14.7 A Sample Function Definition

A function definition uses the @defun and @end defun commands. The name of the function follows immediately after the @defun command and it is followed, on the same line, by the parameter list.

Here is a definition from Calling Functions in The GNU Emacs Lisp Reference Manual.

Function: apply function &rest arguments

apply calls function with arguments, just like funcall but with one difference: the last of arguments is a list of arguments to give to function, rather than a single argument. We also say that this list is appended to the other arguments.

apply returns the result of calling function. As with funcall, function must either be a Lisp function or a primitive function; special forms and macros do not make sense in apply.

(setq f 'list)
    ⇒ list
(apply f 'x 'y 'z)
error→ Wrong type argument: listp, z
(apply '+ 1 2 '(3 4))
    ⇒ 10
(apply '+ '(1 2 3 4))
    ⇒ 10

(apply 'append '((a b c) nil (x y z) nil))
    ⇒ (a b c x y z)

An interesting example of using apply is found in the description of mapcar.

In the Texinfo source file, this example looks like this:

@defun apply function &rest arguments
@code{apply} calls @var{function} with
@var{arguments}, just like @code{funcall} but with one
difference: the last of @var{arguments} is a list of
arguments to give to @var{function}, rather than a single
argument.  We also say that this list is @dfn{appended}
to the other arguments.

@code{apply} returns the result of calling
@var{function}.  As with @code{funcall},
@var{function} must either be a Lisp function or a
primitive function; special forms and macros do not make
sense in @code{apply}.

@example
(setq f 'list)
    @result{} list
(apply f 'x 'y 'z)
@error{} Wrong type argument: listp, z
(apply '+ 1 2 '(3 4))
    @result{} 10
(apply '+ '(1 2 3 4))
    @result{} 10

(apply 'append '((a b c) nil (x y z) nil))
    @result{} (a b c x y z)
@end example

An interesting example of using @code{apply} is found
in the description of @code{mapcar}.
@end defun

In this manual, this function is listed in the Command and Variable Index under apply.

Ordinary variables and user options are described using a format like that for functions except that variables do not take arguments.