(signal_before_change, signal_after_change): Consider a

[emacs.git] / lispref / intro.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 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 2.6.

@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

  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.

@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

Please mail comments and corrections to

@example
bug-lisp-manual@@gnu.org
@end example

@noindent
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 @emph{will} be acted on in due time.  If you want to contact the
Emacs maintainers more quickly, send mail to
@code{bug-gnu-emacs@@gnu.org}.

@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.  @xref{Top,, Common Lisp Extension, cl, 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.

@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 fonts
  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 @code{nil}, uses of
@cindex truth value
@cindex boolean
@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 use @code{()} when we wish to emphasize that it
means the empty list, and we use @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}
(not nil)                     ; @r{Emphasize the truth value @var{false}}
@end example

@cindex @code{t} and truth
@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.  The same is true of
any symbol whose name starts with a colon (@samp{:}).  @xref{Constant
Variables}.

@node Evaluation Notation
@subsection Evaluation Notation
@cindex evaluation notation
@cindex documentation 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.

@example
@group
(progn (print 'foo) (print '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
following argument name will have a value, as a variable, which is a
list of all these remaining arguments.  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 any
variable can be set by the user, certain variables that exist
specifically so that users can change them 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
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 20.3.5 (i486-pc-linux-gnulibc1, X toolkit)
 of Sat Feb 14 1998 on psilocin.gnu.org"
@end group
@end smallexample

Called interactively, the function prints the same information in the
echo area.
@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{} (13623 62065 344633)
@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{"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.  A value with four numeric
components, such as @code{"20.3.9.1"}, 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
20.3, the value is 20.
@end defvar

@defvar emacs-minor-version
The minor version number of Emacs, as an integer.  For Emacs version
20.3, the value is 3.
@end defvar

@node Acknowledgements
@section Acknowledgements

  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 Starb@"ack, Shinichirou Sugou, Kimmo Suominen, Edward Tharp,
Bill Trost, Rickard Westman, Jean White, Matthew Wilding, Carl Witty,
Dale Worley, Rusty Wright, and David D. Zuhn.