;; Data Def:
;; World is one of
;; -- false
;; -- positive number
;; Interpretation:
;; -- false means the rocket hasn't been launched
;; -- number n means the rocket has been launched and has reached height n

;; world-move : World -> World
;; move the world
(define (world-move w)
  (cond
    [(boolean? w) (cond ;; NOTE: this anticipates unions
                    [(odd? (random 100)) HEIGHT]
                    [else false])]
    [(number? w) (- w 3)])) ;; up means smaller y coordinate!!!

;; world-draw : World -> World
;; draw the world Ü
(define (world-draw w)
  (cond
    [(boolean? w) BLANK]
    [(number? w)  (place-image (circle 3 'solid 'red) 20 w BLANK)]))

(define WIDTH 100)
(define HEIGHT 100)
(define BLANK (empty-scene WIDTH HEIGHT))

;; test

(= (world-move 10) 7)
;; how could you test (world-move false)

(image=? (world-draw false) BLANK)
(image=? (world-draw 10) (place-image (circle 1 'solid 'red) 20 10 BLANK))


;; run program run

;; tock : World -> World
;; update the canvas and produce next world
(define (tock w)
  (update (world-draw w) produce (world-move w)))

(big-bang WIDTH HEIGHT .1 false)

(on-tick-event tock)