;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname 17.1.1) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
(require installed-teachpacks/picturing-programs)

; Exercise 17.1.1
; Handlers: tick and draw
; Model is a string -- specifically either "red", "yellow", or "green"
#|
(define (function-on-light-color c)
  ; c     light-color
  (cond [(string=? c "red") ...]
        [(string=? c "yellow") ...]
        [(string=? c "green") ...]))

(define (function-returning-light-color ...)
  (cond [... "red"]
        [... "yellow"]
        [... "green"]))
|#

; Tick handler
; change-light : light-color -> light-color
(check-expect (change-light "green") "yellow")
(check-expect (change-light "yellow") "red")
(check-expect (change-light "red") "green")

(define (change-light color)
  (cond [(string=? color "green") "yellow"]
        [(string=? color "yellow") "red"]
        [(string=? color "red") "green"]))

; Draw handler
; show-light : light-color -> image
(check-expect (show-light "green") (circle 30 "solid" "green"))
(check-expect (show-light "yellow") (circle 30 "solid" "yellow"))
(check-expect (show-light "red") (circle 30 "solid" "red"))
(define (show-light color)
  (circle 30 "solid" color))

(big-bang "green"
          (check-with string?)
          (on-tick change-light 5)
          (on-draw show-light))