I've been trying to program breakout in BSL and I'm stuck currently. I'm trying to reset the game when the ball collides with the bottom, but I do not understand the error messages and tried lots of things now. Do you have any idea?
(require 2htdp/image)
(require 2htdp/universe)
(define WIDTH 200)
(define HEIGHT 200)
(define BALL-IMG (circle 10 "solid" "red"))
(define BALL-RADIUS (/ (image-width BALL-IMG) 2))
(define-struct vel (delta-x delta-y))
; a Vel is a structure: (make-vel Number Number)
; interp. the velocity vector of a moving object
(define-struct ball (loc velocity))
; a Ball is a structure: (make-ball Posn Vel)
; interp. the position and velocity of a object
; Posn Vel -> Posn
; applies q to p and simulates the movement in one clock tick
(check-expect (posn+vel (make-posn 5 6) (make-vel 1 2))
(make-posn 6 8))
(define (posn+vel p q)
(make-posn (+ (posn-x p) (vel-delta-x q))
(+ (posn-y p) (vel-delta-y q))))
; Ball -> Ball
; computes movement of ball in one clock tick
(check-expect (move-ball (make-ball (make-posn 20 30)
(make-vel 5 10)))
(make-ball (make-posn 25 40)
(make-vel 5 10)))
(define (move-ball ball)
(make-ball (posn+vel (ball-loc ball)
(ball-velocity ball))
(ball-velocity ball)))
; A Collision is either
; - "top"
; - "down"
; - "left"
; - "right"
; - "none"
; interp. the location where a ball collides with a wall
; Posn -> Collision
; detects with which of the walls (if any) the ball collides
(check-expect (collision (make-posn 0 12)) "left")
(check-expect (collision (make-posn 15 HEIGHT)) "down")
(check-expect (collision (make-posn WIDTH 12)) "right")
(check-expect (collision (make-posn 15 0)) "top")
(check-expect (collision (make-posn 55 55)) "none")
(define (collision posn)
(cond
[(<= (posn-x posn) BALL-RADIUS) "left"]
[(<= (posn-y posn) BALL-RADIUS) "top"]
[(>= (posn-x posn) (- WIDTH BALL-RADIUS)) "right"]
[(>= (posn-y posn) (- HEIGHT BALL-RADIUS)) "down"]
[else "none"]))
; Vel Collision -> Vel
; computes the velocity of an object after a collision
(check-expect (bounce (make-vel 3 4) "left")
(make-vel -3 4))
(check-expect (bounce (make-vel 3 4) "top")
(make-vel 3 -4))
(check-expect (bounce (make-vel 3 4) "none")
(make-vel 3 4))
(define (bounce vel collision)
(cond [(or (string=? collision "left")
(string=? collision "right"))
(make-vel (- (vel-delta-x vel))
(vel-delta-y vel))]
[(string=? collision "top")
(make-vel (vel-delta-x vel)
(- (vel-delta-y vel)))]
[(string=? collision "down") (tick (render INITIAL-BALL))]
[else vel]))
; WorldState is a Ball
; WorldState -> Image
; renders ball at its position
(check-expect (image? (render INITIAL-BALL)) #true)
(define (render ball)
(place-image BALL-IMG
(posn-x (ball-loc ball))
(posn-y (ball-loc ball))
(empty-scene WIDTH HEIGHT)))
; WorldState -> WorldState
; moves ball to its next location
(check-expect (tick (make-ball (make-posn 20 12) (make-vel 1 2)))
(make-ball (make-posn 21 14) (make-vel 1 2)))
(define (tick ball)
(move-ball (make-ball (ball-loc ball)
(bounce (ball-velocity ball)
(collision (ball-loc ball))))))
(define INITIAL-BALL (make-ball (make-posn 20 12)
(make-vel 1 2)))
; start with: (main INITIAL-BALL)
(define (main ws)
(big-bang ws (on-tick tick 0.01) (to-draw render)))
In your bounce function, change your string=? collision down branch to return
(make-vel (vel-delta-x vel) (- (vel-deltay vel))). Right now it's returning an image, which your tick function has no idea how to handle. Cheers!