;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-advanced-reader.ss" "lang")((modname |36.4|) (read-case-sensitive #t) (teachpacks ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp") (lib "gui.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #t #t none #f ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp") (lib "gui.ss" "teachpack" "htdp")))))
;A traffic-light color (TL-color) is either
;1. 'red,
;2. 'yellow, or
;3. 'green.

;;State Variable:
;;current-color : TL-color
(define current-color 'red)

;next : -> void
;Effect: Changes current-color from 'red to 'green, 'green to 'yellow, or 'yellow to 'red depending on what the current-color is.  Produces no output.

(define (next)
  (set! (next-color current-color)))

;next-color : TL-color -> TL-color
;Given acolor, returns the next logical color.

(define (next-color acolor)
  (cond
    [(symbol=? acolor 'red) 'green]
    [(symbol=? acolor 'yellow) 'red]
    [(symbol=? acolor 'green) 'yellow]))