emacs / lispref / tips.texi

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@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 See the file elisp.texi for copying conditions.
@setfilename ../info/tips
@node Tips, GNU Emacs Internals, Calendar, Top
@appendix Tips and Standards
@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.

@menu
* Style Tips::                Writing clean and robust programs.
* Compilation Tips::          Making compiled code run fast.
* 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

  Here are some tips for avoiding common errors in writing 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.

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.

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
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
alternative common prefixes, so long as they make sense.

Separate the prefix from the rest of the symbol name with a hyphen,
@samp{-}.  This will be consistent with Emacs itself and with most Emacs
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.

@item
If a file requires certain other library 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 this expression before the first use of the
macro:

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

@noindent
(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}.

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}.

@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
If a user option variable records a true-or-false condition, give it a
name that ends in @samp{-flag}.

@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.

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 major modes in Emacs 18 so they would follow this
convention was a lot of work.  Abandoning this convention would make
that work go to waste, and inconvenience users.

@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
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.

@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}}.)

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
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
Modes should redefine @kbd{mouse-2} as a command to follow some sort of
reference in the text of a buffer, if users usually would not want to
alter the text in that buffer by hand.  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.

@item
It is a bad idea to define aliases for the Emacs primitives.  Use the
standard names instead.

@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.

@item
If a file does replace any of the functions or library programs of
standard Emacs, prominent comments at the beginning of the file should
say which functions are replaced, and how the behavior of the
replacements differs from that of the originals.

@item
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
will be 14 characters or less, which is short enough to fit on all kinds
of Unix systems.

@item
Don't use @code{next-line} or @code{previous-line} in programs; nearly
always, @code{forward-line} is more convenient as well as more
predictable and robust.  @xref{Text Lines}.

@item
Don't call functions that set the mark, unless setting the mark is one
of the intended features of your program.  The mark is a user-level
feature, so it is incorrect to change the mark except to supply a value
for the user's benefit.  @xref{The Mark}.

In particular, don't use these functions:

@itemize @bullet
@item
@code{beginning-of-buffer}, @code{end-of-buffer}
@item
@code{replace-string}, @code{replace-regexp}
@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.

@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 @code{message} function, not @code{princ}.  @xref{The Echo Area}.

@item
When you encounter an error condition, call the function @code{error}
(or @code{signal}).  The function @code{error} does not return.
@xref{Signaling Errors}.

Do not use @code{message}, @code{throw}, @code{sleep-for},
or @code{beep} to report errors.

@item
An error message should start with a capital letter but should not end
with a period.

@item
Many commands that take a long time to execute display a message that
says @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.

@item
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}.

@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 program-generated buffers.)  The users will find Emacs more
coherent if all libraries use the same conventions.

@item
Try to avoid compiler warnings about undefined free variables, by adding
@code{defvar} definitions for these variables.

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.
Therefore, you should rename the 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
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 them in one or two significant places.

@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.
@end itemize

@node Compilation Tips
@section Tips for Making Compiled Code Fast
@cindex execution speed
@cindex speedups

  Here are ways of improving the execution speed of byte-compiled
Lisp programs.

@itemize @bullet
@item
@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.

@item
Use iteration rather than recursion whenever possible.
Function calls are slow in Emacs Lisp even when a compiled function
is calling another compiled function.

@item
Using the primitive list-searching functions @code{memq}, @code{member},
@code{assq}, or @code{assoc} is even faster than explicit iteration.  It
may be worth rearranging a data structure so that one of these primitive
search functions can be used.

@item
Certain built-in functions are handled specially in byte-compiled code, 
avoiding the need for an ordinary function call.  It is a good idea to
use these functions rather than alternatives.  To see whether a function
is handled specially by the compiler, examine its @code{byte-compile}
property.  If the property is non-@code{nil}, then the function is
handled specially.

For example, the following input will show you that @code{aref} is
compiled specially (@pxref{Array Functions}) while @code{elt} is not
(@pxref{Sequence 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
program's running time, make the function inline.  This eliminates
the function call overhead.  Since making a function inline reduces
the flexibility of changing the program, don't do it unless it gives
a noticeable speedup in something slow enough that users care about
the speed.  @xref{Inline Functions}.
@end itemize

@node Documentation Tips
@section Tips for Documentation Strings

  Here are some tips for the writing of documentation strings.

@itemize @bullet
@item
Every command, function, or variable intended for users to know about
should have a documentation string.

@item
An internal variable or subroutine of a Lisp program might as well have
a documentation string.  In earlier Emacs versions, you could save space
by using a comment instead of a documentation string, but that is no
longer the case.

@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.

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.

@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.

@item
Write documentation strings in the active voice, not the passive, and in
the present tense, not the future.  For instance, use ``Return a list
containing A and B.'' instead of ``A list containing A and B will be
returned.''

@item
Avoid using the word ``cause'' (or its equivalents) unnecessarily.
Instead of, ``Cause Emacs to display text in boldface,'' write just
``Display text in boldface.''

@item
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.

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
line.  This looks nice in the source code, but looks bizarre when users
view the documentation.  Remember that the indentation before the
starting double-quote is not part of the string!

@item
A variable's documentation string should start with @samp{*} if the
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 that
all non-@code{nil} values are equivalent and indicate explicitly what
@code{nil} and non-@code{nil} mean.

@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}, because the
actual argument name is @code{divisor}.

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.

@item
@iftex
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 @code{t} and @code{nil} without single-quotes.
@end iftex
@ifinfo
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

@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
Emacs displays the documentation string, it substitutes whatever key is
currently bound to @code{forward-char}.  (This is normally @samp{C-f},
but it may be some other character if the user has moved key bindings.)
@xref{Keys in Documentation}.

@item
In documentation strings for a major mode, you will want to refer to the
key bindings of that mode's local map, rather than global ones.
Therefore, use the construct @samp{\\<@dots{}>} once in the
documentation string to specify which key map to use.  Do this before
the first use of @samp{\\[@dots{}]}.  The text inside the
@samp{\\<@dots{}>} should be the name of the variable containing the
local keymap for the major mode.

It is not practical to use @samp{\\[@dots{}]} very many times, because
display of the documentation string will become slow.  So use this to
describe the most important commands in your major mode, and then use
@samp{\\@{@dots{}@}} to display the rest of the mode's keymap.
@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
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