A lambda expression itself is not a function; it must be associated with its module, or more correctly a lexical environment. This conjunction is called a “closure”; it is the closure that may be called as a function.
Evaluation turns a lambda expression to its closure. Consider the following example:
(lambda (x) (1+ x)) ⇒#<closure 1780f10 @ module.foo> ;; Anonymous closure is printed with internal pointer value. (functionp (lambda (x) (1+ x))) ⇒ t (functionp '(lambda (x) (1+ x))) ⇒ () (eq (lambda ()) (lambda ())) ⇒ () ; Each time `lambda' generates a new instance of closure.
A closure can access bindings which belong to the same module as the closure. A closure can also have its own private bindings. (They’re hidden from the outside, thus “closure”.)
N.B. It seems that
define in a closure which is not top-level
creates a binding in the closure’s module, NOT inside of the closure.
(define (count-0) (define val 0) (setq val (1+ val))) (count-0) ⇒ 1 (count-0) ⇒ 1 ;; Each time `define' resets the value to 0. (define (count) (unless (boundp 'val) (define val 0)) (setq val (1+ val))) (count) ⇒ 1 (count) ⇒ 2 ;; Ok, but val ⇒ 2 ;; Not invisible.
Returns true if arg is a closure.
Returns the name of closure fun, a string. Or if it’s unnamed,
Returns the structure fun belongs to.
Sets the structure closure belongs to to structure.
It is often useful to pass a function definition to other
functions, instead of function’s name, especially for example to
delete-if. It is called an “anonymous
function”, and a lambda expression is all what’s needed.
The following example removes all elements from the list which are even and greater than 20.
(setq list (delete-if (lambda (x) (and (zerop (% x 2)) (> x 20))) list))
There’re some low-level closure handling functions. For example, in
certain cases it may be necessary to create a non-constant function,
for example by using backquoting (see Backquoting). In these cases
make-closure function may be used to create a function
object from a lambda expression.
Return the closure of arg and the current lexical environment.
Returns the function object associated with the lexical closure closure.
Sets the function value of closure to
(define (foo) 'foo-sym) (define (bar) 'bar-sym) (set-closure-function foo (closure-function bar)) (foo) ⇒ foo-sym