When you define a variable whose value is a function, or a list of functions, use a name that ends in ‘-function’ or ‘-functions’, respectively.
There are several other variable name conventions; here is a complete list:
The variable is a normal hook (see Hooks).
The value is a function.
The value is a list of functions.
The value is a form (an expression).
The value is a list of forms (expressions).
The value is a predicate—a function of one argument that returns
nil for success and
nil for failure.
The value is significant only as to whether it is
nil or not.
Since such variables often end up acquiring more values over time,
this convention is not strongly recommended.
The value is a program name.
The value is a whole shell command.
The value specifies options for a command.
The variable is intended for internal use and is defined in the file prefix.el. (Emacs code contributed before 2018 may follow other conventions, which are being phased out.)
The variable is intended for internal use and is defined in C code. (Emacs code contributed before 2018 may follow other conventions, which are being phased out.)
When you define a variable, always consider whether you should mark it as safe or risky; see File Local Variables.
When defining and initializing a variable that holds a complicated
value (such as a keymap with bindings in it), it’s best to put the
entire computation of the value into the
defvar, like this:
(defvar my-mode-map (let ((map (make-sparse-keymap))) (define-key map "\C-c\C-a" 'my-command) … map) docstring)
This method has several benefits. First, if the user quits while
loading the file, the variable is either still uninitialized or
initialized properly, never in-between. If it is still uninitialized,
reloading the file will initialize it properly. Second, reloading the
file once the variable is initialized will not alter it; that is
important if the user has run hooks to alter part of the contents
(such as, to rebind keys). Third, evaluating the
with C-M-x will reinitialize the map completely.
Putting so much code in the
defvar form has one disadvantage:
it puts the documentation string far away from the line which names the
variable. Here’s a safe way to avoid that:
(defvar my-mode-map nil docstring) (unless my-mode-map (let ((map (make-sparse-keymap))) (define-key map "\C-c\C-a" 'my-command) … (setq my-mode-map map)))
This has all the same advantages as putting the initialization inside
defvar, except that you must type C-M-x twice, once on
each form, if you do want to reinitialize the variable.