This chapter describes no additional features of Emacs Lisp. Instead it gives advice on making effective use of the features described in the previous chapters, and describes conventions Emacs Lisp Surfers should follow.
Here are conventions that you should follow when writing Emacs Lisp code intended for widespread use:
copy-list. Believe it or not, there is more than one
plausible way to define copy-list. Play it safe;
append your name prefix to produce a name like
foo-copy-list or mylib-copy-list instead.
If you write a function that you think ought to be added to Emacs
under a certain name, such as twiddle-files, don't
call it by that name in your program. Call it
mylib-twiddle-files in your program, and send mail to
`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
alternative common prefixes, so long as they make sense. Separate
the prefix from the rest of the symbol name with a hyphen,
`-'. This will be consistent with Emacs itself and
with most Emacs Lisp programs.
provide in
each separate library program, at least if there is more than one
entry point to the program.
require to make sure they are
loaded.
(eval-when-compile (require 'bar))(And the library bar should contain
(provide
'bar), to make the require work.)
This will cause bar to be loaded when you byte-compile
foo. Otherwise, you risk compiling foo
without the necessary macro loaded, and that would produce compiled
code that won't work right. See section Macros and Byte Compilation. Using
eval-when-compile avoids loading bar when
the compiled version of foo is used.
framep and
frame-live-p.
whatever-mode which turns the feature on or
off, and make it autoload (see section 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.
next-line or previous-line
in programs; nearly always, forward-line is more
convenient as well as more predictable and robust. See section Motion by Text Lines.
beginning-of-buffer,
end-of-bufferreplace-string, replace-regexpmessage function, not princ. See
section The Echo Area.
error (or signal). The function
error does not return. See section How to Signal an Error. Do not use
message, throw, sleep-for,
or beep to report errors.
edit-options command does: switch to another
buffer and let the user switch back at will. See section Recursive Editing.
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 variable names. Therefore, you should rename the variable to
start with the name prefix used for the other functions and
variables in your package.
indent-sexp) using the default indentation
parameters.
;; Copyright (C) year name ;; This program is free software; you can redistribute it and/or ;; modify it under the terms of the GNU General Public License as ;; published by the Free Software Foundation; either version 2 of ;; the License, or (at your option) any later version. ;; This program is distributed in the hope that it will be ;; useful, but WITHOUT ANY WARRANTY; without even the implied ;; warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR ;; PURPOSE. See the GNU General Public License for more details. ;; You should have received a copy of the GNU General Public ;; License along with this program; if not, write to the Free ;; Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, ;; MA 02111-1307 USAIf you have signed papers to assign the copyright to the Foundation, then use `Free Software Foundation, Inc.' as name. Otherwise, use your name.
Here are ways of improving the execution speed of byte-compiled Lisp programs.
memq,
member, assq, or assoc is
even faster than explicit iteration. It can be worth rearranging a
data structure so that one of these primitive search functions can
be used.
byte-compile property. If the
property is non-nil, then the function is handled
specially. For example, the following input will show you that
aref is compiled specially (see section Functions that Operate on Arrays):
(get 'aref 'byte-compile)
=> byte-compile-two-args
Here are some tips and conventions for the writing of documentation strings. You can check many of these conventions by running the command M-x checkdoc-minor-mode.
nil values are equivalent and
indicate explicitly what nil and non-nil
mean.
/ refers to its second argument as
`DIVISOR', because the actual argument name is
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.
t and nil
without single-quotes. 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
`variable', `option',
`function', or `command', immediately
before the symbol name. (Case makes no difference in recognizing
these indicator words.) For example, if you write
This function sets the variable `buffer-file-name'.then the hyperlink will refer only to the variable documentation of
buffer-file-name, and not to its function
documentation. If a symbol has a function definition and/or a
variable definition, but those are irrelevant to the use of the
symbol that you are documenting, you can write the word
`symbol' before the symbol name to prevent making any
hyperlink. For example,
If the argument KIND-OF-RESULT is the symbol `list', this function returns a list of all the objects that satisfy the criterion.does not make a hyperlink to the documentation, irrelevant here, of the function
list.
forward-char. (This is normally
`C-f', but it may be some other character if the user
has moved key bindings.) See section Substituting Key Bindings in
Documentation.
We recommend these conventions for where to put comments and how to indent them:
indent-for-comment) command automatically inserts
such a `;' in the right place, or aligns such a
comment if it is already present. This and following examples are
taken from the Emacs sources.
(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
(prog1 (setq auto-fill-function
...
...
;; update mode line
(force-mode-line-update)))
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.;;; This Lisp code is run in Emacs ;;; when it is to operate as a server ;;; for other processes.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.
(defun foo (a) ;;; This is no longer necessary. ;;; (force-mode-line-update) (message "Finished with %s" a))
;;;; The kill ring
The indentation commands of the Lisp modes in Emacs, such as
M-; (indent-for-comment) and TAB
(lisp-indent-line), automatically indent comments
according to these conventions, depending on the number of
semicolons. See section `Manipulating Comments' in The GNU
Emacs Manual.
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:
;;; lisp-mnt.el --- minor mode for Emacs Lisp maintainers ;; Copyright (C) 1992 Free Software Foundation, Inc. ;; Author: Eric S. Raymond <esr@snark.thyrsus.com> ;; Maintainer: Eric S. Raymond <esr@snark.thyrsus.com> ;; Created: 14 Jul 1992 ;; Version: 1.2 ;; Keywords: docs ;; This file is part of GNU Emacs. ... ;; Free Software Foundation, Inc., 59 Temple Place - Suite 330, ;; Boston, MA 02111-1307, USA.
The very first line should have this format:
;;; filename --- description
The description should be complete in one line.
After the copyright notice come several header comment lines, each beginning with `;; header-name:'. Here is a table of the conventional possibilities for header-name:
;; and a
tab character, like this:
;; Author: Ashwin Ram <Ram-Ashwin@cs.yale.edu> ;; Dave Sill <de5@ornl.gov> ;; Dave Brennan <brennan@hal.com> ;; Eric Raymond <esr@snark.thyrsus.com>
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.
Just about every Lisp library ought to have the `Author' and `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:
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