[plt-scheme] Mutation and worlds with world/universe.ss

This is indeed correct. I recommend the following arrangement:

 ;; space-invader-state% -> space-invader-state% 
 (define (main sis0)
  (big-bang 0
            (on-tick (lambda (x) 
		       (begin
			 (tock sis0)
			 (+ 1 x))))
            (on-key (lambda (x ke)
		      (begin
			(react sis0 ke)
			(+ 1 x))))
            (on-draw (lambda (x) (render sis0)))
            (stop-when (lambda (x) (win-or-lose? sis0)))))

I will add a more appropriate big-bang!! form to universe soon, including an abstract class so that one can teach the functional -> imperative transition in the context of a class-based data representation 

-- Matthias





On Nov 13, 2009, at 9:53 PM, Nadeem Abdul Hamid wrote:

> It seems like the world/universe.ss teachpacks optimize things so that if the world structure has not changed, the window is not redrawn. This causes difficulty when the world is updated via mutation; for example, consider:
> 
> ;; A Board is an N x N, Vector-of-Vector-of-Boolean
> 
> ;; A World is a structure: (make-world Board)
> (define-struct world (board))
> 
> ...
> 
> ;; handle-click : World Number Number MouseEv -> World
> (define (handle-click w x y me)
>  (local [(define p (x/y->cell (world-board w) x y))]
>    (cond [(mouse=? me "button-down")
>           (begin (board-flip! (world-board w) (posn-x p) (posn-y p))
>                  w)]
>          [else w])))
> 
> (big-bang (make-world TEST-BOARD)
>          (on-draw draw-world)
>          (on-mouse handle-click))
> 
> 
> This does not update the display upon clicking in the window, although when you stop the animation, it can be seen that the state has in fact changed.
> 
> However, if the world is recreated in the handler with another piece of data that changes then the display gets updated as expected:
> 
> ;; A World is a structure: (make-world Number Board)
> (define-struct world (n board))
> 
> ...
> 
> ;; handle-click : World Number Number MouseEv -> World
> (define (handle-click w x y me)
>  (local [(define p (x/y->cell (world-board w) x y))]
>    (cond [(mouse=? me "button-down")
>           (begin (board-flip! (world-board w) (posn-x p) (posn-y p))
>                  (make-world (add1 (world-n w)) (world-board w)))]
>          [else w])))
> 
> 
> 
> Is there any way to handle this other than recreating a new world structure? Is there another better way to program worlds with mutation?
> 
> Thanks,
> nadeem
> 
> 
> 
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