@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, Antinews, Top
+@node Tips, GNU Emacs Internals, GPL, Top
@appendix Tips and Conventions
@cindex tips
@cindex standards of coding style
@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
+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.
This recommendation applies even to names for traditional Lisp
-primitives that are not primitives in Emacs Lisp---even to
+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}
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
+time, with @code{(eval-when-compile (require 'cl))}.
@item
When defining a major mode, please follow the major mode
If a user option variable records a true-or-false condition, give it a
name that ends in @samp{-flag}.
+@item
+If the purpose of a variable is to store a single function, give it a
+name that ends in @samp{-function}. If the purpose of a variable is
+to store a list of functions (i.e., the variable is a hook), please
+follow the naming conventions for hooks. @xref{Hooks}.
+
@item
@cindex reserved keys
@cindex keys, reserved
-Please do not define @kbd{C-c @var{letter}} as a key in your major
-modes. These sequences are reserved for users; they are the
-@strong{only} sequences reserved for users, so do not block them.
+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.
-Instead, define sequences consisting of @kbd{C-c} followed by a control
-character, a digit, or certain punctuation characters. These sequences
-are reserved for major modes.
+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.
-Changing all the Emacs major modes to follow this convention was a lot
-of work. Abandoning this convention would make that work go to waste,
-and inconvenience users.
+@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{@}},
not absolutely prohibited, but if you do that, the major mode binding
may be shadowed from time to time by minor modes.
-@item
-Function keys @key{F5} through @key{F9} without modifier keys are
-reserved for users to define.
-
@item
Do not bind @kbd{C-h} following any prefix character (including
@kbd{C-c}). If you don't bind @kbd{C-h}, it is automatically available
@kbd{@key{ESC} @key{ESC}} instead.
@item
-Applications should not bind mouse events based on button 1 with the
-shift key held down. These events include @kbd{S-mouse-1},
-@kbd{M-S-mouse-1}, @kbd{C-S-mouse-1}, and so on. They are reserved for
-users.
-
-@item
+@cindex mouse-2
+@cindex references, following
Special major modes used for read-only text should usually redefine
@kbd{mouse-2} and @key{RET} to trace some sort of reference in the text.
Modes such as Dired, Info, Compilation, and Occur redefine it in this
way.
-@item
-When a package provides a modification of ordinary Emacs behavior, it is
-good to include a command to enable and disable the feature, Provide a
-command named @code{@var{whatever}-mode} which turns the feature on or
-off, and make it autoload (@pxref{Autoload}). Design the package so
-that simply loading it has no visible effect---that should not enable
-the feature. Users will request the feature by invoking the command.
+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}.
+
+@cindex unloading packages
+If loading the file adds functions to hooks, define a function
+@code{@var{feature}-unload-hook}, where @var{feature} is the name of
+the feature the package provides, and make it undo any such changes.
+Using @code{unload-feature} to unload the file will run this function.
+@xref{Unloading}.
@item
It is a bad idea to define aliases for the Emacs primitives. Use the
standard names instead.
+@item
+If a package needs to define an alias or a new function for
+compatibility with some other version of Emacs, name it with the package
+prefix, not with the raw name with which it occurs in the other version.
+Here is an example from Gnus, which provides many examples of such
+compatibility issues.
+
+@example
+(defalias 'gnus-point-at-bol
+ (if (fboundp 'point-at-bol)
+ 'point-at-bol
+ 'line-beginning-position))
+@end example
+
@item
Redefining (or advising) an Emacs primitive is discouraged. It may do
the right thing for a particular program, but there is no telling what
say which functions are replaced, and how the behavior of the
replacements differs from that of the originals.
+@item
+Avoid using macros that define functions and variables with names that
+are constructed. It is best for maintenance when the name of the
+function or variable being defined is given explicitly in the source
+code, as the second element of the list---as it is when you use
+@code{defun}, @code{defalias}, @code{defvar} and @code{defcustom}.
+
@item
Please keep the names of your Emacs Lisp source files to 13 characters
or less. This way, if the files are compiled, the compiled files' names
An error message should start with a capital letter but should not end
with a period.
+@item
+In @code{interactive}, if you use a Lisp expression to produce a list
+of arguments, don't try to provide the ``correct'' default values for
+region or position arguments. Instead, provide @code{nil} for those
+arguments if they were not specified, and have the function body
+compute the default value when the argument is @code{nil}. For
+instance, write this:
+
+@example
+(defun foo (pos)
+ (interactive
+ (list (if @var{specified} @var{specified-pos})))
+ (unless pos (setq pos @var{default-pos}))
+ ...)
+@end example
+
+@noindent
+rather than this:
+
+@example
+(defun foo (pos)
+ (interactive
+ (list (if @var{specified} @var{specified-pos}
+ @var{default-pos})))
+ ...)
+@end example
+
+@noindent
+This is so that repetition of the command will recompute
+these defaults based on the current circumstances.
+
+You do not need to take such precautions when you use interactive
+specs @samp{d}, @samp{m} and @samp{r}, because they make special
+arrangements to recompute the argument values on repetition of the
+command.
+
@item
Many commands that take a long time to execute display a message that
-says @samp{Operating...} when they start, and change it to
+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 at the end.
+@emph{no} period after @samp{done}.
@item
Try to avoid using recursive edits. Instead, do what the Rmail @kbd{e}
@item
Try to avoid compiler warnings about undefined free variables, by adding
-@code{defvar} definitions for these variables.
+dummy @code{defvar} definitions for these variables, like this:
+
+@example
+(defvar foo)
+@end example
-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:
+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)
- (defvar bar-baz))
+ (require 'foo))
@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 variable names.
-Therefore, you should rename the variable to start with the name prefix
-used for the other functions and variables in your package.
+@item
+If you bind a variable in one function, and use it or set it in
+another function, the compiler warns about the latter function unless
+the variable has a definition. But adding a definition would be
+unclean if the variable has a short name, since Lisp packages should
+not define short variable names. The right thing to do is to rename
+this variable to start with the name prefix used for the other
+functions and variables in your package.
@item
Indent each function with @kbd{C-M-q} (@code{indent-sexp}) using the
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.
+Otherwise, use your name. See also @xref{Library Headers}.
@end itemize
@node Compilation Tips
@item
@cindex profiling
@cindex timing programs
-@cindex @file{profile.el}
@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.
+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.
search functions can be used.
@item
-Certain built-in functions are handled specially in byte-compiled code,
+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}
@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
longer the case---documentation strings now take up very little space in
a running Emacs.
+@item
+Format the documentation string so that it fits in an Emacs window on an
+80-column screen. It is a good idea for most lines to be no wider than
+60 characters. The first line should not be wider than 67 characters
+or it will look bad in the output of @code{apropos}.
+
+You can fill the text if that looks good. However, rather than blindly
+filling the entire documentation string, you can often make it much more
+readable by choosing certain line breaks with care. Use blank lines
+between topics if the documentation string is long.
+
@item
The first line of the documentation string should consist of one or two
complete sentences that stand on their own as a summary. @kbd{M-x
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 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.
+For a function, the first line should briefly answer the question,
+``What does this function do?'' For a variable, the first line should
+briefly answer the question, ``What does this value mean?''
+
+Don't limit the documentation string to one line; use as many lines as
+you need to explain the details of how to use the function or
+variable. Please use complete sentences for the rest of the text too.
+
+@item
+The first line should mention all the important arguments of the
+function, and should mention them in the order that they are written
+in a function call. If the function has many arguments, then it is
+not feasible to mention them all in the first line; in that case, the
+first line should mention the first few arguments, including the most
+important arguments.
@item
For consistency, phrase the verb in the first sentence of a function's
-documentation string as an imperative--for instance, use ``Return the
+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.
+is indicative and has a proper subject.
@item
Write documentation strings in the active voice, not the passive, and in
@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 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
@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
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
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:
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 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
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 in single quotes, preceded by @samp{info node} or @samp{Info
-node}. The Info file name defaults to @samp{emacs}. For example,
+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'.
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.
+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.
+they remain at the left margin. By default, Outline minor mode does
+not consider a comment to be a heading (even if it starts with at
+least three semicolons) if the semicolons are followed by at least two
+spaces. Thus, if you add an introductory comment to the commented out
+code, make sure to indent it by at least two spaces after the three
+semicolons.
@smallexample
(defun foo (a)
-;;; This is no longer necessary.
+;;; 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
@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
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 ;;; Documentation:
+This was used in some files in place of @samp{;;; Commentary:},
+but it is deprecated.
@item ;;; Change Log:
This begins change log information stored in the library file (if you
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