@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993 Free Software Foundation, Inc.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1995, 1998, 1999, 2002, 2003,
+@c 2004, 2005 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/tips
-@node Tips, GNU Emacs Internals, Calendar, Top
-@appendix Tips and Standards
+@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.
+ 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
-* Style Tips:: Writing clean and robust programs.
+* 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 Style Tips
-@section Writing Clean Lisp Programs
+@node Coding Conventions
+@section Emacs Lisp Coding Conventions
- Here are some tips for avoiding common errors in writing Lisp code
-intended for widespread use:
+ 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. Then take care to begin the
-names of all global variables, constants, and functions with the chosen
-prefix. This helps avoid name conflicts.
+Simply loading the 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
+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. (Occasionally, for a command
+name intended for users to use, it is cleaner if some words come
+before the package name prefix.)
This recommendation applies even to names for traditional Lisp
-primitives that are not primitives in Emacs Lisp---even to @code{cadr}.
-Believe it or not, there is more than one plausible way to define
-@code{cadr}. Play it safe; append your name prefix to produce a name
-like @code{foo-cadr} or @code{mylib-cadr} instead.
+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@@prep.ai.mit.edu} suggesting we add
+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
+If one prefix is insufficient, your package can 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,
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.
+Put a call to @code{provide} at the end of each separate Lisp file.
+
+@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.
@item
If one file @var{foo} uses a macro defined in another file @var{bar},
-@var{foo} should contain @code{(require '@var{bar})} before the first
-use of the macro. (And @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}.
+@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
-If you define a major mode, make sure to run a hook variable using
-@code{run-hooks}, just as the existing major modes do. @xref{Hooks}.
+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, with @code{(eval-when-compile (require 'cl))}.
@item
-Please do not define @kbd{C-c @var{letter}} as a key in your major
-modes. These sequences are reserved for users; they are the
-@strong{only} sequences reserved for users, so we cannot do without
-them.
+When defining a major mode, please follow the major mode
+conventions. @xref{Major Mode Conventions}.
-Instead, define sequences consisting of @kbd{C-c} followed by a
-non-letter. These sequences are reserved for major modes.
+@item
+When defining a minor mode, please follow the minor mode
+conventions. @xref{Minor Mode Conventions}.
-Changing all the major modes in Emacs 18 so they would follow this
-convention was a lot of work. Abandoning this convention would waste
-that work and inconvenience the users.
+@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
-You should 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.
+If a user option variable records a true-or-false condition, give it a
+name that ends in @samp{-flag}.
@item
-You should 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}}.)
+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}.
-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
+@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. 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
-It is a bad idea to define aliases for the Emacs primitives.
-Use the standard names instead.
+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 an Emacs primitive is an even worse idea.
-It may do the right thing for a particular program, but
-there is no telling what other programs might break as a result.
+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
replacements differs from that of the originals.
@item
-If a file requires certain standard library programs to be loaded
-beforehand, then the comments at the beginning of the file should say
-so.
+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
will be 14 characters or less, which is short enough to fit on all kinds
of Unix systems.
+@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
+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 and copying permission notice on the
+file if you distribute copies. Use a notice 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., 51 Franklin Street, Fifth Floor, Boston,
+;; MA 02110-1301 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. See also @xref{Library Headers}.
+@end itemize
+
+@node Key Binding Conventions
+@section Key Binding Conventions
+
+@itemize @bullet
+@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.
+
+In addition, they should mark the text as a kind of ``link'' so that
+@kbd{mouse-1} will follow it also. @xref{Links and Mouse-1}.
+
+@item
+@cindex reserved keys
+@cindex keys, reserved
+Please do not 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
+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
+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
+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.
+@end itemize
+
+@node Programming Tips
+@section Emacs Programming Tips
+
+ 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 use functions that set the mark in your Lisp code (unless you are
-writing a command to set the mark). 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}.
+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 these functions:
+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, without any
-of the other features intended for interactive users, you can replace
-these functions with one or two lines of simple Lisp code.
+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 print a message in the echo area is with
+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
or @code{beep} to report errors.
@item
-Avoid using recursive edits. Instead, do what the Rmail @kbd{w} 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}.
+An error message should start with a capital letter but should not end
+with a period.
@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 library. (In fact, in Emacs names
-of this form are conventionally used for program-generated buffers.) The
-users will find Emacs more coherent if all libraries use the same
-conventions.
+A question asked in the minibuffer with @code{y-or-n-p} or
+@code{yes-or-no-p} should start with a capital letter and end with
+@samp{? }.
@item
-Indent each function with @kbd{C-M-q} (@code{indent-sexp}) using the
-default indentation parameters.
+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
-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 them in one or two significant places.
+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
-Please put a copyright notice on the file if you give copies to anyone.
-Use the same lines that appear at the top of the Lisp files in Emacs
-itself. If you have not signed papers to assign the copyright to the
-Foundation, then place your name in the copyright notice in place of the
-Foundation's name.
+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}.
@end itemize
@node Compilation Tips
@cindex speedups
Here are ways of improving the execution speed of byte-compiled
-lisp programs.
+Lisp programs.
@itemize @bullet
@item
@cindex profiling
@cindex timing programs
-@cindex @file{profile.el}
-Use the @file{profile} library to profile your program. See the file
-@file{profile.el} for instructions.
+@cindex @file{elp.el}
+Profile your program with the @file{elp} library. See the file
+@file{elp.el} for instructions.
+
+@item
+@cindex @file{benchmark.el}
+@cindex benchmarking
+Check the speed of individual Emacs Lisp forms using the
+@file{benchmark} library. See the functions @code{benchmark-run} and
+@code{benchmark-run-compiled} in @file{benchmark.el}.
@item
Use iteration rather than recursion whenever possible.
is calling another compiled function.
@item
-Using the primitive list-searching functions @code{memq}, @code{assq} or
-@code{assoc} is even faster than explicit iteration. It may be worth
-rearranging a data structure so that one of these primitive search
-functions can be used.
+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 by the byte compiler
+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}
handled specially.
For example, the following input will show you that @code{aref} is
-compiled specially (@pxref{Array Functions}) while @code{elt} is not
-(@pxref{Sequence Functions}):
+compiled specially (@pxref{Array Functions}):
-@smallexample
+@example
@group
(get 'aref 'byte-compile)
@result{} byte-compile-two-args
@end group
-
-@group
-(get 'elt 'byte-compile)
- @result{} nil
-@end group
-@end smallexample
+@end example
@item
-If calling a small function accounts for a substantial part of your
+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 for users to care about
+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
+
+@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
+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 usually is a mistake but it's not a mistake in this one case,
+is to put a call to @code{with-no-warnings} around it.
+@end itemize
+
@node Documentation Tips
@section Tips for Documentation Strings
- Here are some tips for the writing of 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
+Every command, function, or variable intended for users to know about
should have a documentation string.
@item
-An internal subroutine of a Lisp program need not have a documentation
-string, and you can save space by using a comment instead.
+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 which stand on their own as a summary. In particular,
-start the line with a capital letter and end with a period.
-For instance, use ``Return the cons of A and B.'' in preference to
-``Returns the cons of A and B@.''
+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.
-The documentation string can have additional lines which expand on the
-details of how to use the function or variable. The additional lines
-should be made up of complete sentences also, but they may be filled if
-that looks good.
+@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
``Display text in boldface.''
@item
-Do not start or end a documentation string with whitespace.
+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
-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 can be wider if necessary to fit the
-information that ought to be there.
+Do not start or end a documentation string with whitespace.
-However, rather than simply filling the entire documentation string, you
-can make it much more readable by choosing line breaks with care.
-Use blank lines between topics if the documentation string is long.
-
@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
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 want to set interactively often. If
-the value is a long list, or a function, or if the variable would only
-be set in init files, then don't start the documentation string with
+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 both
-that the variable only has two meaningfully distinct values and which value
-means ``yes''.
+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{/} refers to its second argument as @samp{DIVISOR}.
+@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 use all caps for meta-syntactic variables, such as when you show
-the decomposition of a list or vector into subunits, some of which may
-vary.
+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
around it. For example: @samp{`lambda'}. There are two exceptions:
write @code{t} and @code{nil} without single-quotes.
@end iftex
-@ifinfo
+@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 normally do use
-single-quotes for those symbols.)
-@end ifinfo
+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
+
+Finally, to create a hyperlink to URLs, write the URL in single
+quotes, 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 @samp{\\[forward-char]}. When the
-documentation string is printed, Emacs will substitute whatever key is
-currently bound to @code{forward-char}. This will usually be
-@samp{C-f}, but if the user has moved key bindings, it will be the
-correct key for that user. @xref{Keys in Documentation}.
+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
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
-Don't use the term ``Elisp'', since that is or was a trademark.
-Use the term ``Emacs Lisp''.
@end itemize
@node Comment Tips
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 inserted.
+aligns such a comment if it is already present.
-(The following examples are taken from the Emacs sources.)
+This and following examples are taken from the Emacs sources.
@smallexample
@group
@item ;;
Comments that start with two semicolons, @samp{;;}, should be aligned to
-the same level of indentation as the code. Such comments are used 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:
(prog1 (setq auto-fill-function
@dots{}
@dots{}
- ;; update mode-line
+ ;; update mode line
(force-mode-line-update)))
@end group
@end smallexample
-These comments are also written before a function definition to explain
-what the function does and how to call it properly.
+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. Such comments are not used within function
-definitions, but are used to make more general comments. For example:
+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
-@group
-;;; This Lisp code is run in Emacs
-;;; when it is to operate as a server
-;;; for other processes.
-@end group
+(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
@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})
+(@code{indent-for-comment}) and @key{TAB} (@code{lisp-indent-line}),
automatically indent comments according to these conventions,
-depending on the the number of semicolons. @xref{Comments,,
+depending on the number of semicolons. @xref{Comments,,
Manipulating Comments, emacs, The GNU Emacs Manual}.
- If you wish to ``comment out'' a number of lines of code, use triple
-semicolons at the beginnings of the lines.
-
- Any character may be included in a comment, but it is advisable to
-precede a character with syntactic significance in Lisp (such as
-@samp{\} or unpaired @samp{(} or @samp{)}) with a @samp{\}, to prevent
-it from confusing the Emacs commands for editing Lisp.
-
@node Library Headers
@section Conventional Headers for Emacs Libraries
@cindex header comments
@cindex library header comments
- Emacs 19 has conventions for using special comments in Lisp libraries
+ 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. First, an example:
+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
;; Keywords: docs
;; This file is part of GNU Emacs.
-@var{copying conditions}@dots{}
+@dots{}
+;; Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+;; Boston, MA 02110-1301, USA.
@end group
@end smallexample
@end example
@noindent
-The description should be complete in one line.
+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
+each beginning with @samp{;; @var{header-name}:}. Here is a table of
the conventional possibilities for @var{header-name}:
@table @samp
author of the library.
If there are multiple authors, you can list them on continuation lines
-led by @code{;;<TAB>}, like this:
+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>
+;; 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 ``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.
+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
@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.
+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
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. Here is a table of them:
+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.
+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:
+@item ;;; Change Log:
This begins change log information stored in the library file (if you
-store the change history there). For most of the 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 do
-not have a @samp{;;; Change log:} line.
+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.
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
HCoop / bpt / emacs.git / blobdiff