@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1995, 1998 Free Software Foundation, Inc.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1995, 1998, 1999
+@c Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/tips
-@node Tips, GNU Emacs Internals, System Interface, Top
+@node Tips, GNU Emacs Internals, GPL, Top
@appendix Tips and Conventions
@cindex tips
@cindex standards of coding style
previous chapters, and describes conventions Emacs Lisp programmers
should follow.
+ You can automatically check some of the conventions described below by
+running the command @kbd{M-x checkdoc RET} when visiting a Lisp file.
+It cannot check all of the conventions, and not all the warnings it
+gives necessarily correspond to problems, but it is worth examining them
+all.
+
@menu
* Coding Conventions:: Conventions for clean and robust programs.
* Compilation Tips:: Making compiled code run fast.
@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.
+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 with the chosen prefix. This helps avoid name conflicts.
This recommendation applies even to names for traditional Lisp
primitives that are not primitives in Emacs Lisp---even to
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
Using @code{eval-when-compile} avoids loading @var{bar} when
the compiled version of @var{foo} is @emph{used}.
+@item
+Please don't require the @code{cl} package of Common Lisp extensions at
+run time. Use of this package is optional, and it is not part of the
+standard Emacs namespace. If your package loads @code{cl} at run time,
+that could cause name clashes for users who don't use that package.
+
+However, there is no problem with using the @code{cl} package at compile
+time, for the sake of macros. You do that like this:
+
+@example
+(eval-when-compile (require 'cl))
+@end example
+
@item
When defining a major mode, please follow the major mode
conventions. @xref{Major Mode Conventions}.
name that ends in @samp{-flag}.
@item
+@cindex reserved keys
+@cindex keys, reserved
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 do not block them.
any context prevents recognition of escape sequences as function keys in
that context.
+@item
+Anything which acts like a temporary mode or state which the user can
+enter and leave should define @kbd{@key{ESC} @key{ESC}} or
+@kbd{@key{ESC} @key{ESC} @key{ESC}} as a way to escape.
+
+For a state which accepts ordinary Emacs commands, or more generally any
+kind of state in which @key{ESC} followed by a function key or arrow key
+is potentially meaningful, then you must not define @kbd{@key{ESC}
+@key{ESC}}, since that would preclude recognizing an escape sequence
+after @key{ESC}. In these states, you should define @kbd{@key{ESC}
+@key{ESC} @key{ESC}} as the way to escape. Otherwise, define
+@kbd{@key{ESC} @key{ESC}} instead.
+
@item
Applications should not bind mouse events based on button 1 with the
shift key held down. These events include @kbd{S-mouse-1},
users.
@item
+@cindex mouse-2
+@cindex references, following
Special major modes used for read-only text should usually redefine
@kbd{mouse-2} and @key{RET} to trace some sort of reference in the text.
Modes such as Dired, Info, Compilation, and Occur redefine it in this
command named @code{@var{whatever}-mode} which turns the feature on or
off, and make it autoload (@pxref{Autoload}). Design the package so
that simply loading it has no visible effect---that should not enable
-the feature. Users will request the feature by invoking the command.
+the feature.@footnote{Consider that the package may be loaded
+arbitrarily by Custom for instance.} Users will request the feature by
+invoking the command.
@item
It is a bad idea to define aliases for the Emacs primitives. Use the
standard names instead.
+@item
+If a package needs to define an alias or a new function for
+compatibility with some other version of Emacs, name if with the package
+prefix, not with the raw name with which it occurs in the other version.
+Here is an example from Gnus, which provides many examples of such
+compatibility issues.
+
+@example
+(defalias 'gnus-point-at-bol
+ (if (fboundp 'point-at-bol)
+ 'point-at-bol
+ 'line-beginning-position))
+@end example
+
@item
Redefining (or advising) an Emacs primitive is discouraged. It may do
the right thing for a particular program, but there is no telling what
Try to avoid compiler warnings about undefined free variables, by adding
@code{defvar} definitions for these variables.
+Sometimes adding a @code{require} for another package is useful to avoid
+compilation warnings for variables and functions defined in that
+package. If you do this, often it is better if the @code{require} acts
+only at compile time. Here's how to do that:
+
+@example
+(eval-when-compile
+ (require 'foo)
+ (defvar bar-baz))
+@end example
+
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 often these variables have short names,
-and it is not clean for Lisp packages to define such variables names.
+and it is not clean for Lisp packages to define such variable names.
Therefore, you should rename the variable to start with the name prefix
used for the other functions and variables in your package.
@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{profile} library or the @file{elp}
+library. See the files @file{profile.el} and @file{elp.el} for
+instructions.
@item
Use iteration rather than recursion whenever possible.
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}):
@example
@group
(get 'aref 'byte-compile)
@result{} byte-compile-two-args
@end group
-
-@group
-(get 'elt 'byte-compile)
- @result{} nil
-@end group
@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
@node Documentation Tips
@section Tips for Documentation Strings
-@tindex checkdoc-minor-mode
@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
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.
+longer the case---documentation strings now take up very little space in
+a running Emacs.
@item
The first line of the documentation string should consist of one or two
complete sentences that stand on their own as a summary. @kbd{M-x
-apropos} displays just the first line, and if it doesn't stand on its
-own, the result looks bad. In particular, start the first line with a
-capital letter and end with a period.
+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.
-The documentation string can have additional lines that 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.
+The documentation string is not limited 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 in the additional lines.
@item
-For consistency, phrase the verb in the first sentence of a
-documentation string as an infinitive with ``to'' omitted. 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 they have proper subjects.
+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
+has a proper subject.
@item
Write documentation strings in the active voice, not the passive, and in
Instead of, ``Cause Emacs to display text in boldface,'' write just
``Display text in boldface.''
+@item
+When a command is meaningful only in a certain mode or situation,
+do mention that in the documentation string. For example,
+the documentation of @code{dired-find-file} is:
+
+@example
+In Dired, visit the file or directory named on this line.
+@end example
+
@item
Do not start or end a documentation string with whitespace.
@item
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.
+60 characters. The first line should not be wider than 67 characters
+or it will look bad in the output of @code{apropos}.
-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.
+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
@strong{Do not} indent subsequent lines of a documentation string so
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}, because the
-actual argument name is @code{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 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
+If a line in a documentation string begins with an open-parenthesis,
+write a backslash before the open-parenthesis, like this:
-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.
+@example
+The argument FOO can be either a number
+\(a buffer position) or a string (a file name).
+@end example
+
+This prevents the open-parenthesis from being treated as the start of a
+defun (@pxref{Defuns,, Defuns, emacs, The GNU Emacs Manual}).
@item
@iftex
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 use a different
convention, with single-quotes for all symbols.)
-@end ifinfo
+@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.
-For example:
+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
-The value of `swim-speed' specifies how fast to swim.
-Possible values are t for high speed, nil for low speed,
-and `medium' for medium speed.
+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}.
+
+To make a hyperlink to Info documentation, write the name of the Info
+node in single quotes, preceded by @samp{info node} or @samp{Info
+node}. The Info file name defaults to @samp{emacs}. For example,
+
+@smallexample
+See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'.
+@end smallexample
+
@item
Don't write key sequences directly in documentation strings. Instead,
use the @samp{\\[@dots{}]} construct to stand for them. For example,
@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 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:
+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.
+;; This Lisp code is run in Emacs
+;; when it is to operate as a server
+;; for other processes.
@end group
@end smallexample
+Every function that has no documentation string (presumably one that is
+used only internally within the package it belongs to), should instead
+have a two-semicolon comment right before the function, explaining what
+the function does and how to call it properly. Explain precisely what
+each argument means and how the function interprets its possible values.
+
+@item ;;;
+Comments that start with three semicolons, @samp{;;;}, should start at
+the left margin. These are used, occasionally, for comments within
+functions that should start at the margin. We also use them sometimes
+for comments that are between functions---whether to use two or three
+semicolons there is a matter of style.
+
Another use for triple-semicolon comments is for commenting out lines
-within a function. We use triple-semicolons for this precisely so that
+within a function. We use three semicolons for this precisely so that
they remain at the left margin.
@smallexample
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
@item Keywords
This line lists keywords for the @code{finder-by-keyword} help command.
+Please use that command to see a list of the meaningful keywords.
+
This field is important; it's how people will find your package when
they're looking for things by topic area. To separate the keywords, you
can use spaces, commas, or both.
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 has been used in some files in place of @samp{;;; Commentary:},
+but @samp{;;; Commentary:} is preferred.
-@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.