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 is 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
declare form specifying function
metadata (see Declare Form). interactive, if present,
should be an
interactive form specifying how the function is to
be called interactively (see Interactive Call).
The return value of
defun is undefined.
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.
defun redefines even primitive functions such as
without 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.
This function defines the symbol name as a function, with definition definition (which can be any valid Lisp function). Its return value is undefined.
If doc is non-
nil, it becomes the function documentation
of name. Otherwise, any documentation provided by
definition is used.
defalias normally uses
fset to set the definition.
If name has a
defalias-fset-function property, however,
the associated value is used as a function to call in place of
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
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.