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Xref the Major Mode Conventions and Minor Mode Conventions nodes. Other smaller changes in it.
689 lines
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689 lines
25 KiB
Plaintext
@c -*-texinfo-*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993 Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../info/tips
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@node Tips, GNU Emacs Internals, Calendar, Top
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@appendix Tips and Conventions
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@cindex tips
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@cindex standards of coding style
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@cindex coding standards
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This chapter describes no additional features of Emacs Lisp. Instead
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it gives advice on making effective use of the features described in the
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previous chapters, and describes conventions Emacs Lisp programmers
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should follow.
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@menu
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* Coding Conventions:: Conventions for clean and robust programs.
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* Compilation Tips:: Making compiled code run fast.
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* Documentation Tips:: Writing readable documentation strings.
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* Comment Tips:: Conventions for writing comments.
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* Library Headers:: Standard headers for library packages.
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@end menu
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@node Coding Conventions
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@section Emacs Lisp Coding Conventions
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Here are conventions that you should follow when writing Emacs Lisp
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code intended for widespread use:
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@itemize @bullet
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@item
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Since all global variables share the same name space, and all functions
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share another name space, you should choose a short word to distinguish
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your program from other Lisp programs. Then take care to begin the
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names of all global variables, constants, and functions with the chosen
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prefix. This helps avoid name conflicts.
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This recommendation applies even to names for traditional Lisp
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primitives that are not primitives in Emacs Lisp---even to @code{cadr}.
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Believe it or not, there is more than one plausible way to define
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@code{cadr}. Play it safe; append your name prefix to produce a name
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like @code{foo-cadr} or @code{mylib-cadr} instead.
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If you write a function that you think ought to be added to Emacs under
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a certain name, such as @code{twiddle-files}, don't call it by that name
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in your program. Call it @code{mylib-twiddle-files} in your program,
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and send mail to @samp{bug-gnu-emacs@@prep.ai.mit.edu} suggesting we add
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it to Emacs. If and when we do, we can change the name easily enough.
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If one prefix is insufficient, your package may use two or three
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alternative common prefixes, so long as they make sense.
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Separate the prefix from the rest of the symbol name with a hyphen,
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@samp{-}. This will be consistent with Emacs itself and with most Emacs
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Lisp programs.
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@item
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It is often useful to put a call to @code{provide} in each separate
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library program, at least if there is more than one entry point to the
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program.
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@item
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If a file requires certain other library programs to be loaded
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beforehand, then the comments at the beginning of the file should say
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so. Also, use @code{require} to make sure they are loaded.
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@item
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If one file @var{foo} uses a macro defined in another file @var{bar},
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@var{foo} should contain this expression before the first use of the
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macro:
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@example
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(eval-when-compile (require '@var{bar}))
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@end example
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@noindent
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(And @var{bar} should contain @code{(provide '@var{bar})}, to make the
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@code{require} work.) This will cause @var{bar} to be loaded when you
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byte-compile @var{foo}. Otherwise, you risk compiling @var{foo} without
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the necessary macro loaded, and that would produce compiled code that
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won't work right. @xref{Compiling Macros}.
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Using @code{eval-when-compile} avoids loading @var{bar} when
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the compiled version of @var{foo} is @emph{used}.
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@item
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When defining a major mode, please follow the major mode
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conventions. @xref{Major Mode Conventions}.
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@item
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When defining a minor mode, please follow the minor mode
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conventions. @xref{Minor Mode Conventions}.
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@item
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If the purpose of a function is to tell you whether a certain condition
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is true or false, give the function a name that ends in @samp{p}. If
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the name is one word, add just @samp{p}; if the name is multiple words,
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add @samp{-p}. Examples are @code{framep} and @code{frame-live-p}.
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@item
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If a user option variable records a true-or-false condition, give it a
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name that ends in @samp{-flag}.
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@item
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Please do not define @kbd{C-c @var{letter}} as a key in your major
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modes. These sequences are reserved for users; they are the
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@strong{only} sequences reserved for users, so we cannot do without
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them.
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Instead, define sequences consisting of @kbd{C-c} followed by a control
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character, a digit, or certain punctuation characters. These sequences
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are reserved for major modes.
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Changing all the major modes in Emacs 18 so they would follow this
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convention was a lot of work. Abandoning this convention would make
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that work go to waste, and inconvenience users.
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@item
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Sequences consisting of @kbd{C-c} followed by @kbd{@{}, @kbd{@}},
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@kbd{<}, @kbd{>}, @kbd{:} or @kbd{;} are also reserved for major modes.
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@item
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Sequences consisting of @kbd{C-c} followed by any other punctuation
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character are allocated for minor modes. Using them in a major mode is
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not absolutely prohibited, but if you do that, the major mode binding
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may be shadowed from time to time by minor modes.
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@item
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You should not bind @kbd{C-h} following any prefix character (including
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@kbd{C-c}). If you don't bind @kbd{C-h}, it is automatically available
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as a help character for listing the subcommands of the prefix character.
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@item
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You should not bind a key sequence ending in @key{ESC} except following
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another @key{ESC}. (That is, it is ok to bind a sequence ending in
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@kbd{@key{ESC} @key{ESC}}.)
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The reason for this rule is that a non-prefix binding for @key{ESC} in
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any context prevents recognition of escape sequences as function keys in
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that context.
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@item
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Applications should not bind mouse events based on button 1 with the
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shift key held down. These events include @kbd{S-mouse-1},
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@kbd{M-S-mouse-1}, @kbd{C-S-mouse-1}, and so on. They are reserved for
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users.
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@item
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Modes should redefine @kbd{mouse-2} as a command to follow some sort of
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reference in the text of a buffer, if users usually would not want to
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alter the text in that buffer by hand. Modes such as Dired, Info,
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Compilation, and Occur redefine it in this way.
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@item
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When a package provides a modification of ordinary Emacs behavior, it is
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good to include a command to enable and disable the feature, Provide a
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command named @code{@var{whatever}-mode} which turns the feature on or
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off, and make it autoload (@pxref{Autoload}). Design the package so
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that simply loading it has no visible effect---that should not enable
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the feature. Users will request the feature by invoking the command.
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@item
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It is a bad idea to define aliases for the Emacs primitives. Use the
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standard names instead.
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@item
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Redefining (or advising) an Emacs primitive is discouraged. It may do
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the right thing for a particular program, but there is no telling what
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other programs might break as a result.
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@item
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If a file does replace any of the functions or library programs of
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standard Emacs, prominent comments at the beginning of the file should
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say which functions are replaced, and how the behavior of the
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replacements differs from that of the originals.
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@item
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Please keep the names of your Emacs Lisp source files to 13 characters
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or less. This way, if the files are compiled, the compiled files' names
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will be 14 characters or less, which is short enough to fit on all kinds
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of Unix systems.
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@item
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Don't use @code{next-line} or @code{previous-line} in programs; nearly
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always, @code{forward-line} is more convenient as well as more
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predictable and robust. @xref{Text Lines}.
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@item
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Don't call functions that set the mark, unless setting the mark is one
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of the intended features of your program. The mark is a user-level
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feature, so it is incorrect to change the mark except to supply a value
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for the user's benefit. @xref{The Mark}.
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In particular, don't use these functions:
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@itemize @bullet
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@item
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@code{beginning-of-buffer}, @code{end-of-buffer}
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@item
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@code{replace-string}, @code{replace-regexp}
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@end itemize
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If you just want to move point, or replace a certain string, without any
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of the other features intended for interactive users, you can replace
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these functions with one or two lines of simple Lisp code.
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@item
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Use lists rather than vectors, except when there is a particular reason
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to use a vector. Lisp has more facilities for manipulating lists than
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for vectors, and working with lists is usually more convenient.
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Vectors are advantageous for tables that are substantial in size and are
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accessed in random order (not searched front to back), provided there is
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no need to insert or delete elements (only lists allow that).
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@item
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The recommended way to print a message in the echo area is with
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the @code{message} function, not @code{princ}. @xref{The Echo Area}.
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@item
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When you encounter an error condition, call the function @code{error}
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(or @code{signal}). The function @code{error} does not return.
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@xref{Signaling Errors}.
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Do not use @code{message}, @code{throw}, @code{sleep-for},
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or @code{beep} to report errors.
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@item
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An error message should start with a capital letter but should not end
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with a period.
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@item
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Many commands that take a long time to execute display a message that
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says @samp{Operating...} when they start, and change it to
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@samp{Operating...done} when they finish. Please keep the style of
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these messages uniform: @emph{no} space around the ellipsis, and
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@emph{no} period at the end.
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@item
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Try to avoid using recursive edits. Instead, do what the Rmail @kbd{e}
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command does: use a new local keymap that contains one command defined
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to switch back to the old local keymap. Or do what the
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@code{edit-options} command does: switch to another buffer and let the
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user switch back at will. @xref{Recursive Editing}.
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@item
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In some other systems there is a convention of choosing variable names
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that begin and end with @samp{*}. We don't use that convention in Emacs
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Lisp, so please don't use it in your programs. (Emacs uses such names
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only for program-generated buffers.) The users will find Emacs more
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coherent if all libraries use the same conventions.
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@item
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Try to avoid compiler warnings about undefined free variables, by adding
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@code{defvar} definitions for these variables.
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If you bind a variable in one function, and use it or set it in another
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function, the compiler warns about the latter function unless the
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variable has a definition. But often these variables have short names,
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and it is not clean for Lisp packages to define such variables names.
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Therefore, you should rename the variable to start with the name prefix
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used for the other functions and variables in your package.
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@item
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Indent each function with @kbd{C-M-q} (@code{indent-sexp}) using the
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default indentation parameters.
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@item
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Don't make a habit of putting close-parentheses on lines by themselves;
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Lisp programmers find this disconcerting. Once in a while, when there
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is a sequence of many consecutive close-parentheses, it may make sense
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to split them in one or two significant places.
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@item
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Please put a copyright notice on the file if you give copies to anyone.
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Use the same lines that appear at the top of the Lisp files in Emacs
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itself. If you have not signed papers to assign the copyright to the
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Foundation, then place your name in the copyright notice in place of the
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Foundation's name.
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@end itemize
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@node Compilation Tips
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@section Tips for Making Compiled Code Fast
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@cindex execution speed
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@cindex speedups
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Here are ways of improving the execution speed of byte-compiled
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Lisp programs.
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@itemize @bullet
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@item
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@cindex profiling
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@cindex timing programs
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@cindex @file{profile.el}
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Use the @file{profile} library to profile your program. See the file
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@file{profile.el} for instructions.
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@item
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Use iteration rather than recursion whenever possible.
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Function calls are slow in Emacs Lisp even when a compiled function
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is calling another compiled function.
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@item
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Using the primitive list-searching functions @code{memq}, @code{member},
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@code{assq}, or @code{assoc} is even faster than explicit iteration. It
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may be worth rearranging a data structure so that one of these primitive
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search functions can be used.
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@item
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Certain built-in functions are handled specially in byte-compiled code,
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avoiding the need for an ordinary function call. It is a good idea to
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use these functions rather than alternatives. To see whether a function
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is handled specially by the compiler, examine its @code{byte-compile}
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property. If the property is non-@code{nil}, then the function is
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handled specially.
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For example, the following input will show you that @code{aref} is
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compiled specially (@pxref{Array Functions}) while @code{elt} is not
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(@pxref{Sequence Functions}):
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@example
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@group
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(get 'aref 'byte-compile)
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@result{} byte-compile-two-args
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@end group
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@group
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(get 'elt 'byte-compile)
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@result{} nil
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@end group
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@end example
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@item
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If calling a small function accounts for a substantial part of your
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program's running time, make the function inline. This eliminates
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the function call overhead. Since making a function inline reduces
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the flexibility of changing the program, don't do it unless it gives
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a noticeable speedup in something slow enough that users care about
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the speed. @xref{Inline Functions}.
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@end itemize
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@node Documentation Tips
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@section Tips for Documentation Strings
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Here are some tips for the writing of documentation strings.
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@itemize @bullet
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@item
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Every command, function, or variable intended for users to know about
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should have a documentation string.
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@item
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An internal variable or subroutine of a Lisp program might as well have
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a documentation string. In earlier Emacs versions, you could save space
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by using a comment instead of a documentation string, but that is no
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longer the case.
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@item
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The first line of the documentation string should consist of one or two
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complete sentences that stand on their own as a summary. @kbd{M-x
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apropos} displays just the first line, and if it doesn't stand on its
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own, the result looks bad. In particular, start the first line with a
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capital letter and end with a period.
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The documentation string can have additional lines that expand on the
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details of how to use the function or variable. The additional lines
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should be made up of complete sentences also, but they may be filled if
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that looks good.
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@item
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For consistency, phrase the verb in the first sentence of a
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documentation string as an infinitive with ``to'' omitted. For
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instance, use ``Return the cons of A and B.'' in preference to ``Returns
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the cons of A and B@.'' Usually it looks good to do likewise for the
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rest of the first paragraph. Subsequent paragraphs usually look better
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if they have proper subjects.
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@item
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Write documentation strings in the active voice, not the passive, and in
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the present tense, not the future. For instance, use ``Return a list
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containing A and B.'' instead of ``A list containing A and B will be
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returned.''
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@item
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Avoid using the word ``cause'' (or its equivalents) unnecessarily.
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Instead of, ``Cause Emacs to display text in boldface,'' write just
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``Display text in boldface.''
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@item
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Do not start or end a documentation string with whitespace.
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@item
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Format the documentation string so that it fits in an Emacs window on an
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80-column screen. It is a good idea for most lines to be no wider than
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60 characters. The first line can be wider if necessary to fit the
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information that ought to be there.
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However, rather than simply filling the entire documentation string, you
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can make it much more readable by choosing line breaks with care.
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Use blank lines between topics if the documentation string is long.
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@item
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@strong{Do not} indent subsequent lines of a documentation string so
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that the text is lined up in the source code with the text of the first
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line. This looks nice in the source code, but looks bizarre when users
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view the documentation. Remember that the indentation before the
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starting double-quote is not part of the string!
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@item
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When the user tries to use a disabled command, Emacs displays just the
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first paragraph of its documentation string---everything through the
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first blank line. If you wish, you can choose which information to
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include before the first blank line so as to make this display useful.
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@item
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A variable's documentation string should start with @samp{*} if the
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variable is one that users would often want to set interactively. If
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the value is a long list, or a function, or if the variable would be set
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only in init files, then don't start the documentation string with
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@samp{*}. @xref{Defining Variables}.
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@item
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The documentation string for a variable that is a yes-or-no flag should
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start with words such as ``Non-nil means@dots{}'', to make it clear that
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all non-@code{nil} values are equivalent and indicate explicitly what
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@code{nil} and non-@code{nil} mean.
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@item
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When a function's documentation string mentions the value of an argument
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of the function, use the argument name in capital letters as if it were
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a name for that value. Thus, the documentation string of the function
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@code{/} refers to its second argument as @samp{DIVISOR}, because the
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actual argument name is @code{divisor}.
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Also use all caps for meta-syntactic variables, such as when you show
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the decomposition of a list or vector into subunits, some of which may
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vary.
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@item
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@iftex
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When a documentation string refers to a Lisp symbol, write it as it
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would be printed (which usually means in lower case), with single-quotes
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around it. For example: @samp{`lambda'}. There are two exceptions:
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write @code{t} and @code{nil} without single-quotes.
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@end iftex
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@ifinfo
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When a documentation string refers to a Lisp symbol, write it as it
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would be printed (which usually means in lower case), with single-quotes
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around it. For example: @samp{lambda}. There are two exceptions: write
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t and nil without single-quotes. (In this manual, we normally do use
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single-quotes for those symbols.)
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@end ifinfo
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@item
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Don't write key sequences directly in documentation strings. Instead,
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use the @samp{\\[@dots{}]} construct to stand for them. For example,
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instead of writing @samp{C-f}, write the construct
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@samp{\\[forward-char]}. When Emacs displays the documentation string,
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it substitutes whatever key is currently bound to @code{forward-char}.
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(This is normally @samp{C-f}, but it may be some other character if the
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user has moved key bindings.) @xref{Keys in Documentation}.
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@item
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In documentation strings for a major mode, you will want to refer to the
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key bindings of that mode's local map, rather than global ones.
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Therefore, use the construct @samp{\\<@dots{}>} once in the
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documentation string to specify which key map to use. Do this before
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the first use of @samp{\\[@dots{}]}. The text inside the
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@samp{\\<@dots{}>} should be the name of the variable containing the
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local keymap for the major mode.
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It is not practical to use @samp{\\[@dots{}]} very many times, because
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display of the documentation string will become slow. So use this to
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describe the most important commands in your major mode, and then use
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@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
|
|
|
|
Every function that has no documentation string (because it is use only
|
|
internally within the package it belongs to), should have instead 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 used outside function definitions to
|
|
make general statements explaining the design principles of the program.
|
|
For example:
|
|
|
|
@smallexample
|
|
@group
|
|
;;; This Lisp code is run in Emacs
|
|
;;; when it is to operate as a server
|
|
;;; for other processes.
|
|
@end group
|
|
@end smallexample
|
|
|
|
Another use for triple-semicolon comments is for commenting out lines
|
|
within a function. We use triple-semicolons for this precisely so that
|
|
they remain at the left margin.
|
|
|
|
@smallexample
|
|
(defun foo (a)
|
|
;;; This is no longer necessary.
|
|
;;; (force-mode-line-update)
|
|
(message "Finished with %s" a))
|
|
@end smallexample
|
|
|
|
@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 19 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:
|
|
|
|
@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.
|
|
@var{copying permissions}@dots{}
|
|
@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.
|
|
|
|
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.
|
|
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. 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.
|
|
|
|
@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.
|
|
|
|
@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
|