At Couples and Co. programmers create lists of couples from lists of symbols and numbers. The list starts with a symbol and every other of the following items on the list is a symbol, too; the remaining items on the list are numbers. You may assume that all given lists are of even length.

(define-struct couple (left right))
;; A Couple is (make-couple Symbol Number)

Design the function create-couples.

Solution: Grader: Matthias F.

;; [PTS 2, for correct contract, which involves two data 
;;         defs and/or uses of [Listof ...]

;; create-couples : 
;;    [Listof (union Symbol Number)] -> [Listof Couple]
;; create couples from an even length list
;; generative recursion
(define (create-pairs l)
  (cond 
;; [PTS: 1 for the empty condition, 
;;       1 for the ``generative'' recursions]
    [(empty? l) empty]
    [else (cons (make-couple (first l) (second l)) 
                (create-pairs (rest (rest l))))]))

;; EXAMPLES/TESTS ;; [PTS 1, for examples/tests]
(equal? (create-pairs '()) 
        empty)
(equal? (create-pairs '(a 10 b 20))
        (list (make-couple 'a 10) (make-couple 'b 20)))