;;---------- Vending machine example - final version --------------------

(define soda-cost 50)
(define candy-cost 65)

;; amt:  Natural
;; remembers the amount of money in the vending machine
(define amt 0)

;; insert-coin:  Natural -> void
;; inserts the given amount of cents into the machine
;; EFFECT:  changes amt
(define (insert-coin cents)
  (set! amt (+ amt cents)))

;; choose-item:  String -> String
;; consumes the item the user wants and produces a status message
;; EFFECT:  can change the amt in the vending machine
(define (choose-item item)
  
  (cond [(string=? item "soda")
         (if (>= amt soda-cost)
             (local [(define change (- amt soda-cost))]
               (begin
                 (set! amt 0)
                 (format "dispense soda and ~a cents change" change)
                 ))
             (format "need ~a more cents" (- soda-cost amt)))]
        
        [(string=? item "candy")
         (if (>= amt candy-cost)             
             "dispense candy"
             "need more money")]))