On Oct 5, 2010, at 7:30 AM, 김태윤 wrote: > #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.
Not exactly: the "check-with" is just to catch programming errors, and to document that the model is supposed to be a string. If I made a mistake, and my key handler returned something other than a string, it would catch that mistake. > 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. That's sort of right, but a confusing way to look at it. Here's a simpler way. At all times, there is a "model", which is initially the first argument of big-bang. Every time a key is pressed, the key handler is called with the current model and the key; whatever the key handler returns becomes the new model. Every time the model changes (for example, after the key handler returns), the draw handler is called with the current model; whatever the draw handler returns is shown on the screen. > is check-with function must be there? No, this animation would still work without the (check-with string?) . Matthias pointed you to one place to read about this. Another is at http://www.picturingprograms.com/pdf/chap06.pdf . Stephen Bloch sbl...@adelphi.edu _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
