exam9.5.1

;; mht created on Nov 15, 2011

;; data definition:
;; A list-of-numbers is either
;; 1. the empty list, empty, or
;; 2. (cons n lon) where n is a number and lon is a list of numbers.

;; sum : list-of-numbers -> number
;; to compute the sum of the numbers on a-list-of-nums

;; examples: (= (sum empty) 0)
;;           (= (sum (cons 1.00 empty)) 1.00)
;;           (= (sum (cons 1.22 (cons 2.59 empty)) 3.81)

(define (sum a-list-of-nums)
  (cond
    [(empty? a-list-of-nums) 0]
    [else
     (+
      (first a-list-of-nums)
      (sum (rest a-list-of-nums)))]))

;; test
(= (sum empty)
   0)
(= (sum (cons 1.00 empty))
   1.00)
(= (sum (cons 17.05 (cons 1.22 (cons 2.59 empty))))
   20.86)