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
defunis the usual way to define new Lisp functions. It defines the symbol name as a function with argument list args and body forms given by body. Neither name nor args should be quoted.
doc, if present, should be a string specifying the function's documentation string (see Function Documentation). declare, if present, should be a
declareform specifying function metadata (see Declare Form). interactive, if present, should be an
interactiveform specifying how the function is to be called interactively (see Interactive Call).
The return value of
Here are some examples:(defun foo () 5) (foo) ⇒ 5 (defun bar (a &optional b &rest c) (list a b c)) (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 the word at point." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1))
Be careful not to redefine existing functions unintentionally.
defunredefines even primitive functions such as
carwithout any hesitation or notification. Emacs does not prevent you from doing this, because redefining a function is sometimes done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition.
If doc is non-
nil, it becomes the function documentation of name. Otherwise, any documentation provided by definition is used.
The proper place to use
defaliasis where a specific function name is being defined—especially where that name appears explicitly in the source file being loaded. This is because
defaliasrecords which file defined the function, just like
By contrast, in programs that manipulate function definitions for other purposes, it is better to use
fset, which does not keep such records. See Function Cells.
You cannot create a new primitive function with
defalias, but you can use them to change the function definition of
any symbol, even one such as
normal definition is a primitive. However, this is risky: for
instance, it is next to impossible to redefine
breaking Lisp completely. Redefining an obscure function such as
x-popup-menu is less dangerous, but it still may not work as
you expect. If there are calls to the primitive from C code, they
call the primitive's C definition directly, so changing the symbol's
definition will have no effect on them.
defsubst, which defines a function like
and tells the Lisp compiler to perform inline expansion on it.
See Inline Functions.