[racket] could somebody tell me how this code works?

See 
  http://www.ccs.neu.edu/home/matthias/HtDP2e/htdp2e-part1.html#(part._.D.K._design-world)
and for something like 'check-with' see
  http://docs.racket-lang.org/teachpack/2htdpuniverse.html#(form._((lib._2htdp/universe..rkt)._check-with))


On Oct 5, 2010, at 7:30 AM, 김태윤 wrote:

> hello 
> I am a scheme beginner
> I am learning big-bang function with the book, how to design programs 
> but I still have no idea how big-bang function exactly work.
> 
> #lang racket
> (require 2htdp/image)
> (require 2htdp/universe)
> (define (handle-draw model) (text model 18 "blue"))
> (define (handle-key old-model key) key)
> (big-bang ""                           ; initial model is empty string
>          (check-with string?)         ; model is a string
>          (on-draw handle-draw 400 50) ; display text in 150x50 pixel window
>          (on-key handle-key))
> 
> could somebody please tell me how this code exactly work?
> my guessing is this 
> 
> there is only one checking function. 
> if it is true, keep big-bang going. 
> if it is false, it stops.
> the first argument of big-bang "" is going to be sent to handle-draw by on-draw
> if key pressed, that handle-draw's return value is going to be sent to handle-key as second argument by on-key
> big-bang's first argument "" is changed to the handle-key's return value.
> and big-bang start again from the beginning with that value as first argument
> 
> I still have no idea about the old-model argument.
> is check-with function must be there?
> 
> thanks in advanced
> 
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