Function for list is undefined

Question

I'm having problems with clisp.I'm tryng just to get the sum of the numbers in a list,but it gives me this error:

 EVAL-the function L is undefined

when i call

(sum '(2 (c 6)))

Here is the code:

(defun sum(l)
        (cond
           ((null l) 0)
           ((NUMBERP (car l)) (+ (car l) (sum(cdr l)) ))
            (t (sum(cdr(l))))
        )
)  

Answer 1

(cdr(l)) invokes the function l and applies cdr to the value it returns.

You should write (cdr l) instead:

(t (sum (cdr l))))

Answer 2

(defun sum (x) (if (null x) 0 
                    (if (numberp x) x 
                         (+ (car x) (sum (cdr x))))))

by the way you could do similarly with product

(defun prod (x) (if (null x) 1 
                    (if (numberp x) x 
                         (* (car x) (prod (cdr x))))))

(eq (prod '(1 2 3 4 5 6 7 8 9 10)) (fact 10))

but define fact first - here you are a defun for fact (the usual one...)

(defun fact (n)
     (if (not (integerp n)) (print "give me an integer" )          
        (if (< n 0) (print "give me a pos num " )          
         (if (= n 0) 1    
             (if (= n 1) 1 
                 (* n (fact (- n 1)
)))))))