symbol-function symbol → contents

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

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.

Accesses the symbol’s function cell.

 (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

defun

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.)

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

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.