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.
Alternatively, you can define a function by providing the code which will inline it as a compiler macro. The following macros make this possible.
Define a function name by providing code that does its inlining, as a compiler macro. The function will accept the argument list args and will have the specified body.
If present, doc should be the function’s documentation string
(see Function Documentation); declare, if present, should be
declare form (see Declare Form) specifying the function’s
Functions defined via
define-inline have several advantages
with respect to macros defined by
mapcar(see Mapping Functions).
cl-defsubst(see Argument Lists in Common Lisp Extensions for GNU Emacs Lisp).
defmacro, a function inlined with
inherits the scoping rules, either dynamic or lexical, from the call
site. See Variable Scoping.
The following macros should be used in the body of a function defined
Quote expression for
define-inline. This is similar to
the backquote (see Backquote), but quotes code and accepts only
This is is similar to
let (see Local Variables): it sets up
local variables as specified by bindings, and then evaluates
body with those bindings in effect. Each element of
bindings should be either a symbol or a list of the form
(var expr); the result is to evaluate
expr and bind var to the result. The tail of
bindings can be either
nil or a symbol which should hold
a list of arguments, in which case each argument is evaluated, and the
symbol is bound to the resulting list.
nil if the value of expression is already
Return the value of expression.
Signal an error, formatting args according to format.
Here’s an example of using
(define-inline myaccessor (obj) (inline-letevals (obj) (inline-quote (if (foo-p ,obj) (aref (cdr ,obj) 3) (aref ,obj 2)))))
This is equivalent to
(defsubst myaccessor (obj) (if (foo-p obj) (aref (cdr obj) 3) (aref obj 2)))