common-lisp Defining anonymous functions


Example

Functions in Common Lisp are first class values. An anonymous function can be created by using lambda. For example, here is a function of 3 arguments which we then call using funcall

CL-USER> (lambda (a b c) (+ a (* b c)))
#<FUNCTION (LAMBDA (A B C)) {10034F484B}>
CL-USER> (defvar *foo* (lambda (a b c) (+ a (* b c))))
*FOO*
CL-USER> (funcall *foo* 1 2 3)
7

Anonymous functions can also be used directly. Common Lisp provides a syntax for it.

((lambda (a b c) (+ a (* b c)))    ; the lambda expression as the first
                                   ; element in a form
  1 2 3)                           ; followed by the arguments

Anonymous functions can also be stored as global functions:

(let ((a-function (lambda (a b c) (+ a (* b c)))))      ; our anonymous function
  (setf (symbol-function 'some-function) a-function))   ; storing it

(some-function 1 2 3)                                   ; calling it with the name

Quoted lambda expressions are not functions

Note that quoted lambda expressions are not functions in Common Lisp. This does not work:

(funcall '(lambda (x) x)
         42)

To convert a quoted lambda expression to a function use coerce, eval or funcall:

CL-USER > (coerce '(lambda (x) x) 'function)
#<anonymous interpreted function 4060000A7C>

CL-USER > (eval '(lambda (x) x))
#<anonymous interpreted function 4060000B9C>

CL-USER > (compile nil '(lambda (x) x))
#<Function 17 4060000CCC>