;; 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-intermediate-lambda-reader.ss" "lang")((modname world) (read-case-sensitive #t) (teachpacks ((lib "world.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "world.ss" "teachpack" "htdp")))))
;; testing world
;; World = Nat

(define world0 100)

(define (world->image w)
  (place-image (circle 3 'solid 'red) 50 w (empty-scene 100 100)))

(define (world->next w) (sub1 w))

(define (world->steer w ke)
  (cond
    [(char? ke) w]
    [(symbol=? ke 'left) 100]
    [(symbol=? ke 'right) 90]
    [else w]))

;; --- 
(check-expect (key-event? 'a) true)
(check-expect (key-event? 0) false)
(check-expect (key-event? #\a) true)

(check-expect (key=? 'a 'b) false)
(check-expect (key=? 'a #\a) false)
(check-expect (key=? 'left 'left) true)

(check-error (key=? 'a 0) "key=?: expected <KeyEvent> as first argument, given: 0")


;; run world run

(big-bang 100 100 .01 world0 true) ;; get ready to create images 

(on-redraw world->image)
(on-tick-event world->next)
(on-key-event world->steer)
(stop-when zero?)