D.1 Emacs Lisp Coding Conventions
Here are conventions that you should follow when writing Emacs Lisp
code intended for widespread use:
- Simply loading a 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.
- You should choose a short word to distinguish your program from other
Lisp programs. The names of all global symbols in your program, that
is the names of variables, constants, and functions, should begin with
that chosen prefix. Separate the prefix from the rest of the name
with a hyphen, ‘-’. This practice helps avoid name conflicts,
since all global variables in Emacs Lisp share the same name space,
and all functions share another name space1. Use two hyphens to separate prefix and name if the symbol is
not meant to be used by other packages.
Occasionally, for a command name intended for users to use, it is more
convenient if some words come before the package's name prefix. And
constructs that define functions, variables, etc., work better if they
start with ‘defun’ or ‘defvar’, so put the name prefix later
on in the name.
This recommendation applies even to names for traditional Lisp
primitives that are not primitives in Emacs Lisp—such as
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
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 ‘email@example.com’ 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 can use two or three
alternative common prefixes, so long as they make sense.
- Put a call to
provide at the end of each separate Lisp file.
See Named Features.
- If a file requires certain other Lisp 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.
See Named Features.
- If a file foo uses a macro defined in another file bar,
but does not use any functions or variables defined in bar, then
foo should contain the following expression:
(eval-when-compile (require 'bar))
This tells Emacs to load bar just before byte-compiling
foo, so that the macro definition is available during
eval-when-compile avoids loading bar
when the compiled version of foo is used. It should be
called before the first use of the macro in the file. See Compiling Macros.
- Avoid loading additional libraries at run time unless they are really
needed. If your file simply cannot work without some other library,
require that library at the top-level and be done
with it. But if your file contains several independent features, and
only one or two require the extra library, then consider putting
require statements inside the relevant functions rather than at
the top-level. Or use
autoload statements to load the extra
library when needed. This way people who don't use those aspects of
your file do not need to load the extra library.
- If you need Common Lisp extensions, use the
rather than the old
cl library. The latter does not
use a clean namespace (i.e., its definitions do not
start with a ‘cl-’ prefix). If your package loads
run time, that could cause name clashes for users who don't use that
There is no problem with using the
cl package at compile
(eval-when-compile (require 'cl)). That's
sufficient for using the macros in the
cl package, because the
compiler expands them before generating the byte-code. It is still
better to use the more modern
cl-lib in this case, though.
- When defining a major mode, please follow the major mode
conventions. See Major Mode Conventions.
- When defining a minor mode, please follow the minor mode
conventions. See 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’ (which stands for “predicate”). If the name is one word,
add just ‘p’; if the name is multiple words, add ‘-p’.
- If the purpose of a variable is to store a single function, give it a
name that ends in ‘-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. See Hooks.
- If loading the file adds functions to hooks, define a function
-unload-hook, where feature is the name of
the feature the package provides, and make it undo any such changes.
unload-feature to unload the file will run this function.
- It is a bad idea to define aliases for the Emacs primitives. Normally
you should use the standard names instead. The case where an alias
may be useful is where it facilitates backwards compatibility or
- 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
(if (fboundp 'point-at-bol)
- Redefining or advising an Emacs primitive is a bad idea. It may do
the right thing for a particular program, but there is no telling what
other programs might break as a result.
- It is likewise a bad idea for one Lisp package to advise a function in
another Lisp package (see Advising Functions).
- Avoid using
eval-after-load in libraries and packages
(see Hooks for Loading). This feature is meant for personal
customizations; using it in a Lisp program is unclean, because it
modifies the behavior of another Lisp file in a way that's not visible
in that file. This is an obstacle for debugging, much like advising a
function in the other package.
- If a file does replace any of the standard functions or library
programs of 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.
- Constructs that define a function or variable should be macros,
not functions, and their names should start with ‘define-’.
The macro should receive the name to be
defined as the first argument. That will help various tools find the
definition automatically. Avoid constructing the names in the macro
itself, since that would confuse these tools.
- 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.) People will find Emacs more
coherent if all libraries use the same conventions.
- The default file coding system for Emacs Lisp source files is UTF-8
(see Text Representations). In the rare event that your program
contains characters which are not in UTF-8, you should specify
an appropriate coding system in the source file's ‘-*-’ line or
local variables list. See Local Variables in Files.
- Indent the file using the default indentation parameters.
- Don't make a habit of putting close-parentheses on lines by
themselves; Lisp programmers find this disconcerting.
- Please put a copyright notice and copying permission notice on the
file if you distribute copies. See Library Headers.