;; Another example of abstraction. We first wrote sum and product, 
;;  and then we abstracted to get combine-nums.

;; combine-nums : list-of-num num (num num -> num) -> num
(define (combine-nums l base-n COMB)
  (cond
    [(empty? l) base-n]
    [(cons? l)  (COMB (first l)  
                      (combine-nums (rest l) base-n COMB))]))

;; sum : list-of-num -> num
;(define (sum l)
;  (cond
;    [(empty? l) 0]
;    [(cons? l)  (+ (first l)  (sum (rest l)))]))
(define (sum l)
  (combine-nums l 0 +))
(sum '(1 2 3)) "should be" 6

;; product : list-of-num -> num
;(define (product l)
;  (cond
;    [(empty? l) 1]
;    [(cons? l) (* (first l) (product (rest l)))]))
(define (product l)
  (combine-nums l 1 *))
(product '(1 2 4)) "should be" 8