12.4 Defining Functions

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.

Special Form: defun name argument-list body-forms

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 section 12.2 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 section 8.1 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-backwards

Be careful not to redefine existing functions unintentionally. defun redefines even primitive functions such as car 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.

Function: defalias name definition

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 because defalias records which file defined the function, just like defun (see section 15.7 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 section 12.9 Inline Functions.