emacs/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 Copying, Introduction, Top, Top
@comment  node-name,  next,  previous,  up
@unnumbered GNU GENERAL PUBLIC LICENSE
@center Version 2, June 1991

@display
Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@unnumberedsec Preamble

  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.

@iftex
@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end iftex
@ifinfo
@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end ifinfo

@enumerate 0
@item
This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term ``modification''.)  Each licensee is addressed as ``you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.

@item
You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.

@item
You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

@enumerate a
@item
You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.

@item
You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.

@item
If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License.  (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
@end enumerate

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
entire whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under
the scope of this License.

@item
You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:

@enumerate a
@item
Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of Sections
1 and 2 above on a medium customarily used for software interchange; or,

@item
Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than your
cost of physically performing source distribution, a complete
machine-readable copy of the corresponding source code, to be
distributed under the terms of Sections 1 and 2 above on a medium
customarily used for software interchange; or,

@item
Accompany it with the information you received as to the offer
to distribute corresponding source code.  (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form with such
an offer, in accord with Subsection b above.)
@end enumerate

The source code for a work means the preferred form of the work for
making modifications to it.  For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
control compilation and installation of the executable.  However, as a
special exception, the source code distributed need not include
anything that is normally distributed (in either source or binary
form) with the major components (compiler, kernel, and so on) of the
operating system on which the executable runs, unless that component
itself accompanies the executable.

If distribution of executable or object code is made by offering
access to copy from a designated place, then offering equivalent
access to copy the source code from the same place counts as
distribution of the source code, even though third parties are not
compelled to copy the source along with the object code.

@item
You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.

@item
You are not required to accept this License, since you have not
signed it.  However, nothing else grants you permission to modify or
distribute the Program or its derivative works.  These actions are
prohibited by law if you do not accept this License.  Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.

@item
Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions.  You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.

@item
If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all.  For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.

If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.

It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
implemented by public license practices.  Many people have made
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.

This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.

@item
If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
original copyright holder who places the Program under this License
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded.  In such case, this License incorporates
the limitation as if written in the body of this License.

@item
The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

Each version is given a distinguishing version number.  If the Program
specifies a version number of this License which applies to it and ``any
later version'', you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation.  If the Program does not specify a version number of
this License, you may choose any version ever published by the Free Software
Foundation.

@item
If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission.  For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this.  Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.

@iftex
@heading NO WARRANTY
@end iftex
@ifinfo
@center NO WARRANTY
@end ifinfo

@item
BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW@.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE@.  THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU@.  SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.

@item
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
@end enumerate

@iftex
@heading END OF TERMS AND CONDITIONS
@end iftex
@ifinfo
@center END OF TERMS AND CONDITIONS
@end ifinfo

@page
@unnumberedsec How to Apply These Terms to Your New Programs

  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.

@smallexample
@var{one line to give the program's name and an idea of what it does.}
Copyright (C) 19@var{yy}  @var{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., 675 Mass Ave, Cambridge, MA 02139, USA.
@end smallexample

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:

@smallexample
Gnomovision version 69, Copyright (C) 19@var{yy} @var{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.
@end smallexample

The hypothetical commands @samp{show w} and @samp{show c} should show
the appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than @samp{show w} and
@samp{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:

@smallexample
@group
Yoyodyne, Inc., hereby disclaims all copyright
interest in the program `Gnomovision'
(which makes passes at compilers) written 
by James Hacker.

@var{signature of Ty Coon}, 1 April 1989
Ty Coon, President of Vice
@end group
@end smallexample

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.

@node Introduction, Lisp Data Types, Copying, Top
@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 describes Emacs Lisp, presuming 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 2.4.

@menu
* Caveats::             Flaws and a request for help.
* Lisp History::        Emacs Lisp is descended from Maclisp.
* Conventions::         How the manual is formatted.
* 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 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.
@end iftex
@ifinfo

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
which you are criticizing.
@end ifinfo

Please mail comments and corrections to

@example
bug-lisp-manual@@prep.ai.mit.edu
@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@@prep.ai.mit.edu}.

@display
 --Bil Lewis, Dan LaLiberte, Richard Stallman
@end display

@node Lisp History
@section Lisp History
@cindex Lisp history

  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.

@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 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.

@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 for examples that print output.
* 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'' are used to 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 appear in this font or form: @code{(list 1 2
3)}.  Names that represent arguments or metasyntactic variables appear
in this font or form: @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 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{Accessing
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 actual 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.  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 @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: integer-or-marker-p, x
@end example

@node Buffer Text Notation
@subsection Buffer Text Notation
@cindex buffer text notation

  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, @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
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.
@ifinfo
The category---function, variable, or whatever---appears at the
beginning of the line.
@end ifinfo
@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 parameters.
The names used for the parameters are also used in the body of the
description.

  The appearance of the keyword @code{&optional} in the parameter list
indicates that the arguments for subsequent parameters may be omitted
(omitted parameters default to @code{nil}).  Do not write
@code{&optional} when you call the function.

  The keyword @code{&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 @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

More generally,

@example
(foo @var{w} @var{x} @var{y}@dots{})
@equiv{}
(+ (- @var{x} @var{w}) @var{y}@dots{})
@end example
@end defun

  Any parameter 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.  Parameters named @var{object} may be of any type.
(@xref{Lisp Data Types}, for a list of Emacs object types.)
Parameters 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 parameters 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 parameters because they can break the argument
list down into separate arguments in more complicated ways.
@samp{@code{@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 1 (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, then @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 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 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.