@c -*-coding: iso-latin-1-*- @c This is part of the GNU Emacs Lisp Reference Manual. @c Copyright (C) 1990-1994, 2001-2011 Free Software Foundation, Inc. @c See the file elisp.texi for copying conditions. @setfilename ../../info/intro @node Introduction, Lisp Data Types, Top, Top @comment node-name, next, previous, up @chapter Introduction 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 @cite{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 @cite{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 @value{VERSION} of the GNU Emacs Lisp Reference Manual, corresponding to Emacs version @value{EMACSVER}. @menu * Caveats:: Flaws and a request for help. * Lisp History:: Emacs Lisp is descended from Maclisp. * Conventions:: How the manual is formatted. * Version Info:: Which Emacs version is running? * Acknowledgements:: The authors, editors, and sponsors of this manual. @end menu @node Caveats @section Caveats @cindex bugs in this manual 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. 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. @iftex As you use this manual, we ask that you mark pages with corrections so you can later look them up and send them to us. 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. @end iftex @ifnottex As you use this manual, we ask that you send corrections as soon as you find them. 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 node name and function or variable name, as appropriate. Also state the number of the edition you are criticizing. @end ifnottex @cindex bugs @cindex suggestions Please send comments and corrections using @kbd{M-x report-emacs-bug}. @node Lisp History @section Lisp History @cindex Lisp history 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. @cindex Maclisp @cindex Common Lisp 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. @pindex cl A certain amount of Common Lisp emulation is available via the @file{cl} library. @inforef{Top, Overview, cl}. 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. @node Conventions @section Conventions 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. @menu * Some Terms:: Explanation of terms we use in this manual. * nil and t:: How the symbols @code{nil} and @code{t} are used. * Evaluation Notation:: The format we use for examples of evaluation. * Printing Notation:: The format we use when examples print text. * Error Messages:: The format we use for examples of errors. * Buffer Text Notation:: The format we use for buffer contents in examples. * Format of Descriptions:: Notation for describing functions, variables, etc. @end menu @node Some Terms @subsection Some Terms 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. @xref{Printed Representation}, 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. @cindex typographic conventions Examples of Lisp code are formatted like this: @code{(list 1 2 3)}. Names that represent metasyntactic variables, or arguments to a function being described, are formatted like this: @var{first-number}. @node nil and t @subsection @code{nil} and @code{t} @cindex truth value @cindex boolean @cindex @code{nil} @cindex false In Lisp, the symbol @code{nil} has three separate meanings: it is a symbol with the name @samp{nil}; it is the logical truth value @var{false}; and it is the empty list---the list of zero elements. When used as a variable, @code{nil} always has the value @code{nil}. As far as the Lisp reader is concerned, @samp{()} and @samp{nil} are identical: they stand for the same object, the symbol @code{nil}. The different ways of writing the symbol are intended entirely for human readers. After the Lisp reader has read either @samp{()} or @samp{nil}, there is no way to determine which representation was actually written by the programmer. In this manual, we write @code{()} when we wish to emphasize that it means the empty list, and we write @code{nil} when we wish to emphasize that it means the truth value @var{false}. That is a good convention to use in Lisp programs also. @example (cons 'foo ()) ; @r{Emphasize the empty list} (setq foo-flag nil) ; @r{Emphasize the truth value @var{false}} @end example @cindex @code{t} @cindex true In contexts where a truth value is expected, any non-@code{nil} value is considered to be @var{true}. However, @code{t} is the preferred way to represent the truth value @var{true}. When you need to choose a value which represents @var{true}, and there is no other basis for choosing, use @code{t}. The symbol @code{t} always has the value @code{t}. In Emacs Lisp, @code{nil} and @code{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 @code{setting-constant} error. @xref{Constant Variables}. @defun booleanp object Return non-@code{nil} if @var{object} is one of the two canonical boolean values: @code{t} or @code{nil}. @end defun @node Evaluation Notation @subsection Evaluation Notation @cindex evaluation notation @cindex documentation notation @cindex notation A Lisp expression that you can evaluate is called a @dfn{form}. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with @samp{@result{}}: @example (car '(1 2)) @result{} 1 @end example @noindent You can read this as ``@code{(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 @samp{@expansion{}}. We may or may not show the result of the evaluation of the expanded form. @example (third '(a b c)) @expansion{} (car (cdr (cdr '(a b c)))) @result{} c @end example Sometimes to help describe one form we show another form that produces identical results. The exact equivalence of two forms is indicated with @samp{@equiv{}}. @example (make-sparse-keymap) @equiv{} (list 'keymap) @end example @node Printing Notation @subsection Printing Notation @cindex printing notation 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 @samp{*scratch*}), the printed text is inserted into the buffer. If you execute the example by other means (such as by evaluating the function @code{eval-region}), the printed text is displayed in the echo area. Examples in this manual indicate printed text with @samp{@print{}}, irrespective of where that text goes. The value returned by evaluating the form (here @code{bar}) follows on a separate line with @samp{@result{}}. @example @group (progn (prin1 'foo) (princ "\n") (prin1 'bar)) @print{} foo @print{} bar @result{} bar @end group @end example @node Error Messages @subsection Error Messages @cindex error message notation Some examples signal errors. This normally displays an error message in the echo area. We show the error message on a line starting with @samp{@error{}}. Note that @samp{@error{}} itself does not appear in the echo area. @example (+ 23 'x) @error{} Wrong type argument: number-or-marker-p, x @end example @node Buffer Text Notation @subsection Buffer Text Notation @cindex buffer text notation Some examples describe modifications to the contents of a buffer, by showing the ``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, @samp{@point{}} indicates the location of point. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place @emph{between} two characters where point is currently located.) @example ---------- Buffer: foo ---------- This is the @point{}contents of foo. ---------- Buffer: foo ---------- (insert "changed ") @result{} nil ---------- Buffer: foo ---------- This is the changed @point{}contents of foo. ---------- Buffer: foo ---------- @end example @node Format of Descriptions @subsection Format of Descriptions @cindex description format 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. @ifnottex The category---function, variable, or whatever---appears at the beginning of the line. @end ifnottex @iftex The category---function, variable, or whatever---is printed next to the right margin. @end iftex The description follows on succeeding lines, sometimes with examples. @menu * A Sample Function Description:: A description of an imaginary function, @code{foo}. * A Sample Variable Description:: A description of an imaginary variable, @code{electric-future-map}. @end menu @node A Sample Function Description @subsubsection A Sample Function Description @cindex function descriptions @cindex command descriptions @cindex macro descriptions @cindex special form descriptions 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 @code{&optional} in the argument list indicates that the subsequent arguments may be omitted (omitted arguments default to @code{nil}). Do not write @code{&optional} when you call the function. The keyword @code{&rest} (which must be followed by a single argument name) indicates that any number of arguments can follow. The single argument name following @code{&rest} will receive, as its value, a list of all the remaining arguments passed to the function. Do not write @code{&rest} when you call the function. Here is a description of an imaginary function @code{foo}: @defun foo integer1 &optional integer2 &rest integers The function @code{foo} subtracts @var{integer1} from @var{integer2}, then adds all the rest of the arguments to the result. If @var{integer2} is not supplied, then the number 19 is used by default. @example (foo 1 5 3 9) @result{} 16 (foo 5) @result{} 14 @end example @need 1500 More generally, @example (foo @var{w} @var{x} @var{y}@dots{}) @equiv{} (+ (- @var{x} @var{w}) @var{y}@dots{}) @end example @end defun Any argument whose name contains the name of a type (e.g., @var{integer}, @var{integer1} or @var{buffer}) is expected to be of that type. A plural of a type (such as @var{buffers}) often means a list of objects of that type. Arguments named @var{object} may be of any type. (@xref{Lisp Data Types}, for a list of Emacs object types.) Arguments with other sorts of names (e.g., @var{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. @xref{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. @samp{@r{[}@var{optional-arg}@r{]}} means that @var{optional-arg} is optional and @samp{@var{repeated-args}@dots{}} stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure. Here is an example: @defspec count-loop (@var{var} [@var{from} @var{to} [@var{inc}]]) @var{body}@dots{} This imaginary special form implements a loop that executes the @var{body} forms and then increments the variable @var{var} on each iteration. On the first iteration, the variable has the value @var{from}; on subsequent iterations, it is incremented by one (or by @var{inc} if that is given). The loop exits before executing @var{body} if @var{var} equals @var{to}. Here is an example: @example (count-loop (i 0 10) (prin1 i) (princ " ") (prin1 (aref vector i)) (terpri)) @end example If @var{from} and @var{to} are omitted, @var{var} is bound to @code{nil} before the loop begins, and the loop exits if @var{var} is non-@code{nil} at the beginning of an iteration. Here is an example: @example (count-loop (done) (if (pending) (fixit) (setq done t))) @end example In this special form, the arguments @var{from} and @var{to} are optional, but must both be present or both absent. If they are present, @var{inc} may optionally be specified as well. These arguments are grouped with the argument @var{var} into a list, to distinguish them from @var{body}, which includes all remaining elements of the form. @end defspec @node A Sample Variable Description @subsubsection A Sample Variable Description @cindex variable descriptions @cindex option descriptions A @dfn{variable} is a name that can hold a value. Although nearly all variables can be set by the user, certain variables exist specifically so that users can change them; these are called @dfn{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 @code{electric-future-map} variable.@refill @defvar electric-future-map The value of this variable is a full keymap used by Electric Command Future mode. The functions in this map allow you to edit commands you have not yet thought about executing. @end defvar User option descriptions have the same format, but `Variable' is replaced by `User Option'. @node Version Info @section Version Information These facilities provide information about which version of Emacs is in use. @deffn Command emacs-version &optional here This function returns a string describing the version of Emacs that is running. It is useful to include this string in bug reports. @smallexample @group (emacs-version) @result{} "GNU Emacs 23.1 (i686-pc-linux-gnu, GTK+ Version 2.14.4) of 2009-06-01 on cyd.mit.edu" @end group @end smallexample If @var{here} is non-@code{nil}, it inserts the text in the buffer before point, and returns @code{nil}. When this function is called interactively, it prints the same information in the echo area, but giving a prefix argument makes @var{here} non-@code{nil}. @end deffn @defvar emacs-build-time The value of this variable indicates the time at which Emacs was built at the local site. It is a list of three integers, like the value of @code{current-time} (@pxref{Time of Day}). @example @group emacs-build-time @result{} (18846 52016 156039) @end group @end example @end defvar @defvar emacs-version The value of this variable is the version of Emacs being run. It is a string such as @code{"23.1.1"}. 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. A value with four numeric components, such as @code{"22.0.91.1"}, indicates an unreleased test version. @end defvar The following two variables have existed since Emacs version 19.23: @defvar emacs-major-version The major version number of Emacs, as an integer. For Emacs version 23.1, the value is 23. @end defvar @defvar emacs-minor-version The minor version number of Emacs, as an integer. For Emacs version 23.1, the value is 1. @end defvar @node Acknowledgements @section Acknowledgements This manual was written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard@tie{}M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert@tie{}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@tie{}A. Hunt, Jr.@: of Computational Logic, Inc. Additional sections were written by Miles Bader, Lars Brinkhoff, Chong Yidong, Kenichi Handa, Lute Kamstra, Juri Linkov, Glenn Morris, Thien-Thi Nguyen, Dan Nicolaescu, Martin Rudalics, Kim F. Storm, Luc Teirlinck, and Eli Zaretskii. Corrections were supplied by Drew Adams, Juanma Barranquero, 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, Jesper Harder, 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, Stefan Monnier, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potorti, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Jason Rumney, Per Starbäck, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Eduard Wiebe, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn.