symbol-function

symbol-function Accessor

Syntax:

symbol-function symbol → contents

(setf (symbol-function symbol) new-contents**)**

Arguments and Values:

symbol—a symbol.

contents— If the symbol is globally defined as a macro or a special operator , an object of implementation-dependent nature and identity is returned. If the symbol is not globally defined as either a macro or a special operator , and if the symbol is fbound, a function object is returned.

new-contents—a function.

Description:

Accesses the symbol’s function cell.

symbol-function

Examples:

(symbol-function ’car) → #<FUNCTION CAR> 
(symbol-function ’twice) is an error ;because TWICE isn’t defined. 
(defun twice (n) (\* n 2)) → TWICE 
(symbol-function ’twice) → #<FUNCTION TWICE> 
(list (twice 3) 
      (funcall (function twice) 3) 
      (funcall (symbol-function ’twice) 3)) 
→ (6 6 6) 
(flet ((twice (x) (list x x))) 
  (list (twice 3) 
	(funcall (function twice) 3) 
	(funcall (symbol-function ’twice) 3))) 
→ ((3 3) (3 3) 6) 
(setf (symbol-function ’twice) #’(lambda (x) (list x x))) 
→ #<FUNCTION anonymous> 
(list (twice 3) 
      (funcall (function twice) 3) 
      (funcall (symbol-function ’twice) 3)) 
→ ((3 3) (3 3) (3 3)) 
(fboundp ’defun) → true 
(symbol-function ’defun) 
→ implementation-dependent 
(functionp (symbol-function ’defun)) 
→ implementation-dependent 
(defun symbol-function-or-nil (symbol) 
  (if (and (fboundp symbol) 
	   (not (macro-function symbol)) 
	   (not (special-operator-p symbol))) 
      (symbol-function symbol) 
      nil)) → SYMBOL-FUNCTION-OR-NIL 
(symbol-function-or-nil ’car) → #<FUNCTION CAR> 
(symbol-function-or-nil ’defun) → NIL 

Affected By:

defun

Exceptional Situations:

Should signal an error of type type-error if symbol is not a symbol.

Should signal undefined-function if symbol is not fbound and an attempt is made to read its definition. (No such error is signaled on an attempt to write its definition.)

See Also:

fboundp, fmakunbound, macro-function, special-operator-p

Notes:

symbol-function cannot access the value of a lexical function name produced by flet or labels; it can access only the global function value.

setf may be used with symbol-function to replace a global function definition when the symbol’s function definition does not represent a special operator .

(symbol-function symbol) ≡ (fdefinition symbol)

However, fdefinition accepts arguments other than just symbols.

Expanded Reference: symbol-function

Basic usage: retrieving a function definition

symbol-function returns the function object stored in a symbol's function cell.

(defun square (x) (* x x))

(symbol-function 'square)
==> #<FUNCTION SQUARE>

(funcall (symbol-function 'square) 5)
=> 25

Accessing built-in functions

Standard Common Lisp functions can be retrieved through symbol-function.

(symbol-function 'car)
==> #<FUNCTION CAR>
(funcall (symbol-function 'car) '(a b c))
=> A
(funcall (symbol-function '+) 1 2 3)
=> 6

Setting a function definition with setf

You can install a new function definition using setf with symbol-function.

(setf (symbol-function 'double) #'(lambda (x) (* x 2)))
(double 21)
=> 42

symbol-function does not see lexical function bindings

Unlike #'name inside an flet or labels, symbol-function always accesses the global function definition.

(defun greet () "hello globally")

(flet ((greet () "hello locally"))
  (list (greet)
        (funcall (symbol-function 'greet))))
=> ("hello locally" "hello globally")

Checking for undefined functions

Calling symbol-function on a symbol with no function definition signals an undefined-function error. Use fboundp to check first.

(fboundp 'not-yet-defined)
=> NIL

(defun safe-symbol-function (sym)
  (if (and (fboundp sym)
           (not (macro-function sym))
           (not (special-operator-p sym)))
      (symbol-function sym)
      nil))

(safe-symbol-function 'car)
==> #<FUNCTION CAR>
(safe-symbol-function 'not-yet-defined)
=> NIL
(safe-symbol-function 'defun)
=> NIL