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We usually give a name to a function when it is first created. This
is called defining a function, and it is done with the
defun
special form.
defun
is the usual way to define new Lisp functions. It defines the symbol name as a function that looks like this:(lambda argument-list . body-forms)
defun
stores this lambda expression in the function cell of name. It returns the value name, but usually we ignore this value.As described previously (see Lambda Expressions), argument-list is a list of argument names and may include the keywords
&optional
and&rest
. Also, the first two of the body-forms may be a documentation string and an interactive declaration.There is no conflict if the same symbol name is also used as a variable, since the symbol's value cell is independent of the function cell. See Symbol Components.
Here are some examples:
(defun foo () 5) => foo (foo) => 5 (defun bar (a &optional b &rest c) (list a b c)) => bar (bar 1 2 3 4 5) => (1 2 (3 4 5)) (bar 1) => (1 nil nil) (bar) error--> Wrong number of arguments. (defun capitalize-backwards () "Upcase the last letter of a word." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) => capitalize-backwardsBe careful not to redefine existing functions unintentionally.
defun
redefines even primitive functions such ascar
without any hesitation or notification. Redefining a function already defined is often done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition.
This special form defines the symbol name as a function, with definition definition (which can be any valid Lisp function).
The proper place to use
defalias
is where a specific function name is being defined—especially where that name appears explicitly in the source file being loaded. This is becausedefalias
records which file defined the function, just likedefun
(see Unloading).By contrast, in programs that manipulate function definitions for other purposes, it is better to use
fset
, which does not keep such records.
See also defsubst
, which defines a function like defun
and tells the Lisp compiler to open-code it. See Inline Functions.