ex073.rkt

;; 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 ex073) (read-case-sensitive #t) (teachpacks ((lib "image.rkt" "teachpack" "2htdp") (lib "universe.rkt" "teachpack" "2htdp") (lib "batch-io.rkt" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "image.rkt" "teachpack" "2htdp") (lib "universe.rkt" "teachpack" "2htdp") (lib "batch-io.rkt" "teachpack" "2htdp")) #f)))
(define MTS (empty-scene 100 100))
(define DOT (circle 3 "solid" "red"))


; A Posn represents the state of the world.


; Posn -> Image
; adds a red spot to MTS at p
(define (scene+dot p)
  (place-image DOT (posn-x p) (posn-y p) MTS))

(check-expect (scene+dot (make-posn 10 20))
              (place-image DOT 10 20 MTS))
(check-expect (scene+dot (make-posn 88 73))
              (place-image DOT 88 73 MTS))


; Posn -> Posn
; increases the x-coordinate of p by 3
(define (x+ p)
  (posn-up-x p (+ (posn-x p) 3)))

(check-expect (x+ (make-posn 10 0)) (make-posn 13 0))


; Posn -> Posn
; produces a Posn like p with n in the x field
(define (posn-up-x p n) (make-posn n (posn-y p)))

(check-expect (posn-up-x (make-posn 10 0) 5) (make-posn 5 0))


; Posn -> Posn 
(define (main p0)
  (big-bang p0
    [on-tick x+]
;    [on-mouse reset-dot]
    [to-draw scene+dot]))