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[emacs.git] / lispref / tips.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1995, 1998, 1999
@c   Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/tips
@node Tips, GNU Emacs Internals, GPL, Top
@appendix Tips and Conventions
@cindex tips
@cindex standards of coding style
@cindex coding standards

  This chapter describes no additional features of Emacs Lisp.  Instead
it gives advice on making effective use of the features described in the
previous chapters, and describes conventions Emacs Lisp programmers
should follow.

  You can automatically check some of the conventions described below by
running the command @kbd{M-x checkdoc RET} when visiting a Lisp file.
It cannot check all of the conventions, and not all the warnings it
gives necessarily correspond to problems, but it is worth examining them
all.

@menu
* Coding Conventions::        Conventions for clean and robust programs.
* Compilation Tips::          Making compiled code run fast.
* Documentation Tips::        Writing readable documentation strings.
* Comment Tips::              Conventions for writing comments.
* Library Headers::           Standard headers for library packages.
@end menu

@node Coding Conventions
@section Emacs Lisp Coding Conventions

  Here are conventions that you should follow when writing Emacs Lisp
code intended for widespread use:

@itemize @bullet
@item
Since all global variables share the same name space, and all
functions share another name space, you should choose a short word to
distinguish your program from other Lisp programs.@footnote{The
benefits of a Common Lisp-style package system are considered not to
outweigh the costs.}  Then take care to begin the names of all global
variables, constants, and functions in your program with the chosen
prefix.  This helps avoid name conflicts.

This recommendation applies even to names for traditional Lisp
primitives that are not primitives in Emacs Lisp---even to
@code{copy-list}.  Believe it or not, there is more than one plausible
way to define @code{copy-list}.  Play it safe; append your name prefix
to produce a name like @code{foo-copy-list} or @code{mylib-copy-list}
instead.

If you write a function that you think ought to be added to Emacs under
a certain name, such as @code{twiddle-files}, don't call it by that name
in your program.  Call it @code{mylib-twiddle-files} in your program,
and send mail to @samp{bug-gnu-emacs@@gnu.org} suggesting we add
it to Emacs.  If and when we do, we can change the name easily enough.

If one prefix is insufficient, your package may use two or three
alternative common prefixes, so long as they make sense.

Separate the prefix from the rest of the symbol name with a hyphen,
@samp{-}.  This will be consistent with Emacs itself and with most Emacs
Lisp programs.

@item
It is often useful to put a call to @code{provide} in each separate
library program, at least if there is more than one entry point to the
program.

@item
If a file requires certain other library programs to be loaded
beforehand, then the comments at the beginning of the file should say
so.  Also, use @code{require} to make sure they are loaded.

@item
If one file @var{foo} uses a macro defined in another file @var{bar},
@var{foo} should contain this expression before the first use of the
macro:

@example
(eval-when-compile (require '@var{bar}))
@end example

@noindent
(And the library @var{bar} should contain @code{(provide '@var{bar})},
to make the @code{require} work.)  This will cause @var{bar} to be
loaded when you byte-compile @var{foo}.  Otherwise, you risk compiling
@var{foo} without the necessary macro loaded, and that would produce
compiled code that won't work right.  @xref{Compiling Macros}.

Using @code{eval-when-compile} avoids loading @var{bar} when
the compiled version of @var{foo} is @emph{used}.

@item
Please don't require the @code{cl} package of Common Lisp extensions at
run time.  Use of this package is optional, and it is not part of the
standard Emacs namespace.  If your package loads @code{cl} at run time,
that could cause name clashes for users who don't use that package.

However, there is no problem with using the @code{cl} package at compile
time, for the sake of macros.  You do that like this:

@example
(eval-when-compile (require 'cl))
@end example

@item
When defining a major mode, please follow the major mode
conventions.  @xref{Major Mode Conventions}.

@item
When defining a minor mode, please follow the minor mode
conventions.  @xref{Minor Mode Conventions}.

@item
If the purpose of a function is to tell you whether a certain condition
is true or false, give the function a name that ends in @samp{p}.  If
the name is one word, add just @samp{p}; if the name is multiple words,
add @samp{-p}.  Examples are @code{framep} and @code{frame-live-p}.

@item
If a user option variable records a true-or-false condition, give it a
name that ends in @samp{-flag}.

@item
If the purpose of a variable is to store a single function, give it a
name that ends in @samp{-function}.  If the purpose of a variable is
to store a list of functions (i.e., the variable is a hook), please
follow the naming conventions for hooks.  @xref{Hooks}.

@item
@cindex reserved keys
@cindex keys, reserved
Please do not define @kbd{C-c @var{letter}} as a key in your major
modes.  Sequences consisting of @kbd{C-c} and a letter (either upper
or lower case) are reserved for users; they are the @strong{only}
sequences reserved for users, so do not block them.

Changing all the Emacs major modes to respect this convention was a
lot of work; abandoning this convention would make that work go to
waste, and inconvenience users.  Please comply with it.

@item
Sequences consisting of @kbd{C-c} followed by a control character or a
digit are reserved for major modes.

@item
Sequences consisting of @kbd{C-c} followed by @kbd{@{}, @kbd{@}},
@kbd{<}, @kbd{>}, @kbd{:} or @kbd{;} are also reserved for major modes.

@item
Sequences consisting of @kbd{C-c} followed by any other punctuation
character are allocated for minor modes.  Using them in a major mode is
not absolutely prohibited, but if you do that, the major mode binding
may be shadowed from time to time by minor modes.

@item
Function keys @key{F5} through @key{F9} without modifier keys are
reserved for users to define.

@item
Do not bind @kbd{C-h} following any prefix character (including
@kbd{C-c}).  If you don't bind @kbd{C-h}, it is automatically available
as a help character for listing the subcommands of the prefix character.

@item
Do not bind a key sequence ending in @key{ESC} except following
another @key{ESC}.  (That is, it is OK to bind a sequence ending in
@kbd{@key{ESC} @key{ESC}}.)

The reason for this rule is that a non-prefix binding for @key{ESC} in
any context prevents recognition of escape sequences as function keys in
that context.

@item
Anything which acts like a temporary mode or state which the user can
enter and leave should define @kbd{@key{ESC} @key{ESC}} or
@kbd{@key{ESC} @key{ESC} @key{ESC}} as a way to escape.

For a state which accepts ordinary Emacs commands, or more generally any
kind of state in which @key{ESC} followed by a function key or arrow key
is potentially meaningful, then you must not define @kbd{@key{ESC}
@key{ESC}}, since that would preclude recognizing an escape sequence
after @key{ESC}.  In these states, you should define @kbd{@key{ESC}
@key{ESC} @key{ESC}} as the way to escape.  Otherwise, define
@kbd{@key{ESC} @key{ESC}} instead.

@item
Applications should not bind mouse events based on button 1 with the
shift key held down.  These events include @kbd{S-mouse-1},
@kbd{M-S-mouse-1}, @kbd{C-S-mouse-1}, and so on.  They are reserved for
users.

@item
@cindex mouse-2
@cindex references, following
Special major modes used for read-only text should usually redefine
@kbd{mouse-2} and @key{RET} to trace some sort of reference in the text.
Modes such as Dired, Info, Compilation, and Occur redefine it in this
way.

@item
When a package provides a modification of ordinary Emacs behavior, it is
good to include a command to enable and disable the feature, provide a
command named @code{@var{whatever}-mode} which turns the feature on or
off, and make it autoload (@pxref{Autoload}).  Design the package so
that simply loading it has no visible effect---that should not enable
the feature.@footnote{Consider that the package may be loaded
arbitrarily by Custom for instance.}  Users will request the feature by
invoking the command.  It is a good idea to define this command
as a minor mode.

@cindex unloading packages
If loading the file adds functions to hooks, define a function
@code{@var{feature}-unload-hook}, where @var{feature} is the name of
the feature the package provides, and make it undo any such changes.
Using @code{unload-feature} to unload the file will run this function.
@xref{Unloading}.

@item
It is a bad idea to define aliases for the Emacs primitives.  Use the
standard names instead.

@item
If a package needs to define an alias or a new function for
compatibility with some other version of Emacs, name it with the package
prefix, not with the raw name with which it occurs in the other version.
Here is an example from Gnus, which provides many examples of such
compatibility issues.

@example
(defalias 'gnus-point-at-bol
  (if (fboundp 'point-at-bol)
      'point-at-bol
    'line-beginning-position))
@end example

@item
Redefining (or advising) an Emacs primitive is discouraged.  It may do
the right thing for a particular program, but there is no telling what
other programs might break as a result.

@item
If a file does replace any of the functions or library programs of
standard Emacs, prominent comments at the beginning of the file should
say which functions are replaced, and how the behavior of the
replacements differs from that of the originals.

@item
Avoid using macros that define functions and variables with names that
are constructed.  It is best for maintenance when the name of the
function or variable being defined is given explicitly in the source
code, as the second element of the list---as it is when you use
@code{defun}, @code{defalias}, @code{defvar} and @code{defcustom}.

@item
Please keep the names of your Emacs Lisp source files to 13 characters
or less.  This way, if the files are compiled, the compiled files' names
will be 14 characters or less, which is short enough to fit on all kinds
of Unix systems.

@item
Don't use @code{next-line} or @code{previous-line} in programs; nearly
always, @code{forward-line} is more convenient as well as more
predictable and robust.  @xref{Text Lines}.

@item
Don't call functions that set the mark, unless setting the mark is one
of the intended features of your program.  The mark is a user-level
feature, so it is incorrect to change the mark except to supply a value
for the user's benefit.  @xref{The Mark}.

In particular, don't use any of these functions:

@itemize @bullet
@item
@code{beginning-of-buffer}, @code{end-of-buffer}
@item
@code{replace-string}, @code{replace-regexp}
@end itemize

If you just want to move point, or replace a certain string, without any
of the other features intended for interactive users, you can replace
these functions with one or two lines of simple Lisp code.

@item
Use lists rather than vectors, except when there is a particular reason
to use a vector.  Lisp has more facilities for manipulating lists than
for vectors, and working with lists is usually more convenient.

Vectors are advantageous for tables that are substantial in size and are
accessed in random order (not searched front to back), provided there is
no need to insert or delete elements (only lists allow that).

@item
The recommended way to print a message in the echo area is with
the @code{message} function, not @code{princ}.  @xref{The Echo Area}.

@item
When you encounter an error condition, call the function @code{error}
(or @code{signal}).  The function @code{error} does not return.
@xref{Signaling Errors}.

Do not use @code{message}, @code{throw}, @code{sleep-for},
or @code{beep} to report errors.

@item
An error message should start with a capital letter but should not end
with a period.

@item
In @code{interactive}, if you use a Lisp expression to produce a list
of arguments, don't try to provide the ``correct'' default values for
region or position arguments.  Instead, provide @code{nil} for those
arguments if they were not specified, and have the function body
compute the default value when the argument is @code{nil}.  For
instance, write this:

@example
(defun foo (pos)
  (interactive
   (list (if @var{specified} @var{specified-pos})))
  (unless pos (setq pos @var{default-pos}))
  ...)
@end example

@noindent
rather than this:

@example
(defun foo (pos)
  (interactive
   (list (if @var{specified} @var{specified-pos}
             @var{default-pos})))
  ...)
@end example

@noindent
This is so that repetition of the command will recompute
these defaults based on the current circumstances.

You do not need to take such precautions when you use interactive
specs @samp{d}, @samp{m} and @samp{r}, because they make special
arrangements to recompute the argument values on repetition of the
command.

@item
Many commands that take a long time to execute display a message that
says something like @samp{Operating...} when they start, and change it to
@samp{Operating...done} when they finish.  Please keep the style of
these messages uniform: @emph{no} space around the ellipsis, and
@emph{no} period after @samp{done}.

@item
Try to avoid using recursive edits.  Instead, do what the Rmail @kbd{e}
command does: use a new local keymap that contains one command defined
to switch back to the old local keymap.  Or do what the
@code{edit-options} command does: switch to another buffer and let the
user switch back at will.  @xref{Recursive Editing}.

@item
In some other systems there is a convention of choosing variable names
that begin and end with @samp{*}.  We don't use that convention in Emacs
Lisp, so please don't use it in your programs.  (Emacs uses such names
only for special-purpose buffers.)  The users will find Emacs more
coherent if all libraries use the same conventions.

@item
Try to avoid compiler warnings about undefined free variables, by adding
dummy @code{defvar} definitions for these variables, like this:

@example
(defvar foo)
@end example

Such a definition has no effect except to tell the compiler
not to warn about uses of the variable @code{foo} in this file.

@item
If you use many functions and variables from a certain file, you can
add a @code{require} for that package to avoid compilation warnings
for them.  It is better if the @code{require} acts only at compile
time.  Here's how to do this:

@example
(eval-when-compile
  (require 'foo))
@end example

@item
If you bind a variable in one function, and use it or set it in
another function, the compiler warns about the latter function unless
the variable has a definition.  But adding a definition would be
unclean if the variable has a short name, since Lisp packages should
not define short variable names.  The right thing to do is to rename
this variable to start with the name prefix used for the other
functions and variables in your package.

@item
Indent each function with @kbd{C-M-q} (@code{indent-sexp}) using the
default indentation parameters.

@item
Don't make a habit of putting close-parentheses on lines by themselves;
Lisp programmers find this disconcerting.  Once in a while, when there
is a sequence of many consecutive close-parentheses, it may make sense
to split the sequence in one or two significant places.

@item
Please put a copyright notice on the file if you give copies to anyone.
Use a message like this one:

@smallexample
;; Copyright (C) @var{year} @var{name}

;; This program is free software; you can redistribute it and/or
;; modify it under the terms of the GNU General Public License as
;; published by the Free Software Foundation; either version 2 of
;; the License, or (at your option) any later version.

;; This program is distributed in the hope that it will be
;; useful, but WITHOUT ANY WARRANTY; without even the implied
;; warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
;; PURPOSE.  See the GNU General Public License for more details.

;; You should have received a copy of the GNU General Public
;; License along with this program; if not, write to the Free
;; Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
;; MA 02111-1307 USA
@end smallexample

If you have signed papers to assign the copyright to the Foundation,
then use @samp{Free Software Foundation, Inc.} as @var{name}.
Otherwise, use your name.
@end itemize

@node Compilation Tips
@section Tips for Making Compiled Code Fast
@cindex execution speed
@cindex speedups

  Here are ways of improving the execution speed of byte-compiled
Lisp programs.

@itemize @bullet
@item
@cindex profiling
@cindex timing programs
@cindex @file{elp.el}
Profile your program with the @file{elp} library.  See the file
@file{elp.el} for instructions.

@item
Use iteration rather than recursion whenever possible.
Function calls are slow in Emacs Lisp even when a compiled function
is calling another compiled function.

@item
Using the primitive list-searching functions @code{memq}, @code{member},
@code{assq}, or @code{assoc} is even faster than explicit iteration.  It
can be worth rearranging a data structure so that one of these primitive
search functions can be used.

@item
Certain built-in functions are handled specially in byte-compiled code,
avoiding the need for an ordinary function call.  It is a good idea to
use these functions rather than alternatives.  To see whether a function
is handled specially by the compiler, examine its @code{byte-compile}
property.  If the property is non-@code{nil}, then the function is
handled specially.

For example, the following input will show you that @code{aref} is
compiled specially (@pxref{Array Functions}):

@example
@group
(get 'aref 'byte-compile)
     @result{} byte-compile-two-args
@end group
@end example

@item
If calling a small function accounts for a substantial part of your
program's running time, make the function inline.  This eliminates
the function call overhead.  Since making a function inline reduces
the flexibility of changing the program, don't do it unless it gives
a noticeable speedup in something slow enough that users care about
the speed.  @xref{Inline Functions}.
@end itemize

@node Documentation Tips
@section Tips for Documentation Strings

@findex checkdoc-minor-mode
  Here are some tips and conventions for the writing of documentation
strings.  You can check many of these conventions by running the command
@kbd{M-x checkdoc-minor-mode}.

@itemize @bullet
@item
Every command, function, or variable intended for users to know about
should have a documentation string.

@item
An internal variable or subroutine of a Lisp program might as well have
a documentation string.  In earlier Emacs versions, you could save space
by using a comment instead of a documentation string, but that is no
longer the case---documentation strings now take up very little space in
a running Emacs.

@item
Format the documentation string so that it fits in an Emacs window on an
80-column screen.  It is a good idea for most lines to be no wider than
60 characters.  The first line should not be wider than 67 characters
or it will look bad in the output of @code{apropos}.

You can fill the text if that looks good.  However, rather than blindly
filling the entire documentation string, you can often make it much more
readable by choosing certain line breaks with care.  Use blank lines
between topics if the documentation string is long.

@item
The first line of the documentation string should consist of one or two
complete sentences that stand on their own as a summary.  @kbd{M-x
apropos} displays just the first line, and if that line's contents don't
stand on their own, the result looks bad.  In particular, start the
first line with a capital letter and end with a period.

For a function, the first line should briefly answer the question,
``What does this function do?''  For a variable, the first line should
briefly answer the question, ``What does this value mean?''

Don't limit the documentation string to one line; use as many lines as
you need to explain the details of how to use the function or
variable.  Please use complete sentences for the rest of the text too.

@item
The first line should mention all the important arguments of the
function, and should mention them in the order that they are written
in a function call.  If the function has many arguments, then it is
not feasible to mention them all in the first line; in that case, the
first line should mention the first few arguments, including the most
important arguments.

@item
For consistency, phrase the verb in the first sentence of a function's
documentation string as an imperative---for instance, use ``Return the
cons of A and B.'' in preference to ``Returns the cons of A and B@.''
Usually it looks good to do likewise for the rest of the first
paragraph.  Subsequent paragraphs usually look better if each sentence
is indicative and has a proper subject.

@item
Write documentation strings in the active voice, not the passive, and in
the present tense, not the future.  For instance, use ``Return a list
containing A and B.'' instead of ``A list containing A and B will be
returned.''

@item
Avoid using the word ``cause'' (or its equivalents) unnecessarily.
Instead of, ``Cause Emacs to display text in boldface,'' write just
``Display text in boldface.''

@item
When a command is meaningful only in a certain mode or situation,
do mention that in the documentation string.  For example,
the documentation of @code{dired-find-file} is:

@example
In Dired, visit the file or directory named on this line.
@end example

@item
Do not start or end a documentation string with whitespace.

@item
@strong{Do not} indent subsequent lines of a documentation string so
that the text is lined up in the source code with the text of the first
line.  This looks nice in the source code, but looks bizarre when users
view the documentation.  Remember that the indentation before the
starting double-quote is not part of the string!

@item
When the user tries to use a disabled command, Emacs displays just the
first paragraph of its documentation string---everything through the
first blank line.  If you wish, you can choose which information to
include before the first blank line so as to make this display useful.

@item
A variable's documentation string should start with @samp{*} if the
variable is one that users would often want to set interactively.  If
the value is a long list, or a function, or if the variable would be set
only in init files, then don't start the documentation string with
@samp{*}.  @xref{Defining Variables}.

@item
The documentation string for a variable that is a yes-or-no flag should
start with words such as ``Non-nil means@dots{}'', to make it clear that
all non-@code{nil} values are equivalent and indicate explicitly what
@code{nil} and non-@code{nil} mean.

@item
The documentation string for a function that is a yes-or-no predicate
should start with words such as ``Return t if @dots{}'', to indicate
explicitly what constitutes ``truth''.  The word ``return'' avoids
starting the sentence with lower-case ``t'', which is somewhat
distracting.

@item
When a function's documentation string mentions the value of an argument
of the function, use the argument name in capital letters as if it were
a name for that value.  Thus, the documentation string of the function
@code{eval} refers to its second argument as @samp{FORM}, because the
actual argument name is @code{form}:

@example
Evaluate FORM and return its value.
@end example

Also write metasyntactic variables in capital letters, such as when you
show the decomposition of a list or vector into subunits, some of which
may vary.  @samp{KEY} and @samp{VALUE} in the following example
illustrate this practice:

@example
The argument TABLE should be an alist whose elements
have the form (KEY . VALUE).  Here, KEY is ...
@end example

@item
Never change the case of a Lisp symbol when you mention it in a doc
string.  If the symbol's name is @code{foo}, write ``foo'', not
``Foo'' (which is a different symbol).

This might appear to contradict the policy of writing function
argument values, but there is no real contradiction; the argument
@emph{value} is not the same thing as the @emph{symbol} which the
function uses to hold the value.

If this puts a lower-case letter at the beginning of a sentence
and that annoys you, rewrite the sentence so that the symbol
is not at the start of it.

@item
If a line in a documentation string begins with an open-parenthesis,
write a backslash before the open-parenthesis, like this:

@example
The argument FOO can be either a number
\(a buffer position) or a string (a file name).
@end example

This prevents the open-parenthesis from being treated as the start of a
defun (@pxref{Defuns,, Defuns, emacs, The GNU Emacs Manual}).

@anchor{Docstring hyperlinks}
@item
@iftex
When a documentation string refers to a Lisp symbol, write it as it
would be printed (which usually means in lower case), with single-quotes
around it.  For example: @samp{`lambda'}.  There are two exceptions:
write @code{t} and @code{nil} without single-quotes.
@end iftex
@ifnottex
When a documentation string refers to a Lisp symbol, write it as it
would be printed (which usually means in lower case), with single-quotes
around it.  For example: @samp{lambda}.  There are two exceptions: write
t and nil without single-quotes.  (In this manual, we use a different
convention, with single-quotes for all symbols.)
@end ifnottex

Help mode automatically creates a hyperlink when a documentation string
uses a symbol name inside single quotes, if the symbol has either a
function or a variable definition.  You do not need to do anything
special to make use of this feature.  However, when a symbol has both a
function definition and a variable definition, and you want to refer to
just one of them, you can specify which one by writing one of the words
@samp{variable}, @samp{option}, @samp{function}, or @samp{command},
immediately before the symbol name.  (Case makes no difference in
recognizing these indicator words.)  For example, if you write

@example
This function sets the variable `buffer-file-name'.
@end example

@noindent
then the hyperlink will refer only to the variable documentation of
@code{buffer-file-name}, and not to its function documentation.

If a symbol has a function definition and/or a variable definition, but
those are irrelevant to the use of the symbol that you are documenting,
you can write the word @samp{symbol} before the symbol name to prevent
making any hyperlink.  For example,

@example
If the argument KIND-OF-RESULT is the symbol `list',
this function returns a list of all the objects
that satisfy the criterion.
@end example

@noindent
does not make a hyperlink to the documentation, irrelevant here, of the
function @code{list}.

Normally, no hyperlink is made for a variable without variable
documentation.  You can force a hyperlink for such variables by
preceding them with one of the words @samp{variable} or
@samp{option}.

Hyperlinks for faces are only made if the face name is preceded or
followed by the word @samp{face}.  In that case, only the face
documentation will be shown, even if the symbol is also defined as a
variable or as a function.

To make a hyperlink to Info documentation, write the name of the Info
node (or anchor) in single quotes, preceded by @samp{info node},
@samp{Info node}, @samp{info anchor} or @samp{Info anchor}.  The Info
file name defaults to @samp{emacs}.  For example,

@smallexample
See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'.
@end smallexample

@item
Don't write key sequences directly in documentation strings.  Instead,
use the @samp{\\[@dots{}]} construct to stand for them.  For example,
instead of writing @samp{C-f}, write the construct
@samp{\\[forward-char]}.  When Emacs displays the documentation string,
it substitutes whatever key is currently bound to @code{forward-char}.
(This is normally @samp{C-f}, but it may be some other character if the
user has moved key bindings.)  @xref{Keys in Documentation}.

@item
In documentation strings for a major mode, you will want to refer to the
key bindings of that mode's local map, rather than global ones.
Therefore, use the construct @samp{\\<@dots{}>} once in the
documentation string to specify which key map to use.  Do this before
the first use of @samp{\\[@dots{}]}.  The text inside the
@samp{\\<@dots{}>} should be the name of the variable containing the
local keymap for the major mode.

It is not practical to use @samp{\\[@dots{}]} very many times, because
display of the documentation string will become slow.  So use this to
describe the most important commands in your major mode, and then use
@samp{\\@{@dots{}@}} to display the rest of the mode's keymap.
@end itemize

@node Comment Tips
@section Tips on Writing Comments

  We recommend these conventions for where to put comments and how to
indent them:

@table @samp
@item ;
Comments that start with a single semicolon, @samp{;}, should all be
aligned to the same column on the right of the source code.  Such
comments usually explain how the code on the same line does its job.  In
Lisp mode and related modes, the @kbd{M-;} (@code{indent-for-comment})
command automatically inserts such a @samp{;} in the right place, or
aligns such a comment if it is already present.

This and following examples are taken from the Emacs sources.

@smallexample
@group
(setq base-version-list                 ; there was a base
      (assoc (substring fn 0 start-vn)  ; version to which
             file-version-assoc-list))  ; this looks like
                                        ; a subversion
@end group
@end smallexample

@item ;;
Comments that start with two semicolons, @samp{;;}, should be aligned to
the same level of indentation as the code.  Such comments usually
describe the purpose of the following lines or the state of the program
at that point.  For example:

@smallexample
@group
(prog1 (setq auto-fill-function
             @dots{}
             @dots{}
  ;; update mode line
  (force-mode-line-update)))
@end group
@end smallexample

We also normally use two semicolons for comments outside functions.

@smallexample
@group
;; This Lisp code is run in Emacs
;; when it is to operate as a server
;; for other processes.
@end group
@end smallexample

Every function that has no documentation string (presumably one that is
used only internally within the package it belongs to), should instead
have a two-semicolon comment right before the function, explaining what
the function does and how to call it properly.  Explain precisely what
each argument means and how the function interprets its possible values.

@item ;;;
Comments that start with three semicolons, @samp{;;;}, should start at
the left margin.  These are used, occasionally, for comments within
functions that should start at the margin.  We also use them sometimes
for comments that are between functions---whether to use two or three
semicolons depends on whether the comment should be considered a
``heading'' by Outline minor mode.  By default, comments starting with
at least three semicolons (followed by a single space and a
non-whitespace character) are considered headings, comments starting
with two or less are not.

Another use for triple-semicolon comments is for commenting out lines
within a function.  We use three semicolons for this precisely so that
they remain at the left margin.  By default, Outline minor mode does
not consider a comment to be a heading (even if it starts with at
least three semicolons) if the semicolons are followed by at least two
spaces.  Thus, if you add an introductory comment to the commented out
code, make sure to indent it by at least two spaces after the three
semicolons.

@smallexample
(defun foo (a)
;;;  This is no longer necessary.
;;;  (force-mode-line-update)
  (message "Finished with %s" a))
@end smallexample

When commenting out entire functions, use two semicolons.

@item ;;;;
Comments that start with four semicolons, @samp{;;;;}, should be aligned
to the left margin and are used for headings of major sections of a
program.  For example:

@smallexample
;;;; The kill ring
@end smallexample
@end table

@noindent
The indentation commands of the Lisp modes in Emacs, such as @kbd{M-;}
(@code{indent-for-comment}) and @key{TAB} (@code{lisp-indent-line}),
automatically indent comments according to these conventions,
depending on the number of semicolons.  @xref{Comments,,
Manipulating Comments, emacs, The GNU Emacs Manual}.

@node Library Headers
@section Conventional Headers for Emacs Libraries
@cindex header comments
@cindex library header comments

  Emacs has conventions for using special comments in Lisp libraries
to divide them into sections and give information such as who wrote
them.  This section explains these conventions.

  We'll start with an example, a package that is included in the Emacs
distribution.

  Parts of this example reflect its status as part of Emacs; for
example, the copyright notice lists the Free Software Foundation as the
copyright holder, and the copying permission says the file is part of
Emacs.  When you write a package and post it, the copyright holder would
be you (unless your employer claims to own it instead), and you should
get the suggested copying permission from the end of the GNU General
Public License itself.  Don't say your file is part of Emacs
if we haven't installed it in Emacs yet!

  With that warning out of the way, on to the example:

@smallexample
@group
;;; lisp-mnt.el --- minor mode for Emacs Lisp maintainers

;; Copyright (C) 1992 Free Software Foundation, Inc.
@end group

;; Author: Eric S. Raymond <esr@@snark.thyrsus.com>
;; Maintainer: Eric S. Raymond <esr@@snark.thyrsus.com>
;; Created: 14 Jul 1992
;; Version: 1.2
@group
;; Keywords: docs

;; This file is part of GNU Emacs.
@dots{}
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.
@end group
@end smallexample

  The very first line should have this format:

@example
;;; @var{filename} --- @var{description}
@end example

@noindent
The description should be complete in one line.  If the file
needs a @samp{-*-} specification, put it after @var{description}.

  After the copyright notice come several @dfn{header comment} lines,
each beginning with @samp{;; @var{header-name}:}.  Here is a table of
the conventional possibilities for @var{header-name}:

@table @samp
@item Author
This line states the name and net address of at least the principal
author of the library.

If there are multiple authors, you can list them on continuation lines
led by @code{;;} and a tab character, like this:

@smallexample
@group
;; Author: Ashwin Ram <Ram-Ashwin@@cs.yale.edu>
;;      Dave Sill <de5@@ornl.gov>
;;      Dave Brennan <brennan@@hal.com>
;;      Eric Raymond <esr@@snark.thyrsus.com>
@end group
@end smallexample

@item Maintainer
This line should contain a single name/address as in the Author line, or
an address only, or the string @samp{FSF}.  If there is no maintainer
line, the person(s) in the Author field are presumed to be the
maintainers.  The example above is mildly bogus because the maintainer
line is redundant.

The idea behind the @samp{Author} and @samp{Maintainer} lines is to make
possible a Lisp function to ``send mail to the maintainer'' without
having to mine the name out by hand.

Be sure to surround the network address with @samp{<@dots{}>} if
you include the person's full name as well as the network address.

@item Created
This optional line gives the original creation date of the
file.  For historical interest only.

@item Version
If you wish to record version numbers for the individual Lisp program, put
them in this line.

@item Adapted-By
In this header line, place the name of the person who adapted the
library for installation (to make it fit the style conventions, for
example).

@item Keywords
This line lists keywords for the @code{finder-by-keyword} help command.
Please use that command to see a list of the meaningful keywords.

This field is important; it's how people will find your package when
they're looking for things by topic area.  To separate the keywords, you
can use spaces, commas, or both.
@end table

  Just about every Lisp library ought to have the @samp{Author} and
@samp{Keywords} header comment lines.  Use the others if they are
appropriate.  You can also put in header lines with other header
names---they have no standard meanings, so they can't do any harm.

  We use additional stylized comments to subdivide the contents of the
library file.  These should be separated by blank lines from anything
else.  Here is a table of them:

@table @samp
@item ;;; Commentary:
This begins introductory comments that explain how the library works.
It should come right after the copying permissions, terminated by a
@samp{Change Log}, @samp{History} or @samp{Code} comment line.  This
text is used by the Finder package, so it should make sense in that
context.

@item ;;; Documentation:
This was used in some files in place of @samp{;;; Commentary:},
but it is deprecated.

@item ;;; Change Log:
This begins change log information stored in the library file (if you
store the change history there).  For Lisp files distributed with Emacs,
the change history is kept in the file @file{ChangeLog} and not in the
source file at all; these files generally do not have a @samp{;;; Change
Log:} line.  @samp{History} is an alternative to @samp{Change Log}.

@item ;;; Code:
This begins the actual code of the program.

@item ;;; @var{filename} ends here
This is the @dfn{footer line}; it appears at the very end of the file.
Its purpose is to enable people to detect truncated versions of the file
from the lack of a footer line.
@end table

@ignore
   arch-tag: 9ea911c2-6b1d-47dd-88b7-0a94e8b27c2e
@end ignore