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functionp

functionp Function

Syntax:

functionp object → generalized-boolean

Arguments and Values:

object—an object.

generalized-boolean—a generalized boolean.

Description:

Returns true if object is of type function; otherwise, returns false.

Examples:

(functionp ’append) → false 
(functionp #’append) → true 
(functionp (symbol-function ’append)) → true 
(flet ((f () 1)) (functionp #’f)) → true 
(functionp (compile nil ’(lambda () 259))) → true 
(functionp nil) → false 
(functionp 12) → false 
(functionp ’(lambda (x) (\* x x))) → false 
(functionp #’(lambda (x) (\* x x))) → true

Notes:

(functionp object) ≡ (typep object ’function)

Expanded Reference: functionp​

Testing Various Objects​

functionp returns true if its argument is a function object. Symbols and lambda expressions are not functions -- you need #' or (function ...) to get the actual function object.

;; Function objects
(functionp #'car)
=> T
(functionp #'+)
=> T
(functionp (lambda (x) x))
=> T

;; Not function objects
(functionp 'car)
=> NIL
(functionp 42)
=> NIL
(functionp "hello")
=> NIL
(functionp '(lambda (x) x))
=> NIL

Compiled and Interpreted Functions​

Both compiled and interpreted function objects satisfy functionp.

(functionp (compile nil (lambda (x) (1+ x))))
=> T
(functionp #'(lambda (x) (1+ x)))
=> T

Equivalence to typep​

functionp is equivalent to (typep object 'function).

(let ((f #'list))
  (values (functionp f)
          (typep f 'function)))
=> T
=> T

(let ((s 'list))
  (values (functionp s)
          (typep s 'function)))
=> NIL
=> NIL