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[emacs.git] / doc / lispref / tips.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1993, 1995, 1998-1999, 2001-2015 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Tips
@appendix Tips and Conventions
@cindex tips for writing Lisp
@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.
* Key Binding Conventions::   Which keys should be bound by which programs.
* Programming Tips::          Making Emacs code fit smoothly in Emacs.
* Compilation Tips::          Making compiled code run fast.
* Warning Tips::              Turning off compiler warnings.
* 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

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

@itemize @bullet
@item
Simply loading a package should not change Emacs's editing behavior.
Include a command or commands to enable and disable the feature,
or to invoke it.

This convention is mandatory for any file that includes custom
definitions.  If fixing such a file to follow this convention requires
an incompatible change, go ahead and make the incompatible change;
don't postpone it.

@item
You should choose a short word to distinguish your program from other
Lisp programs.  The names of all global symbols in your program, that
is the names of variables, constants, and functions, should begin with
that chosen prefix.  Separate the prefix from the rest of the name
with a hyphen, @samp{-}.  This practice helps avoid name conflicts,
since all global variables in Emacs Lisp share the same name space,
and all functions share another name space@footnote{The benefits of a
Common Lisp-style package system are considered not to outweigh the
costs.}.  Use two hyphens to separate prefix and name if the symbol is
not meant to be used by other packages.

Occasionally, for a command name intended for users to use, it is more
convenient if some words come before the package's name prefix.  And
constructs that define functions, variables, etc., work better if they
start with @samp{defun} or @samp{defvar}, so put the name prefix later
on in the name.

This recommendation applies even to names for traditional Lisp
primitives that are not primitives in Emacs Lisp---such as
@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 can use two or three
alternative common prefixes, so long as they make sense.

@item
Put a call to @code{provide} at the end of each separate Lisp file.
@xref{Named Features}.

@item
If a file requires certain other Lisp 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.
@xref{Named Features}.

@item
If a file @var{foo} uses a macro defined in another file @var{bar},
but does not use any functions or variables defined in @var{bar}, then
@var{foo} should contain the following expression:

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

@noindent
This tells Emacs to load @var{bar} just before byte-compiling
@var{foo}, so that the macro definition is available during
compilation.  Using @code{eval-when-compile} avoids loading @var{bar}
when the compiled version of @var{foo} is @emph{used}.  It should be
called before the first use of the macro in the file.  @xref{Compiling
Macros}.

@item
Avoid loading additional libraries at run time unless they are really
needed.  If your file simply cannot work without some other library,
then just @code{require} that library at the top-level and be done
with it.  But if your file contains several independent features, and
only one or two require the extra library, then consider putting
@code{require} statements inside the relevant functions rather than at
the top-level.  Or use @code{autoload} statements to load the extra
library when needed.  This way people who don't use those aspects of
your file do not need to load the extra library.

@item
If you need Common Lisp extensions, use the @code{cl-lib} library
rather than the old @code{cl} library.  The latter does not
use a clean namespace (i.e., its definitions do not
start with a @samp{cl-} prefix).  If your package loads @code{cl} at
run time, that could cause name clashes for users who don't use that
package.

There is no problem with using the @code{cl} package at @emph{compile}
time, with @code{(eval-when-compile (require 'cl))}.  That's
sufficient for using the macros in the @code{cl} package, because the
compiler expands them before generating the byte-code.  It is still
better to use the more modern @code{cl-lib} in this case, though.

@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} (which stands for ``predicate'').  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 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 unloading packages, preparing for
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.  Normally
you should use the standard names instead.  The case where an alias
may be useful is where it facilitates backwards compatibility or
portability.

@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 a bad idea.  It may do
the right thing for a particular program, but there is no telling what
other programs might break as a result.

@item
It is likewise a bad idea for one Lisp package to advise a function in
another Lisp package (@pxref{Advising Functions}).

@item
Avoid using @code{eval-after-load} in libraries and packages
(@pxref{Hooks for Loading}).  This feature is meant for personal
customizations; using it in a Lisp program is unclean, because it
modifies the behavior of another Lisp file in a way that's not visible
in that file.  This is an obstacle for debugging, much like advising a
function in the other package.

@item
If a file does replace any of the standard functions or library
programs of 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
Constructs that define a function or variable should be macros,
not functions, and their names should start with @samp{define-}.
The macro should receive the name to be
defined as the first argument.  That will help various tools find the
definition automatically.  Avoid constructing the names in the macro
itself, since that would confuse these tools.

@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.)  People will find Emacs more
coherent if all libraries use the same conventions.

@item
The default file coding system for Emacs Lisp source files is UTF-8
(@pxref{Text Representations}).  In the rare event that your program
contains characters which are @emph{not} in UTF-8, you should specify
an appropriate coding system in the source file's @samp{-*-} line or
local variables list.  @xref{File Variables, , Local Variables in
Files, emacs, The GNU Emacs Manual}.

@item
Indent the file using the default indentation parameters.

@item
Don't make a habit of putting close-parentheses on lines by
themselves; Lisp programmers find this disconcerting.

@item
Please put a copyright notice and copying permission notice on the
file if you distribute copies.  @xref{Library Headers}.

@end itemize

@node Key Binding Conventions
@section Key Binding Conventions
@cindex key binding, conventions for

@itemize @bullet
@item
@cindex mouse-2
@cindex references, following
Many special major modes, like Dired, Info, Compilation, and Occur,
are designed to handle read-only text that contains @dfn{hyper-links}.
Such a major mode should redefine @kbd{mouse-2} and @key{RET} to
follow the links.  It should also set up a @code{follow-link}
condition, so that the link obeys @code{mouse-1-click-follows-link}.
@xref{Clickable Text}.  @xref{Buttons}, for an easy method of
implementing such clickable links.

@item
@cindex reserved keys
@cindex keys, reserved
Don't define @kbd{C-c @var{letter}} as a key in Lisp programs.
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
Function keys @key{F5} through @key{F9} without modifier keys are
also reserved for users to define.

@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
Don't 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
Don't 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
Similarly, don't bind a key sequence ending in @key{C-g}, since that
is commonly used to cancel a key sequence.

@item
Anything that acts like a temporary mode or state that 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 that 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.
@end itemize

@node Programming Tips
@section Emacs Programming Tips
@cindex programming conventions

  Following these conventions will make your program fit better
into Emacs when it runs.

@itemize @bullet
@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}
@item
@code{insert-file}, @code{insert-buffer}
@end itemize

If you just want to move point, or replace a certain string, or insert
a file or buffer's contents, 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 show 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}.

Don't 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
A question asked in the minibuffer with @code{yes-or-no-p} or
@code{y-or-n-p} should start with a capital letter and end with
@samp{? }.

@item
When you mention a default value in a minibuffer prompt,
put it and the word @samp{default} inside parentheses.
It should look like this:

@example
Enter the answer (default 42):
@end example

@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}.  @xref{Progress}, for an easy way
to generate such messages.

@item
Try to avoid using recursive edits.  Instead, do what the Rmail @kbd{e}
command does: use a new local keymap that contains a command defined
to switch back to the old local keymap.  Or simply switch to another
buffer and let the user switch back at will.  @xref{Recursive Editing}.
@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
Profile your program, to find out where the time is being spent.
@xref{Profiling}.

@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

@noindent
Note that in this case (and many others), you must first load the
@file{bytecomp} library, which defines the @code{byte-compile} property.

@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 Warning Tips
@section Tips for Avoiding Compiler Warnings
@cindex byte compiler warnings, how to avoid

@itemize @bullet
@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
Similarly, to avoid a compiler warning about an undefined function
that you know @emph{will} be defined, use a @code{declare-function}
statement (@pxref{Declaring Functions}).

@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.  For instance,

@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
The last resort for avoiding a warning, when you want to do something
that is usually a mistake but you know is not a mistake in your usage,
is to put it inside @code{with-no-warnings}.  @xref{Compiler Errors}.
@end itemize

@node Documentation Tips
@section Tips for Documentation Strings
@cindex documentation strings, conventions and tips

@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.  Documentation strings 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}.

@vindex emacs-lisp-docstring-fill-column
You can fill the text if that looks good.  Emacs Lisp mode fills
documentation strings to the width specified by
@code{emacs-lisp-docstring-fill-column}.  However, you can sometimes
make a documentation string much more readable by adjusting its line
breaks with care.  Use blank lines between sections 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 it 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
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
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
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 first 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} that 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
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!

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

@cindex hyperlinks in documentation strings
Help mode automatically creates a hyperlink when a documentation string
uses a symbol name between grave accent and apostrophe, 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 words @samp{symbol} or @samp{program} 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) between grave accent and apostrophe, 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

Finally, to create a hyperlink to URLs, write the URL between grave
accent and apostrophe, preceded by @samp{URL}. For example,

@smallexample
The home page for the GNU project has more information (see URL
`http://www.gnu.org/').
@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.

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

@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}).

@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
Avoid using ``iff'' (a mathematics term meaning ``if and only if''),
since many people are unfamiliar with it and mistake it for a typo.  In
most cases, the meaning is clear with just ``if''.  Otherwise, try to
find an alternate phrasing that conveys the meaning.

@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
When you define a variable that represents an option users might want
to set, use @code{defcustom}.  @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'', to make it clear that
all non-@code{nil} values are equivalent and indicate explicitly what
@code{nil} and non-@code{nil} mean.
@end itemize

@node Comment Tips
@section Tips on Writing Comments
@cindex comments, Lisp convention for

  We recommend these conventions for comments:

@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 that line does its job.
For example:

@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

If a function has no documentation string, it 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.  It is much better to convert such comments to documentation
strings, though.

@item ;;;
Comments that start with three semicolons, @samp{;;;}, should start at
the left margin.  We use them
for comments which 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 fewer are not.  Historically, triple-semicolon comments have
also been used for commenting out lines within a function, but this use
is discouraged.

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
Generally speaking, the @kbd{M-;} (@code{comment-dwim}) command
automatically starts a comment of the appropriate type; or indents an
existing comment to the right place, 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.  Using a standard format for these items makes it easier for
tools (and people) to extract the relevant information.  This section
explains these conventions, starting with an example:

@smallexample
@group
;;; foo.el --- Support for the Foo programming language

;; Copyright (C) 2010-2015 Your Name
@end group

;; Author: Your Name <yourname@@example.com>
;; Maintainer: Someone Else <someone@@example.com>
;; Created: 14 Jul 2010
@group
;; Keywords: languages
;; Homepage: http://example.com/foo

;; This file is not part of GNU Emacs.

;; This file is free software@dots{}
@dots{}
;; along with this file.  If not, see <http://www.gnu.org/licenses/>.
@end group
@end smallexample

  The very first line should have this format:

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

@noindent
The description should be contained in one line.  If the file
needs a @samp{-*-} specification, put it after @var{description}.
If this would make the first line too long, use a Local Variables
section at the end of the file.

  The copyright notice usually lists your name (if you wrote the
file).  If you have an employer who claims copyright on your work, you
might need to list them instead.  Do not say that the copyright holder
is the Free Software Foundation (or that the file is part of GNU
Emacs) unless your file has been accepted into the Emacs distribution.
For more information on the form of copyright and license notices, see
@uref{http://www.gnu.org/licenses/gpl-howto.html, the guide on the GNU
website}.

  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 email address of at least the principal
author of the library.  If there are multiple authors, list them on
continuation lines led by @code{;;} and a tab or at least two spaces.
We recommend including a contact email address, of the form
@samp{<@dots{}>}.  For example:

@smallexample
@group
;; Author: Your Name <yourname@@example.com>
;;      Someone Else <someone@@example.com>
;;      Another Person <another@@example.com>
@end group
@end smallexample

@item Maintainer
This header has the same format as the Author header.  It lists the
person(s) who currently maintain(s) the file (respond to bug reports,
etc.).

If there is no maintainer line, the person(s) in the Author field
is/are presumed to be the maintainers.  Some files in Emacs use
@samp{FSF} for the maintainer.  This means that the original author is
no longer responsible for the file, and that it is maintained as part
of Emacs.

@item Created
This optional line gives the original creation date of the file, and
is for historical interest only.

@item Version
If you wish to record version numbers for the individual Lisp program,
put them in this line.  Lisp files distributed with Emacs generally do
not have a @samp{Version} header, since the version number of Emacs
itself serves the same purpose.  If you are distributing a collection
of multiple files, we recommend not writing the version in every file,
but only the main one.

@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 how people will find your package when they're looking
for things by topic.  To separate the keywords, you can use spaces,
commas, or both.

The name of this field is unfortunate, since people often assume it is
the place to write arbitrary keywords that describe their package,
rather than just the relevant Finder keywords.

@item Homepage
This line states the homepage of the library.

@item Package-Version
If @samp{Version} is not suitable for use by the package manager, then
a package can define @samp{Package-Version}; it will be used instead.
This is handy if @samp{Version} is an RCS id or something else that
cannot be parsed by @code{version-to-list}.  @xref{Packaging Basics}.

@item Package-Requires
If this exists, it names packages on which the current package depends
for proper operation.  @xref{Packaging Basics}.  This is used by the
package manager both at download time (to ensure that a complete set
of packages is downloaded) and at activation time (to ensure that a
package is only activated if all its dependencies have been).

Its format is a list of lists.  The @code{car} of each sub-list is the
name of a package, as a symbol.  The @code{cadr} of each sub-list is
the minimum acceptable version number, as a string.  For instance:

@smallexample
;; Package-Requires: ((gnus "1.0") (bubbles "2.7.2"))
@end smallexample

The package code automatically defines a package named @samp{emacs}
with the version number of the currently running Emacs.  This can be
used to require a minimal version of Emacs for a package.
@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 from anything else by blank
lines.  Here is a table of them:

@cindex commentary, in a Lisp library
@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 ;;; Change Log:
This begins an optional log of changes to the file over time.  Don't
put too much information in this section---it is better to keep the
detailed logs in a version control system (as Emacs does) or in a
separate @file{ChangeLog} file.  @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