(define WINDOW-WIDTH 100)
(define WINDOW-HEIGHT 100)
(define DIAMETER 10)


; A world is a string.
; Its display is a ball of that color.
; show-world : world -> image
(define (show-world color) 
  (circle DIAMETER "solid" color))
"Examples of show-world:"
(show-world "green") "should be" (circle DIAMETER "solid" "green")
(show-world "red") "should be" (circle DIAMETER "solid" "red")

; The next world is red if this one's green, and vice versa.
; next-world : world -> world
(define (next-world color)
  (cond [(string=? color "red") "green"]
        [(string=? color "green") "red"]))
"Examples of next-world:"
(next-world "green") "should be" "red"
(next-world "red") "should be" "green"

(big-bang WINDOW-WIDTH WINDOW-HEIGHT 1 "red")
(on-update-event show-world)
(on-tick-event next-world)