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 describes Emacs Lisp, presuming 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 2.3.
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@prep.ai.mit.edu
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@prep.ai.mit.edu.
--Bil Lewis, Dan LaLiberte, Richard Stallman
Lisp (LISt Processing language) was first developed in the late 1950's at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it superior 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 1960's 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.
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 of the 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.
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.
nil and t are used.
Throughout this manual, the phrases "the Lisp reader" and "the Lisp printer" are used to refer to those routines in Lisp that convert textual representations of Lisp objects into actual 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 arguments or metasyntactic variables appear
in this font or form: first-number.
nil and t
In Lisp, the symbol nil is overloaded with three 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 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. See section Accessing Variable Values.
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 actual 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: integer-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 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.
foo.
electric-future-map.
In a function description, the name of the function being described appears first. It is followed on the same line by a list of parameters. The names used for the parameters are also used in the body of the description.
The appearance of the keyword &optional in the parameter list
indicates that the arguments for subsequent parameters may be omitted
(omitted parameters default to nil). Do not write
&optional when you call the function.
The keyword &rest (which will always be followed by a single
parameter) indicates that any number of arguments can follow. The value
of the single following parameter will be a list of all these 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 parameter 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. Parameters named object may be of any type. (See section Lisp Data Types, for a list of Emacs object types.) Parameters with other sorts of names (e.g., new-file) are discussed specifically in the description of the function. In some sections, features common to parameters 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 parameters 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, then 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'.
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.