Although functions are usually defined with defun and given
names at the same time, it is sometimes convenient to use an explicit
lambda expression—an anonymous function. Anonymous functions
are valid wherever function names are. They are often assigned as
variable values, or as arguments to functions; for instance, you might
pass one as the function argument to mapcar, which
applies that function to each element of a list (see Mapping Functions). See describe-symbols example, for a realistic example
of this.
When defining a lambda expression that is to be used as an anonymous
function, you can in principle use any method to construct the list.
But typically you should use the lambda macro, or the
function special form, or the #' read syntax:
This macro returns an anonymous function with argument list args, documentation string doc (if any), interactive spec interactive (if any), and body forms given by body.
Under dynamic binding, this macro effectively makes lambda
forms self-quoting: evaluating a form whose CAR is lambda
yields the form itself:
(lambda (x) (* x x))
⇒ (lambda (x) (* x x))
Note that when evaluating under lexical binding the result is a closure object (see Closures).
The lambda form has one other effect: it tells the Emacs
evaluator and byte-compiler that its argument is a function, by using
function as a subroutine (see below).
This special form returns function-object without evaluating it.
In this, it is similar to quote (see Quoting). But unlike
quote, it also serves as a note to the Emacs evaluator and
byte-compiler that function-object is intended to be used as a
function. Assuming function-object is a valid lambda
expression, this has two effects:
When function-object is a symbol and the code is byte compiled, the byte-compiler will warn if that function is not defined or might not be known at run time.
The read syntax #' is a short-hand for using function.
The following forms are all equivalent:
(lambda (x) (* x x)) (function (lambda (x) (* x x))) #'(lambda (x) (* x x))
In the following example, we define a change-property
function that takes a function as its third argument, followed by a
double-property function that makes use of
change-property by passing it an anonymous function:
(defun change-property (symbol prop function)
(let ((value (get symbol prop)))
(put symbol prop (funcall function value))))
(defun double-property (symbol prop) (change-property symbol prop (lambda (x) (* 2 x))))
Note that we do not quote the lambda form.
If you compile the above code, the anonymous function is also compiled. This would not happen if, say, you had constructed the anonymous function by quoting it as a list:
(defun double-property (symbol prop) (change-property symbol prop '(lambda (x) (* 2 x))))
In that case, the anonymous function is kept as a lambda expression in
the compiled code. The byte-compiler cannot assume this list is a
function, even though it looks like one, since it does not know that
change-property intends to use it as a function.