this function gives
keymap no parent at all.
If keymap has submaps (bindings for prefix keys), they too receive new parent keymaps that reflect what parent specifies for those prefix keys.
Here is an example showing how to make a keymap that inherits
from text-mode-map
:
(let ((map (make-sparse-keymap))) (set-keymap-parent map text-mode-map) map)
A prefix key is a key sequence whose binding is a keymap. The
keymap defines what to do with key sequences that extend the prefix key.
For example, C-x is a prefix key, and it uses a keymap that is
also stored in the variable ctl-x-map
. This keymap defines
bindings for key sequences starting with C-x.
Some of the standard Emacs prefix keys use keymaps that are also found in Lisp variables:
esc-map
is the global keymap for the ESC prefix key. Thus,
the global definitions of all meta characters are actually found here.
This map is also the function definition of ESC-prefix
.
help-map
is the global keymap for the C-h prefix key.
mode-specific-map
is the global keymap for the prefix key
C-c. This map is actually global, not mode-specific, but its name
provides useful information about C-c in the output of C-h b
(display-bindings
), since the main use of this prefix key is for
mode-specific bindings.
ctl-x-map
is the global keymap used for the C-x prefix key.
This map is found via the function cell of the symbol
Control-X-prefix
.
mule-keymap
is the global keymap used for the C-x RET
prefix key.
ctl-x-4-map
is the global keymap used for the C-x 4 prefix
key.
ctl-x-5-map
is the global keymap used for the C-x 5 prefix
key.
2C-mode-map
is the global keymap used for the C-x 6 prefix
key.
vc-prefix-map
is the global keymap used for the C-x v prefix
key.
facemenu-keymap
is the global keymap used for the M-g
prefix key.
The keymap binding of a prefix key is used for looking up the event
that follows the prefix key. (It may instead be a symbol whose function
definition is a keymap. The effect is the same, but the symbol serves
as a name for the prefix key.) Thus, the binding of C-x is the
symbol Control-X-prefix
, whose function cell holds the keymap
for C-x commands. (The same keymap is also the value of
ctl-x-map
.)
Prefix key definitions can appear in any active keymap. The definitions of C-c, C-x, C-h and ESC as prefix keys appear in the global map, so these prefix keys are always available. Major and minor modes can redefine a key as a prefix by putting a prefix key definition for it in the local map or the minor mode's map. See section Active Keymaps.
If a key is defined as a prefix in more than one active map, then its various definitions are in effect merged: the commands defined in the minor mode keymaps come first, followed by those in the local map's prefix definition, and then by those from the global map.
In the following example, we make C-p a prefix key in the local
keymap, in such a way that C-p is identical to C-x. Then
the binding for C-p C-f is the function find-file
, just
like C-x C-f. The key sequence C-p 6 is not found in any
active keymap.
(use-local-map (make-sparse-keymap)) => nil (local-set-key "\C-p" ctl-x-map) => nil (key-binding "\C-p\C-f") => find-file (key-binding "\C-p6") => nil
This function also sets symbol as a variable, with the keymap as its value. It returns symbol.
Emacs normally contains many keymaps; at any given time, just a few of them are active in that they participate in the interpretation of user input. These are the global keymap, the current buffer's local keymap, and the keymaps of any enabled minor modes.
The global keymap holds the bindings of keys that are defined
regardless of the current buffer, such as C-f. The variable
global-map
holds this keymap, which is always active.
Each buffer may have another keymap, its local keymap, which may
contain new or overriding definitions for keys. The current buffer's
local keymap is always active except when overriding-local-map
overrides it. Text properties can specify an alternative local map for
certain parts of the buffer; see section Properties with Special Meanings.
Each minor mode can have a keymap; if it does, the keymap is active when the minor mode is enabled.
The variable overriding-local-map
, if non-nil
, specifies
another local keymap that overrides the buffer's local map and all the
minor mode keymaps.
All the active keymaps are used together to determine what command to execute when a key is entered. Emacs searches these maps one by one, in order of decreasing precedence, until it finds a binding in one of the maps. The procedure for searching a single keymap is called key lookup; see section Key Lookup.
Normally, Emacs first searches for the key in the minor mode maps, in
the order specified by minor-mode-map-alist
; if they do not
supply a binding for the key, Emacs searches the local map; if that too
has no binding, Emacs then searches the global map. However, if
overriding-local-map
is non-nil
, Emacs searches that map
first, before the global map.
Since every buffer that uses the same major mode normally uses the
same local keymap, you can think of the keymap as local to the mode. A
change to the local keymap of a buffer (using local-set-key
, for
example) is seen also in the other buffers that share that keymap.
The local keymaps that are used for Lisp mode and some other major
modes exist even if they have not yet been used. These local maps are
the values of variables such as lisp-mode-map
. For most major
modes, which are less frequently used, the local keymap is constructed
only when the mode is used for the first time in a session.
The minibuffer has local keymaps, too; they contain various completion and exit commands. See section Introduction to Minibuffers.
Emacs has other keymaps that are used in a different way--translating
events within read-key-sequence
. See section Translating Input Events.
See section Standard Keymaps, for a list of standard keymaps.
self-insert-command
to all of the printing characters.
It is normal practice to change the bindings in the global map, but you should not assign this variable any value other than the keymap it starts out with.
global-map
unless you change one or the
other.
(current-global-map) => (keymap [set-mark-command beginning-of-line ... delete-backward-char])
nil
if it has none. In the following example, the keymap for the
`*scratch*' buffer (using Lisp Interaction mode) is a sparse keymap
in which the entry for ESC, ASCII code 27, is another sparse
keymap.
(current-local-map) => (keymap (10 . eval-print-last-sexp) (9 . lisp-indent-line) (127 . backward-delete-char-untabify) (27 keymap (24 . eval-defun) (17 . indent-sexp)))
nil
.
It is very unusual to change the global keymap.
nil
, then the buffer has no local
keymap. use-local-map
returns nil
. Most major mode
commands use this function.
(variable . keymap)
The keymap keymap is active whenever variable has a
non-nil
value. Typically variable is the variable that
enables or disables a minor mode. See section Keymaps and Minor Modes.
Note that elements of minor-mode-map-alist
do not have the same
structure as elements of minor-mode-alist
. The map must be the
CDR of the element; a list with the map as the CADR will not
do. The CADR can be either a keymap (a list) or a symbol
whose function definition is a keymap.
When more than one minor mode keymap is active, their order of priority
is the order of minor-mode-map-alist
. But you should design
minor modes so that they don't interfere with each other. If you do
this properly, the order will not matter.
See section Keymaps and Minor Modes, for more information about minor
modes. See also minor-mode-key-binding
(see section Functions for Key Lookup).
minor-mode-map-alist
: (variable
. keymap)
.
If a variable appears as an element of
minor-mode-overriding-map-alist
, the map specified by that
element totally replaces any map specified for the same variable in
minor-mode-map-alist
.
minor-mode-overriding-map-alist
is automatically buffer-local in
all buffers.
nil
, this variable holds a keymap to use instead of the
buffer's local keymap and instead of all the minor mode keymaps. This
keymap, if any, overrides all other maps that would have been active,
except for the current global map.
nil
, this variable holds a keymap to use instead of
overriding-local-map
, the buffer's local keymap and all the minor
mode keymaps.
This variable is always local to the current terminal and cannot be buffer-local. See section Multiple Displays. It is used to implement incremental search mode.
nil
, the value of
overriding-local-map
or overriding-terminal-local-map
can
affect the display of the menu bar. The default value is nil
, so
those map variables have no effect on the menu bar.
Note that these two map variables do affect the execution of key sequences entered using the menu bar, even if they do not affect the menu bar display. So if a menu bar key sequence comes in, you should clear the variables before looking up and executing that key sequence. Modes that use the variables would typically do this anyway; normally they respond to events that they do not handle by "unreading" them and exiting.
read-event
. See section Special Events.
Key lookup is the process of finding the binding of a key sequence from a given keymap. Actual execution of the binding is not part of key lookup.
Key lookup uses just the event type of each event in the key sequence;
the rest of the event is ignored. In fact, a key sequence used for key
lookup may designate mouse events with just their types (symbols)
instead of with entire mouse events (lists). See section Input Events. Such
a "key-sequence" is insufficient for command-execute
to run,
but it is sufficient for looking up or rebinding a key.
When the key sequence consists of multiple events, key lookup processes the events sequentially: the binding of the first event is found, and must be a keymap; then the second event's binding is found in that keymap, and so on until all the events in the key sequence are used up. (The binding thus found for the last event may or may not be a keymap.) Thus, the process of key lookup is defined in terms of a simpler process for looking up a single event in a keymap. How that is done depends on the type of object associated with the event in that keymap.
Let's use the term keymap entry to describe the value found by
looking up an event type in a keymap. (This doesn't include the item
string and other extra elements in menu key bindings, because
lookup-key
and other key lookup functions don't include them in
the returned value.) While any Lisp object may be stored in a keymap as
a keymap entry, not all make sense for key lookup. Here is a table of
the meaningful kinds of keymap entries:
nil
nil
means that the events used so far in the lookup form an
undefined key. When a keymap fails to mention an event type at all, and
has no default binding, that is equivalent to a binding of nil
for that event type.
keymap
, then the list
is a keymap, and is treated as a keymap (see above).
lambda
, then the list is a
lambda expression. This is presumed to be a command, and is treated as
such (see above).
(othermap . othertype)When key lookup encounters an indirect entry, it looks up instead the binding of othertype in othermap and uses that. This feature permits you to define one key as an alias for another key. For example, an entry whose CAR is the keymap called
esc-map
and whose CDR is 32 (the code for SPC) means, "Use the global
binding of Meta-SPC, whatever that may be."
command-execute
(see section Interactive Call).
The symbol undefined
is worth special mention: it means to treat
the key as undefined. Strictly speaking, the key is defined, and its
binding is the command undefined
; but that command does the same
thing that is done automatically for an undefined key: it rings the bell
(by calling ding
) but does not signal an error.
undefined
is used in local keymaps to override a global key
binding and make the key "undefined" locally. A local binding of
nil
would fail to do this because it would not override the
global binding.
In short, a keymap entry may be a keymap, a command, a keyboard macro,
a symbol that leads to one of them, or an indirection or nil
.
Here is an example of a sparse keymap with two characters bound to
commands and one bound to another keymap. This map is the normal value
of emacs-lisp-mode-map
. Note that 9 is the code for TAB,
127 for DEL, 27 for ESC, 17 for C-q and 24 for
C-x.
(keymap (9 . lisp-indent-line) (127 . backward-delete-char-untabify) (27 keymap (17 . indent-sexp) (24 . eval-defun)))
Here are the functions and variables pertaining to key lookup.
lookup-key
. Here are examples:
(lookup-key (current-global-map) "\C-x\C-f") => find-file (lookup-key (current-global-map) "\C-x\C-f12345") => 2
If the string or vector key is not a valid key sequence according to the prefix keys specified in keymap, it must be "too long" and have extra events at the end that do not fit into a single key sequence. Then the value is a number, the number of events at the front of key that compose a complete key.
If accept-defaults is non-nil
, then lookup-key
considers default bindings as well as bindings for the specific events
in key. Otherwise, lookup-key
reports only bindings for
the specific sequence key, ignoring default bindings except when
you explicitly ask about them. (To do this, supply t
as an
element of key; see section Format of Keymaps.)
If key contains a meta character, that character is implicitly
replaced by a two-character sequence: the value of
meta-prefix-char
, followed by the corresponding non-meta
character. Thus, the first example below is handled by conversion into
the second example.
(lookup-key (current-global-map) "\M-f") => forward-word (lookup-key (current-global-map) "\ef") => forward-word
Unlike read-key-sequence
, this function does not modify the
specified events in ways that discard information (see section Key Sequence Input). In particular, it does not convert letters to lower case and
it does not change drag events to clicks.
ding
, but does
not cause an error.
nil
if
key is undefined in the keymaps.
The argument accept-defaults controls checking for default
bindings, as in lookup-key
(above).
An error is signaled if key is not a string or a vector.
(key-binding "\C-x\C-f") => find-file
nil
if it is undefined there.
The argument accept-defaults controls checking for default bindings,
as in lookup-key
(above).
nil
if it is undefined there.
The argument accept-defaults controls checking for default bindings,
as in lookup-key
(above).
(modename . binding)
, where modename is the
variable that enables the minor mode, and binding is key's
binding in that mode. If key has no minor-mode bindings, the
value is nil
.
If the first binding found is not a prefix definition (a keymap or a symbol defined as a keymap), all subsequent bindings from other minor modes are omitted, since they would be completely shadowed. Similarly, the list omits non-prefix bindings that follow prefix bindings.
The argument accept-defaults controls checking for default
bindings, as in lookup-key
(above).
As long as the value of meta-prefix-char
remains 27, key
lookup translates M-b into ESC b, which is normally
defined as the backward-word
command. However, if you set
meta-prefix-char
to 24, the code for C-x, then Emacs will
translate M-b into C-x b, whose standard binding is the
switch-to-buffer
command. Here is an illustration:
meta-prefix-char ; The default value. => 27 (key-binding "\M-b") => backward-word ?\C-x ; The print representation => 24 ; of a character. (setq meta-prefix-char 24) => 24 (key-binding "\M-b") => switch-to-buffer ; Now, typing M-b is ; like typing C-x b. (setq meta-prefix-char 27) ; Avoid confusion! => 27 ; Restore the default value!
The way to rebind a key is to change its entry in a keymap. If you
change a binding in the global keymap, the change is effective in all
buffers (though it has no direct effect in buffers that shadow the
global binding with a local one). If you change the current buffer's
local map, that usually affects all buffers using the same major mode.
The global-set-key
and local-set-key
functions are
convenient interfaces for these operations (see section Commands for Binding Keys). You can also use define-key
, a more general
function; then you must specify explicitly the map to change.
In writing the key sequence to rebind, it is good to use the special
escape sequences for control and meta characters (see section String Type).
The syntax `\C-' means that the following character is a control
character and `\M-' means that the following character is a meta
character. Thus, the string "\M-x"
is read as containing a
single M-x, "\C-f"
is read as containing a single
C-f, and "\M-\C-x"
and "\C-\M-x"
are both read as
containing a single C-M-x. You can also use this escape syntax in
vectors, as well as others that aren't allowed in strings; one example
is `[?\C-\H-x home]'. See section Character Type.
The key definition and lookup functions accept an alternate syntax for
event types in a key sequence that is a vector: you can use a list
containing modifier names plus one base event (a character or function
key name). For example, (control ?a)
is equivalent to
?\C-a
and (hyper control left)
is equivalent to
C-H-left
. One advantage of such lists is that the precise
numeric codes for the modifier bits don't appear in compiled files.
For the functions below, an error is signaled if keymap is not a keymap or if key is not a string or vector representing a key sequence. You can use event types (symbols) as shorthand for events that are lists.
define-key
is binding.
Every prefix of key must be a prefix key (i.e., bound to a keymap)
or undefined; otherwise an error is signaled. If some prefix of
key is undefined, then define-key
defines it as a prefix
key so that the rest of key can be defined as specified.
If there was previously no binding for key in keymap, the new binding is added at the beginning of keymap. The order of bindings in a keymap makes no difference in most cases, but it does matter for menu keymaps (see section Menu Keymaps).
Here is an example that creates a sparse keymap and makes a number of bindings in it:
(setq map (make-sparse-keymap)) => (keymap) (define-key map "\C-f" 'forward-char) => forward-char map => (keymap (6 . forward-char)) ;; Build sparse submap for C-x and bind f in that. (define-key map "\C-xf" 'forward-word) => forward-word map => (keymap (24 keymap ; C-x (102 . forward-word)) ; f (6 . forward-char)) ; C-f ;; Bind C-p to thectl-x-map
. (define-key map "\C-p" ctl-x-map) ;;ctl-x-map
=> [nil ... find-file ... backward-kill-sentence] ;; Bind C-f tofoo
in thectl-x-map
. (define-key map "\C-p\C-f" 'foo) => 'foo map => (keymap ; Notefoo
inctl-x-map
. (16 keymap [nil ... foo ... backward-kill-sentence]) (24 keymap (102 . forward-word)) (6 . forward-char))
Note that storing a new binding for C-p C-f actually works by
changing an entry in ctl-x-map
, and this has the effect of
changing the bindings of both C-p C-f and C-x C-f in the
default global map.
nil
.
For example, this redefines C-x C-f, if you do it in an Emacs with standard bindings:
(substitute-key-definition 'find-file 'find-file-read-only (current-global-map))
If oldmap is non-nil
, then its bindings determine which
keys to rebind. The rebindings still happen in keymap, not in
oldmap. Thus, you can change one map under the control of the
bindings in another. For example,
(substitute-key-definition 'delete-backward-char 'my-funny-delete my-map global-map)
puts the special deletion command in my-map
for whichever keys
are globally bound to the standard deletion command.
Here is an example showing a keymap before and after substitution:
(setq map '(keymap (?1 . olddef-1) (?2 . olddef-2) (?3 . olddef-1))) => (keymap (49 . olddef-1) (50 . olddef-2) (51 . olddef-1)) (substitute-key-definition 'olddef-1 'newdef map) => nil map => (keymap (49 . newdef) (50 . olddef-2) (51 . newdef))
undefined
. This makes ordinary insertion of
text impossible. suppress-keymap
returns nil
.
If nodigits is nil
, then suppress-keymap
defines
digits to run digit-argument
, and - to run
negative-argument
. Otherwise it makes them undefined like the
rest of the printing characters.
The suppress-keymap
function does not make it impossible to
modify a buffer, as it does not suppress commands such as yank
and quoted-insert
. To prevent any modification of a buffer, make
it read-only (see section Read-Only Buffers).
Since this function modifies keymap, you would normally use it
on a newly created keymap. Operating on an existing keymap
that is used for some other purpose is likely to cause trouble; for
example, suppressing global-map
would make it impossible to use
most of Emacs.
Most often, suppress-keymap
is used to initialize local
keymaps of modes such as Rmail and Dired where insertion of text is not
desirable and the buffer is read-only. Here is an example taken from
the file `emacs/lisp/dired.el', showing how the local keymap for
Dired mode is set up:
(setq dired-mode-map (make-keymap)) (suppress-keymap dired-mode-map) (define-key dired-mode-map "r" 'dired-rename-file) (define-key dired-mode-map "\C-d" 'dired-flag-file-deleted) (define-key dired-mode-map "d" 'dired-flag-file-deleted) (define-key dired-mode-map "v" 'dired-view-file) (define-key dired-mode-map "e" 'dired-find-file) (define-key dired-mode-map "f" 'dired-find-file) ...
This section describes some convenient interactive interfaces for
changing key bindings. They work by calling define-key
.
People often use global-set-key
in their `.emacs' file for
simple customization. For example,
(global-set-key "\C-x\C-\\" 'next-line)
or
(global-set-key [?\C-x ?\C-\\] 'next-line)
or
(global-set-key [(control ?x) (control ?\\)] 'next-line)
redefines C-x C-\ to move down a line.
(global-set-key [M-mouse-1] 'mouse-set-point)
redefines the first (leftmost) mouse button, typed with the Meta key, to set point where you click.
(global-set-key key definition) == (define-key (current-global-map) key definition)
One use of this function is in preparation for defining a longer key that uses key as a prefix--which would not be allowed if key has a non-prefix binding. For example:
(global-unset-key "\C-l") => nil (global-set-key "\C-l\C-l" 'redraw-display) => nil
This function is implemented simply using define-key
:
(global-unset-key key) == (define-key (current-global-map) key nil)
(local-set-key key definition) == (define-key (current-local-map) key definition)
(local-unset-key key) == (define-key (current-local-map) key nil)
This section describes functions used to scan all the current keymaps for the sake of printing help information.
(key .
map)
, where key is a prefix key whose definition in
keymap is map.
The elements of the alist are ordered so that the key increases
in length. The first element is always ("" . keymap)
,
because the specified keymap is accessible from itself with a prefix of
no events.
If prefix is given, it should be a prefix key sequence; then
accessible-keymaps
includes only the submaps whose prefixes start
with prefix. These elements look just as they do in the value of
(accessible-keymaps)
; the only difference is that some elements
are omitted.
In the example below, the returned alist indicates that the key
ESC, which is displayed as `^[', is a prefix key whose
definition is the sparse keymap (keymap (83 . center-paragraph)
(115 . foo))
.
(accessible-keymaps (current-local-map)) =>(("" keymap (27 keymap ; Note this keymap for ESC is repeated below. (83 . center-paragraph) (115 . center-line)) (9 . tab-to-tab-stop)) ("^[" keymap (83 . center-paragraph) (115 . foo)))
In the following example, C-h is a prefix key that uses a sparse
keymap starting with (keymap (118 . describe-variable)...)
.
Another prefix, C-x 4, uses a keymap which is also the value of
the variable ctl-x-4-map
. The event mode-line
is one of
several dummy events used as prefixes for mouse actions in special parts
of a window.
(accessible-keymaps (current-global-map)) => (("" keymap [set-mark-command beginning-of-line ... delete-backward-char]) ("^H" keymap (118 . describe-variable) ... (8 . help-for-help)) ("^X" keymap [x-flush-mouse-queue ... backward-kill-sentence]) ("^[" keymap [mark-sexp backward-sexp ... backward-kill-word]) ("^X4" keymap (15 . display-buffer) ...) ([mode-line] keymap (S-mouse-2 . mouse-split-window-horizontally) ...))
These are not all the keymaps you would see in actuality.
where-is
command
(see section `Help' in The GNU Emacs Manual). It returns a list
of key sequences (of any length) that are bound to command in a
set of keymaps.
The argument command can be any object; it is compared with all
keymap entries using eq
.
If keymap is nil
, then the maps used are the current active
keymaps, disregarding overriding-local-map
(that is, pretending
its value is nil
). If keymap is non-nil
, then the
maps searched are keymap and the global keymap.
Usually it's best to use overriding-local-map
as the expression
for keymap. Then where-is-internal
searches precisely the
keymaps that are active. To search only the global map, pass
(keymap)
(an empty keymap) as keymap.
If firstonly is non-ascii
, then the value is a single
string representing the first key sequence found, rather than a list of
all possible key sequences. If firstonly is t
, then the
value is the first key sequence, except that key sequences consisting
entirely of ASCII characters (or meta variants of ASCII
characters) are preferred to all other key sequences.
If noindirect is non-nil
, where-is-internal
doesn't
follow indirect keymap bindings. This makes it possible to search for
an indirect definition itself.
(where-is-internal 'describe-function) => ("\^hf" "\^hd")
If prefix is non-nil
, it should be a prefix key; then the
listing includes only keys that start with prefix.
The listing describes meta characters as ESC followed by the corresponding non-meta character.
When several characters with consecutive ASCII codes have the
same definition, they are shown together, as
`firstchar..lastchar'. In this instance, you need to
know the ASCII codes to understand which characters this means.
For example, in the default global map, the characters `SPC
.. ~' are described by a single line. SPC is ASCII 32,
~ is ASCII 126, and the characters between them include all
the normal printing characters, (e.g., letters, digits, punctuation,
etc.); all these characters are bound to self-insert-command
.
A keymap can define a menu as well as bindings for keyboard keys and mouse button. Menus are usually actuated with the mouse, but they can work with the keyboard also.
A keymap is suitable for menu use if it has an overall prompt
string, which is a string that appears as an element of the keymap.
(See section Format of Keymaps.) The string should describe the purpose of
the menu. The easiest way to construct a keymap with a prompt string is
to specify the string as an argument when you call make-keymap
or
make-sparse-keymap
(see section Creating Keymaps).
The order of items in the menu is the same as the order of bindings in
the keymap. Since define-key
puts new bindings at the front, you
should define the menu items starting at the bottom of the menu and
moving to the top, if you care about the order. When you add an item to
an existing menu, you can specify its position in the menu using
define-key-after
(see section Modifying Menus).
The simpler and older way to define a menu keymap binding looks like this:
(item-string . real-binding)
The CAR, item-string, is the string to be displayed in the menu. It should be short--preferably one to three words. It should describe the action of the command it corresponds to.
You can also supply a second string, called the help string, as follows:
(item-string help-string . real-binding)
Currently Emacs does not actually use help-string; it knows only how to ignore help-string in order to extract real-binding. In the future we may use help-string as extended documentation for the menu item, available on request.
As far as define-key
is concerned, item-string and
help-string are part of the event's binding. However,
lookup-key
returns just real-binding, and only
real-binding is used for executing the key.
If real-binding is nil
, then item-string appears in
the menu but cannot be selected.
If real-binding is a symbol and has a non-nil
menu-enable
property, that property is an expression that
controls whether the menu item is enabled. Every time the keymap is
used to display a menu, Emacs evaluates the expression, and it enables
the menu item only if the expression's value is non-nil
. When a
menu item is disabled, it is displayed in a "fuzzy" fashion, and
cannot be selected.
The menu bar does not recalculate which items are enabled every time you
look at a menu. This is because the X toolkit requires the whole tree
of menus in advance. To force recalculation of the menu bar, call
force-mode-line-update
(see section Mode Line Format).
You've probably noticed that menu items show the equivalent keyboard key sequence (if any) to invoke the same command. To save time on recalculation, menu display caches this information in a sublist in the binding, like this:
(item-string [help-string] (key-binding-data) . real-binding)
Don't put these sublists in the menu item yourself; menu display calculates them automatically. Don't mention keyboard equivalents in the item strings themselves, since that is redundant.
An extended-format menu item is a more flexible and also cleaner
alternative to the simple format. It consists of a list that starts
with the symbol menu-item
. To define a non-selectable string,
the item looks like this:
(menu-item item-name)
where a string consisting of two or more dashes specifies a separator line.
To define a real menu item which can be selected, the extended format item looks like this:
(menu-item item-name real-binding . item-property-list)
Here, item-name is an expression which evaluates to the menu item string. Thus, the string need not be a constant. The third element, real-binding, is the command to execute. The tail of the list, item-property-list, has the form of a property list which contains other information. Here is a table of the properties that are supported:
:enable FORM
nil
means yes).
:visible FORM
nil
means yes). If the item
does not appear, then the menu is displayed as if this item were
not defined at all.
:help help
:button (type . selected)
:toggle
or
:radio
. The CDR, selected, should be a form; the
result of evaluating it says whether this button is currently selected.
A toggle is a menu item which is labeled as either "on" or "off"
according to the value of selected. The command itself should
toggle selected, setting it to t
if it is nil
,
and to nil
if it is t
. Here is how the menu item
to toggle the debug-on-error
flag is defined:
(menu-item "Debug on Error" toggle-debug-on-error :button (:toggle . (and (boundp 'debug-on-error) debug-on-error))This works because
toggle-debug-on-error
is defined as a command
which toggles the variable debug-on-error
.
Radio buttons are a group of menu items, in which at any time one
and only one is "selected." There should be a variable whose value
says which one is selected at any time. The selected form for
each radio button in the group should check whether the variable has the
right value for selecting that button. Clicking on the button should
set the variable so that the button you clicked on becomes selected.
:key-sequence key-sequence
:key-sequence nil
:keys
property and finds the keyboard
equivalent anyway.
:keys string
:filter filter-fn
Sometimes it is useful to make menu items that use the "same"
command but with different enable conditions. The best way to do this
in Emacs now is with extended menu items; before that feature existed,
it could be done by defining alias commands and using them in menu
items. Here's an example that makes two aliases for
toggle-read-only
and gives them different enable conditions:
(defalias 'make-read-only 'toggle-read-only) (put 'make-read-only 'menu-enable '(not buffer-read-only)) (defalias 'make-writable 'toggle-read-only) (put 'make-writable 'menu-enable 'buffer-read-only)
When using aliases in menus, often it is useful to display the
equivalent key bindings for the "real" command name, not the aliases
(which typically don't have any key bindings except for the menu
itself). To request this, give the alias symbol a non-nil
menu-alias
property. Thus,
(put 'make-read-only 'menu-alias t) (put 'make-writable 'menu-alias t)
causes menu items for make-read-only
and make-writable
to
show the keyboard bindings for toggle-read-only
.
The usual way to make a menu keymap produce a menu is to make it the definition of a prefix key. (A Lisp program can explicitly pop up a menu and receive the user's choice--see section Pop-Up Menus.)
If the prefix key ends with a mouse event, Emacs handles the menu keymap by popping up a visible menu, so that the user can select a choice with the mouse. When the user clicks on a menu item, the event generated is whatever character or symbol has the binding that brought about that menu item. (A menu item may generate a series of events if the menu has multiple levels or comes from the menu bar.)
It's often best to use a button-down event to trigger the menu. Then the user can select a menu item by releasing the button.
A single keymap can appear as multiple menu panes, if you explicitly arrange for this. The way to do this is to make a keymap for each pane, then create a binding for each of those maps in the main keymap of the menu. Give each of these bindings an item string that starts with `@'. The rest of the item string becomes the name of the pane. See the file `lisp/mouse.el' for an example of this. Any ordinary bindings with `@'-less item strings are grouped into one pane, which appears along with the other panes explicitly created for the submaps.
X toolkit menus don't have panes; instead, they can have submenus. Every nested keymap becomes a submenu, whether the item string starts with `@' or not. In a toolkit version of Emacs, the only thing special about `@' at the beginning of an item string is that the `@' doesn't appear in the menu item.
You can also produce multiple panes or submenus from separate keymaps. The full definition of a prefix key always comes from merging the definitions supplied by the various active keymaps (minor mode, local, and global). When more than one of these keymaps is a menu, each of them makes a separate pane or panes (when Emacs does not use an X-toolkit) or a separate submenu (when using an X-toolkit). See section Active Keymaps.
When a prefix key ending with a keyboard event (a character or function key) has a definition that is a menu keymap, the user can use the keyboard to choose a menu item.
Emacs displays the menu alternatives (the item strings of the bindings)
in the echo area. If they don't all fit at once, the user can type
SPC to see the next line of alternatives. Successive uses of
SPC eventually get to the end of the menu and then cycle around to
the beginning. (The variable menu-prompt-more-char
specifies
which character is used for this; SPC is the default.)
When the user has found the desired alternative from the menu, he or she should type the corresponding character--the one whose binding is that alternative.
This way of using menus in an Emacs-like editor was inspired by the Hierarkey system.
Here is a complete example of defining a menu keymap. It is the definition of the `Print' submenu in the `Tools' menu in the menu bar, and it uses the simple menu item format (see section Simple Menu Items). First we create the keymap, and give it a name:
(defvar menu-bar-print-menu (make-sparse-keymap "Print"))
Next we define the menu items:
(define-key menu-bar-print-menu [ps-print-region] '("Postscript Print Region" . ps-print-region-with-faces)) (define-key menu-bar-print-menu [ps-print-buffer] '("Postscript Print Buffer" . ps-print-buffer-with-faces)) (define-key menu-bar-print-menu [separator-ps-print] '("--")) (define-key menu-bar-print-menu [print-region] '("Print Region" . print-region)) (define-key menu-bar-print-menu [print-buffer] '("Print Buffer" . print-buffer))
Note the symbols which the bindings are "made for"; these appear
inside square brackets, in the key sequence being defined. In some
cases, this symbol is the same as the command name; sometimes it is
different. These symbols are treated as "function keys", but they are
not real function keys on the keyboard. They do not affect the
functioning of the menu itself, but they are "echoed" in the echo area
when the user selects from the menu, and they appear in the output of
where-is
and apropos
.
The binding whose definition is ("--")
is a separator line.
Like a real menu item, the separator has a key symbol, in this case
separator-ps-print
. If one menu has two separators, they must
have two different key symbols.
Here is code to define enable conditions for two of the commands in the menu:
(put 'print-region 'menu-enable 'mark-active) (put 'ps-print-region-with-faces 'menu-enable 'mark-active)
Here is how we make this menu appear as an item in the parent menu:
(define-key menu-bar-tools-menu [print] (cons "Print" menu-bar-print-menu))
Note that this incorporates the submenu keymap, which is the value of
the variable menu-bar-print-menu
, rather than the symbol
menu-bar-print-menu
itself. Using that symbol in the parent menu
item would be meaningless because menu-bar-print-menu
is not a
command.
If you wanted to attach the same print menu to a mouse click, you can do it this way:
(define-key global-map [C-S-down-mouse-1] menu-bar-print-menu)
We could equally well use an extended menu item (see section Extended Menu Items) for print-region
, like this:
(define-key menu-bar-print-menu [print-region] '(menu-item "Print Region" print-region :enable (mark-active)))
With the extended menu item, the enable condition is specified inside the menu item itself. If we wanted to make this item disappear from the menu entirely when the mark is inactive, we could do it this way:
(define-key menu-bar-print-menu [print-region] '(menu-item "Print Region" print-region :visible (mark-active)))
Most window systems allow each frame to have a menu bar---a
permanently displayed menu stretching horizontally across the top of the
frame. The items of the menu bar are the subcommands of the fake
"function key" menu-bar
, as defined by all the active keymaps.
To add an item to the menu bar, invent a fake "function key" of your
own (let's call it key), and make a binding for the key sequence
[menu-bar key]
. Most often, the binding is a menu keymap,
so that pressing a button on the menu bar item leads to another menu.
When more than one active keymap defines the same fake function key for the menu bar, the item appears just once. If the user clicks on that menu bar item, it brings up a single, combined menu containing all the subcommands of that item--the global subcommands, the local subcommands, and the minor mode subcommands.
The variable overriding-local-map
is normally ignored when
determining the menu bar contents. That is, the menu bar is computed
from the keymaps that would be active if overriding-local-map
were nil
. See section Active Keymaps.
In order for a frame to display a menu bar, its menu-bar-lines
parameter must be greater than zero. Emacs uses just one line for the
menu bar itself; if you specify more than one line, the other lines
serve to separate the menu bar from the windows in the frame. We
recommend 1 or 2 as the value of menu-bar-lines
. See section Window Frame Parameters.
Here's an example of setting up a menu bar item:
(modify-frame-parameters (selected-frame) '((menu-bar-lines . 2))) ;; Make a menu keymap (with a prompt string) ;; and make it the menu bar item's definition. (define-key global-map [menu-bar words] (cons "Words" (make-sparse-keymap "Words"))) ;; Define specific subcommands in this menu. (define-key global-map [menu-bar words forward] '("Forward word" . forward-word)) (define-key global-map [menu-bar words backward] '("Backward word" . backward-word))
A local keymap can cancel a menu bar item made by the global keymap by
rebinding the same fake function key with undefined
as the
binding. For example, this is how Dired suppresses the `Edit' menu
bar item:
(define-key dired-mode-map [menu-bar edit] 'undefined)
edit
is the fake function key used by the global map for the
`Edit' menu bar item. The main reason to suppress a global
menu bar item is to regain space for mode-specific items.
This variable holds a list of fake function keys for items to display at
the end of the menu bar rather than in normal sequence. The default
value is (help-menu)
; thus, the `Help' menu item normally appears
at the end of the menu bar, following local menu items.
When you insert a new item in an existing menu, you probably want to
put it in a particular place among the menu's existing items. If you
use define-key
to add the item, it normally goes at the front of
the menu. To put it elsewhere in the menu, use define-key-after
:
define-key
, but position the binding in map after
the binding for the event after. The argument key should be
of length one--a vector or string with just one element. But
after should be a single event type--a symbol or a character, not
a sequence. The new binding goes after the binding for after. If
after is t
, then the new binding goes last, at the end of
the keymap.
Here is an example:
(define-key-after my-menu [drink] '("Drink" . drink-command) 'eat)
makes a binding for the fake function key DRINK and puts it right after the binding for EAT.
Here is how to insert an item called `Work' in the `Signals'
menu of Shell mode, after the item break
:
(define-key-after (lookup-key shell-mode-map [menu-bar signals]) [work] '("Work" . work-command) 'break)
A mode is a set of definitions that customize Emacs and can be turned on and off while you edit. There are two varieties of modes: major modes, which are mutually exclusive and used for editing particular kinds of text, and minor modes, which provide features that users can enable individually.
This chapter describes how to write both major and minor modes, how to indicate them in the mode line, and how they run hooks supplied by the user. For related topics such as keymaps and syntax tables, see section Keymaps, and section Syntax Tables.
Major modes specialize Emacs for editing particular kinds of text. Each buffer has only one major mode at a time.
The least specialized major mode is called Fundamental mode.
This mode has no mode-specific definitions or variable settings, so each
Emacs command behaves in its default manner, and each option is in its
default state. All other major modes redefine various keys and options.
For example, Lisp Interaction mode provides special key bindings for
C-j (eval-print-last-sexp
), TAB
(lisp-indent-line
), and other keys.
When you need to write several editing commands to help you perform a specialized editing task, creating a new major mode is usually a good idea. In practice, writing a major mode is easy (in contrast to writing a minor mode, which is often difficult).
If the new mode is similar to an old one, it is often unwise to modify the old one to serve two purposes, since it may become harder to use and maintain. Instead, copy and rename an existing major mode definition and alter the copy--or define a derived mode (see section Defining Derived Modes). For example, Rmail Edit mode, which is in `emacs/lisp/rmailedit.el', is a major mode that is very similar to Text mode except that it provides three additional commands. Its definition is distinct from that of Text mode, but was derived from it.
Rmail Edit mode offers an example of changing the major mode temporarily for a buffer, so it can be edited in a different way (with ordinary Emacs commands rather than Rmail commands). In such cases, the temporary major mode usually has a command to switch back to the buffer's usual mode (Rmail mode, in this case). You might be tempted to present the temporary redefinitions inside a recursive edit and restore the usual ones when the user exits; but this is a bad idea because it constrains the user's options when it is done in more than one buffer: recursive edits must be exited most-recently-entered first. Using an alternative major mode avoids this limitation. See section Recursive Editing.
The standard GNU Emacs Lisp library directory contains the code for several major modes, in files such as `text-mode.el', `texinfo.el', `lisp-mode.el', `c-mode.el', and `rmail.el'. You can study these libraries to see how modes are written. Text mode is perhaps the simplest major mode aside from Fundamental mode. Rmail mode is a complicated and specialized mode.
The code for existing major modes follows various coding conventions, including conventions for local keymap and syntax table initialization, global names, and hooks. Please follow these conventions when you define a new major mode:
describe-mode
) in your mode will display this string.
The documentation string may include the special documentation
substrings, `\[command]', `\{keymap}', and
`\<keymap>', that enable the documentation to adapt
automatically to the user's own key bindings. See section Substituting Key Bindings in Documentation.
kill-all-local-variables
. This is what gets rid of the
buffer-local variables of the major mode previously in effect.
major-mode
to the
major mode command symbol. This is how describe-mode
discovers
which documentation to print.
mode-name
to the
"pretty" name of the mode, as a string. This string appears in the
mode line.
use-local-map
to install this local map. See section Active Keymaps, for more information.
This keymap should be stored permanently in a global variable named
modename-mode-map
. Normally the library that defines the
mode sets this variable.
See section Tips for Defining Variables Robustly, for advice about how to write the code to set
up the mode's keymap variable.
modename-mode-syntax-table
. See section Syntax Tables.
modename-mode-abbrev-table
. See section Abbrev Tables.
font-lock-defaults
(see section Font Lock Mode).
imenu-generic-expression
or
imenu-create-index-function
(see section Imenu).
defvar
or defcustom
to set mode-related variables, so
that they are not reinitialized if they already have a value. (Such
reinitialization could discard customizations made by the user.)
make-local-variable
in the major mode command, not
make-variable-buffer-local
. The latter function would make the
variable local to every buffer in which it is subsequently set, which
would affect buffers that do not use this mode. It is undesirable for a
mode to have such global effects. See section Buffer-Local Variables.
It's OK to use make-variable-buffer-local
, if you wish, for a
variable used only within a single Lisp package.
modename-mode-hook
. The major mode command should run that
hook, with run-hooks
, as the very last thing it
does. See section Hooks.
indented-text-mode
runs text-mode-hook
as
well as indented-text-mode-hook
. It may run these other hooks
immediately before the mode's own hook (that is, after everything else),
or it may run them earlier.
change-major-mode-hook
(see section Creating and Deleting Buffer-Local Bindings).
mode-class
with value special
, put on as follows:
(put 'funny-mode 'mode-class 'special)This tells Emacs that new buffers created while the current buffer has Funny mode should not inherit Funny mode. Modes such as Dired, Rmail, and Buffer List use this feature.
auto-mode-alist
to select
the mode for those file names. If you define the mode command to
autoload, you should add this element in the same file that calls
autoload
. Otherwise, it is sufficient to add the element in the
file that contains the mode definition. See section How Emacs Chooses a Major Mode.
autoload
form
and an example of how to add to auto-mode-alist
, that users can
include in their `.emacs' files.
Text mode is perhaps the simplest mode besides Fundamental mode. Here are excerpts from `text-mode.el' that illustrate many of the conventions listed above:
;; Create mode-specific tables. (defvar text-mode-syntax-table nil "Syntax table used while in text mode.") (if text-mode-syntax-table () ; Do not change the table if it is already set up. (setq text-mode-syntax-table (make-syntax-table)) (modify-syntax-entry ?\" ". " text-mode-syntax-table) (modify-syntax-entry ?\\ ". " text-mode-syntax-table) (modify-syntax-entry ?' "w " text-mode-syntax-table)) (defvar text-mode-abbrev-table nil "Abbrev table used while in text mode.") (define-abbrev-table 'text-mode-abbrev-table ()) (defvar text-mode-map nil) ; Create a mode-specific keymap. (if text-mode-map () ; Do not change the keymap if it is already set up. (setq text-mode-map (make-sparse-keymap)) (define-key text-mode-map "\t" 'indent-relative) (define-key text-mode-map "\es" 'center-line) (define-key text-mode-map "\eS" 'center-paragraph))
Here is the complete major mode function definition for Text mode:
(defun text-mode () "Major mode for editing text intended for humans to read@enddots{} Special commands: \\{text-mode-map} Turning on text-mode runs the hook `text-mode-hook'." (interactive) (kill-all-local-variables) (use-local-map text-mode-map) (setq local-abbrev-table text-mode-abbrev-table) (set-syntax-table text-mode-syntax-table) (make-local-variable 'paragraph-start) (setq paragraph-start (concat "[ \t]*$\\|" page-delimiter)) (make-local-variable 'paragraph-separate) (setq paragraph-separate paragraph-start) (setq mode-name "Text") (setq major-mode 'text-mode) (run-hooks 'text-mode-hook)) ; Finally, this permits the user to ; customize the mode with a hook.
The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp Interaction mode) have more features than Text mode and the code is correspondingly more complicated. Here are excerpts from `lisp-mode.el' that illustrate how these modes are written.
;; Create mode-specific table variables.
(defvar lisp-mode-syntax-table nil "")
(defvar emacs-lisp-mode-syntax-table nil "")
(defvar lisp-mode-abbrev-table nil "")
(if (not emacs-lisp-mode-syntax-table) ; Do not change the table
; if it is already set.
(let ((i 0))
(setq emacs-lisp-mode-syntax-table (make-syntax-table))
;; Set syntax of chars up to 0 to class of chars that are
;; part of symbol names but not words.
;; (The number 0 is 48
in the ASCII character set.)
(while (< i ?0)
(modify-syntax-entry i "_ " emacs-lisp-mode-syntax-table)
(setq i (1+ i)))
...
;; Set the syntax for other characters.
(modify-syntax-entry ? " " emacs-lisp-mode-syntax-table)
(modify-syntax-entry ?\t " " emacs-lisp-mode-syntax-table)
...
(modify-syntax-entry ?\( "() " emacs-lisp-mode-syntax-table)
(modify-syntax-entry ?\) ")( " emacs-lisp-mode-syntax-table)
...))
;; Create an abbrev table for lisp-mode.
(define-abbrev-table 'lisp-mode-abbrev-table ())
Much code is shared among the three Lisp modes. The following function sets various variables; it is called by each of the major Lisp mode functions:
(defun lisp-mode-variables (lisp-syntax) (cond (lisp-syntax (set-syntax-table lisp-mode-syntax-table))) (setq local-abbrev-table lisp-mode-abbrev-table) ...
Functions such as forward-paragraph
use the value of the
paragraph-start
variable. Since Lisp code is different from
ordinary text, the paragraph-start
variable needs to be set
specially to handle Lisp. Also, comments are indented in a special
fashion in Lisp and the Lisp modes need their own mode-specific
comment-indent-function
. The code to set these variables is the
rest of lisp-mode-variables
.
(make-local-variable 'paragraph-start) (setq paragraph-start (concat page-delimiter "\\|$" )) (make-local-variable 'paragraph-separate) (setq paragraph-separate paragraph-start) ... (make-local-variable 'comment-indent-function) (setq comment-indent-function 'lisp-comment-indent))
Each of the different Lisp modes has a slightly different keymap. For
example, Lisp mode binds C-c C-z to run-lisp
, but the other
Lisp modes do not. However, all Lisp modes have some commands in
common. The following code sets up the common commands:
(defvar shared-lisp-mode-map () "Keymap for commands shared by all sorts of Lisp modes.") (if shared-lisp-mode-map () (setq shared-lisp-mode-map (make-sparse-keymap)) (define-key shared-lisp-mode-map "\e\C-q" 'indent-sexp) (define-key shared-lisp-mode-map "\177" 'backward-delete-char-untabify))
And here is the code to set up the keymap for Lisp mode:
(defvar lisp-mode-map () "Keymap for ordinary Lisp mode@enddots{}") (if lisp-mode-map () (setq lisp-mode-map (make-sparse-keymap)) (set-keymap-parent lisp-mode-map shared-lisp-mode-map) (define-key lisp-mode-map "\e\C-x" 'lisp-eval-defun) (define-key lisp-mode-map "\C-c\C-z" 'run-lisp))
Finally, here is the complete major mode function definition for Emacs Lisp mode.
(defun lisp-mode ()
"Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
Commands:
Delete converts tabs to spaces as it moves back.
Blank lines separate paragraphs. Semicolons start comments.
\\{lisp-mode-map}
Note that `run-lisp' may be used either to start an inferior Lisp job
or to switch back to an existing one.
Entry to this mode calls the value of `lisp-mode-hook'
if that value is non-nil."
(interactive)
(kill-all-local-variables)
(use-local-map lisp-mode-map) ; Select the mode's keymap.
(setq major-mode 'lisp-mode) ; This is how describe-mode
; finds out what to describe.
(setq mode-name "Lisp") ; This goes into the mode line.
(lisp-mode-variables t) ; This defines various variables.
(setq imenu-case-fold-search t)
(set-syntax-table lisp-mode-syntax-table)
(run-hooks 'lisp-mode-hook)) ; This permits the user to use a
; hook to customize the mode.
Based on information in the file name or in the file itself, Emacs automatically selects a major mode for the new buffer when a file is visited. It also processes local variables specified in the file text.
fundamental-mode
function does not
run any hooks; you're not supposed to customize it. (If you want Emacs
to behave differently in Fundamental mode, change the global
state of Emacs.)
set-auto-mode
,
then it runs hack-local-variables
to parse, and bind or
evaluate as appropriate, the file's local variables.
If the find-file argument to normal-mode
is non-nil
,
normal-mode
assumes that the find-file
function is calling
it. In this case, it may process a local variables list at the end of
the file and in the `-*-' line. The variable
enable-local-variables
controls whether to do so. See section `Local Variables in Files' in The GNU Emacs Manual, for
the syntax of the local variables section of a file.
If you run normal-mode
interactively, the argument
find-file is normally nil
. In this case,
normal-mode
unconditionally processes any local variables list.
normal-mode
uses condition-case
around the call to the
major mode function, so errors are caught and reported as a `File
mode specification error', followed by the original error message.
t
means process the local variables
lists unconditionally; nil
means ignore them; anything else means
ask the user what to do for each file. The default value is t
.
In addition to this list, any variable whose name has a non-nil
risky-local-variable
property is also ignored.
t
means process them
unconditionally; nil
means ignore them; anything else means ask
the user what to do for each file. The default value is maybe
.
auto-mode-alist
), on the
`#!' line (using interpreter-mode-alist
), or on the
file's local variables list. However, this function does not look for
the `mode:' local variable near the end of a file; the
hack-local-variables
function does that. See section `How Major Modes are Chosen' in The GNU Emacs Manual.
fundamental-mode
.
If the value of default-major-mode
is nil
, Emacs uses
the (previously) current buffer's major mode for the major mode of a new
buffer. However, if that major mode symbol has a mode-class
property with value special
, then it is not used for new buffers;
Fundamental mode is used instead. The modes that have this property are
those such as Dired and Rmail that are useful only with text that has
been specially prepared.
default-major-mode
. If that variable is nil
, it uses
the current buffer's major mode (if that is suitable).
The low-level primitives for creating buffers do not use this function,
but medium-level commands such as switch-to-buffer
and
find-file-noselect
use it whenever they create buffers.
lisp-interaction-mode
.
(regexp .
mode-function)
.
For example,
(("\\`/tmp/fol/" . text-mode) ("\\.texinfo\\'" . texinfo-mode) ("\\.texi\\'" . texinfo-mode) ("\\.el\\'" . emacs-lisp-mode) ("\\.c\\'" . c-mode) ("\\.h\\'" . c-mode) ...)
When you visit a file whose expanded file name (see section Functions that Expand Filenames) matches a regexp, set-auto-mode
calls the
corresponding mode-function. This feature enables Emacs to select
the proper major mode for most files.
If an element of auto-mode-alist
has the form (regexp
function t)
, then after calling function, Emacs searches
auto-mode-alist
again for a match against the portion of the file
name that did not match before. This feature is useful for
uncompression packages: an entry of the form ("\\.gz\\'"
function t)
can uncompress the file and then put the uncompressed
file in the proper mode according to the name sans `.gz'.
Here is an example of how to prepend several pattern pairs to
auto-mode-alist
. (You might use this sort of expression in your
`.emacs' file.)
(setq auto-mode-alist (append ;; File name (within directory) starts with a dot. '(("/\\.[^/]*\\'" . fundamental-mode) ;; File name has no dot. ("[^\\./]*\\'" . fundamental-mode) ;; File name ends in `.C'. ("\\.C\\'" . c++-mode)) auto-mode-alist))
(interpreter . mode)
; for
example, ("perl" . perl-mode)
is one element present by default.
The element says to use mode mode if the file specifies
an interpreter which matches interpreter. The value of
interpreter is actually a regular expression.
This variable is applicable only when the auto-mode-alist
does
not indicate which major mode to use.
The handling of enable-local-variables
documented for
normal-mode
actually takes place here. The argument force
usually comes from the argument find-file given to
normal-mode
.
The describe-mode
function is used to provide information
about major modes. It is normally called with C-h m. The
describe-mode
function uses the value of major-mode
,
which is why every major mode function needs to set the
major-mode
variable.
The describe-mode
function calls the documentation
function using the value of major-mode
as an argument. Thus, it
displays the documentation string of the major mode function.
(See section Access to Documentation Strings.)
describe-mode
function uses the
documentation string of the function as the documentation of the major
mode.
It's often useful to define a new major mode in terms of an existing
one. An easy way to do this is to use define-derived-mode
.