How can I modify function bindings in Common Lisp?

Question

Here is something you can do in Scheme:

> (define (sum lst acc)
    (if (null? lst)
        acc
        (sum (cdr lst) (+ acc (car lst)))))
> (define s         


        

Answer 1

Because Common Lisp has a different namespace for functions, you need to use symbol-function or fdefinition.

CL-USER> (defun foo (a)
           (+ 2 a))
FOO
CL-USER> (defun debug-foo (a)
           (format t " DEBUGGING FOO: ~a" a)
           (+ 2 a))
DEBUG-FOO
CL-USER> (defun debug-foo-again (a)
           (format t " DEBUGGING ANOTHER FOO: ~a" a)
           (+ 2 a))
DEBUG-FOO-AGAIN
CL-USER> (foo 4)
6
CL-USER> (setf (symbol-function 'foo) #'debug-foo)
#
CL-USER> (foo 4)
 DEBUGGING FOO: 4
6
CL-USER> (setf (fdefinition 'foo) #'debug-foo-again)
#
CL-USER> (foo 4)
 DEBUGGING ANOTHER FOO: 4
6
CL-USER>