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(elisp)Coding Conventions
Emacs Lisp Coding Conventions
=============================
Here are conventions that you should follow when writing Emacs Lisp
code intended for widespread use:
* 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.(1) 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
`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.
* It is often useful to put a call to `provide' in each separate
library program, at least if there is more than one entry point to
the program.
* 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 `require' to make sure they are loaded.
* If one file FOO uses a macro defined in another file BAR, FOO
should contain this expression before the first use of the macro:
(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. Note: Compiling Macros.
Using `eval-when-compile' avoids loading BAR when the compiled
version of FOO is _used_.
* Please don't require the `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 `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 `cl' package at compile
time, for the sake of macros. You do that like this:
(eval-when-compile (require 'cl))
* When defining a major mode, please follow the major mode
conventions. Note: Major Mode Conventions.
* When defining a minor mode, please follow the minor mode
conventions. Note: Minor Mode Conventions.
* 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
`p'. If the name is one word, add just `p'; if the name is
multiple words, add `-p'. Examples are `framep' and
`frame-live-p'.
* If a user option variable records a true-or-false condition, give
it a name that ends in `-flag'.
* Please do not define `C-c LETTER' as a key in your major modes.
These sequences are reserved for users; they are the *only*
sequences reserved for users, so do not block them.
Instead, define sequences consisting of `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 follow this convention was a
lot of work. Abandoning this convention would make that work go
to waste, and inconvenience users.
* Sequences consisting of `C-c' followed by `{', `}', `<', `>', `:'
or `;' are also reserved for major modes.
* Sequences consisting of `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.
* Function keys <F5> through <F9> without modifier keys are reserved
for users to define.
* Do not bind `C-h' following any prefix character (including
`C-c'). If you don't bind `C-h', it is automatically available as
a help character for listing the subcommands of the prefix
character.
* Do not bind a key sequence ending in <ESC> except following
another <ESC>. (That is, it is OK to bind a sequence ending in
`<ESC> <ESC>'.)
The reason for this rule is that a non-prefix binding for <ESC> in
any context prevents recognition of escape sequences as function
keys in that context.
* Anything which acts like a temporary mode or state which the user
can enter and leave should define `<ESC> <ESC>' or `<ESC> <ESC>
<ESC>' as a way to escape.
For a state which accepts ordinary Emacs commands, or more
generally any kind of state in which <ESC> followed by a function
key or arrow key is potentially meaningful, then you must not
define `<ESC> <ESC>', since that would preclude recognizing an
escape sequence after <ESC>. In these states, you should define
`<ESC> <ESC> <ESC>' as the way to escape. Otherwise, define
`<ESC> <ESC>' instead.
* Applications should not bind mouse events based on button 1 with
the shift key held down. These events include `S-mouse-1',
`M-S-mouse-1', `C-S-mouse-1', and so on. They are reserved for
users.
* Special major modes used for read-only text should usually redefine
`mouse-2' and <RET> to trace some sort of reference in the text.
Modes such as Dired, Info, Compilation, and Occur redefine it in
this way.
* 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 `WHATEVER-mode' which turns the feature on
or off, and make it autoload (Note: Autoload). Design the
package so that simply loading it has no visible effect--that
should not enable the feature.(2) Users will request the feature
by invoking the command.
* It is a bad idea to define aliases for the Emacs primitives. Use
the standard names instead.
* 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.
(defalias 'gnus-point-at-bol
(if (fboundp 'point-at-bol)
'point-at-bol
'line-beginning-position))
* Redefining (or advising) an Emacs primitive is discouraged. It
may do the right thing for a particular program, but there is no
telling what other programs might break as a result.
* 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.
* 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.
* Don't use `next-line' or `previous-line' in programs; nearly
always, `forward-line' is more convenient as well as more
predictable and robust. Note: Text Lines.
* 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. Note: The Mark.
In particular, don't use any of these functions:
* `beginning-of-buffer', `end-of-buffer'
* `replace-string', `replace-regexp'
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.
* 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).
* The recommended way to print a message in the echo area is with
the `message' function, not `princ'. Note: The Echo Area.
* When you encounter an error condition, call the function `error'
(or `signal'). The function `error' does not return. Note:
Signaling Errors.
Do not use `message', `throw', `sleep-for', or `beep' to report
errors.
* An error message should start with a capital letter but should not
end with a period.
* In `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 `nil' for those
arguments if they were not specified, and have the function body
compute the default value when the argument is `nil'. For
instance, write this:
(defun foo (pos)
(interactive
(list (if SPECIFIED SPECIFIED-POS)))
(unless pos (setq pos DEFAULT-POS))
...)
rather than this:
(defun foo (pos)
(interactive
(list (if SPECIFIED SPECIFIED-POS
DEFAULT-POS)))
...)
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 `d', `m' and `r', because they make special arrangements to
recompute the argument values on repetition of the command.
* Many commands that take a long time to execute display a message
that says something like `Operating...' when they start, and
change it to `Operating...done' when they finish. Please keep the
style of these messages uniform: _no_ space around the ellipsis,
and _no_ period after `done'.
* Try to avoid using recursive edits. Instead, do what the Rmail `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
`edit-options' command does: switch to another buffer and let the
user switch back at will. Note: Recursive Editing.
* In some other systems there is a convention of choosing variable
names that begin and end with `*'. We don't use that convention
in Emacs Lisp, so please don't use it in your programs. (Emacs
uses such names only for special-purpose buffers.) The users will
find Emacs more coherent if all libraries use the same conventions.
* Try to avoid compiler warnings about undefined free variables, by
adding `defvar' definitions for these variables.
Sometimes adding a `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 `require' acts
only at compile time. Here's how to do that:
(eval-when-compile
(require 'foo)
(defvar bar-baz))
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 each function with `C-M-q' (`indent-sexp') using the
default indentation parameters.
* 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 the sequence in one
or two significant places.
* Please put a copyright notice on the file if you give copies to
anyone. Use a message like this one:
;; 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 USA
If you have signed papers to assign the copyright to the
Foundation, then use `Free Software Foundation, Inc.' as NAME.
Otherwise, use your name.
---------- Footnotes ----------
(1) The benefits of a Common Lisp-style package system are
considered not to outweigh the costs.
(2) Consider that the package may be loaded arbitrarily by Custom
for instance.
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