[plt-scheme] I miss end-of-time...

Paul,

  -- cmd-k is the drscheme emergency key for killing the current  
evaluation _and_ the entire evaluator
  -- one could bind "k" in every animation like this:

(require 2htdp/universe)
(require 2htdp/image)


(big-bang 0
           (on-tick add1)
           (on-draw (lambda (n) (text (if (boolean? n) " " (number- 
 >string n)) 11 'red)))
           (stop-when boolean?)
           (on-key  (lambda (n key)
                      (if (key=? key "k") false n))))


and then nothing bad would happen.





On Mar 11, 2010, at 3:05 PM, <engineer at alum.mit.edu> wrote:

> One of my students has the habit of tying his "k" key to (exit).  It  
> gives
> him a quick non-mousy way to close his world and state windows
> simultaneously.  I just tried it.  It works but leaves Dr. Scheme in  
> a weird
> state.  The Definitions window works and is immediately runnable and/ 
> or
> editable, but the Interactions window is disabled.  I wouldn't like  
> the
> Interactions window being disabled all the time, and I don't know  
> what other
> ramifications there may be to ending a World by (exit)-ing.  But it  
> serves
> my student's purposes.
>
> -Paul
>
> -----Original Message-----
> From: plt-scheme-bounces at list.cs.brown.edu
> [mailto:plt-scheme-bounces at list.cs.brown.edu] On Behalf Of Stephen  
> Bloch
> Sent: Thursday, March 11, 2010 2:34 PM
> To: PLT-Scheme Mailing List
> Subject: [plt-scheme] I miss end-of-time...
>
> In recent versions of the world/universe teachpacks, the "end-of-time"
> function has been removed.  This makes some animations considerably  
> harder
> or more unnatural to write.
>
> Try this: write an animation of a disk whose radius starts at 0 and  
> grows by
> 1 pixel per second.  The animation stops when the user presses any  
> key on
> the keyboard.
>
> You could do this with a struct that contains a "quitting?" field,  
> but my
> students won't see structs for several more weeks. So the model has  
> to be
> just a number.  Now how could the key handler indicate, with a  
> number, that
> the animation should stop?
>
> Well, we could use a negative number, since it'll never go negative  
> in the
> normal course of events... but since the draw handler is called  
> after the
> tick/mouse/key handlers but before the stop-when handler, that means  
> we're
> going to call a function that draws a disk of specified radius with a
> negative number, and it'll crash.  We could write the draw handler  
> to check
> whether the number is negative before trying to draw a disk of that  
> radius,
> but (a) that adds unnecessary complexity, and (b) my students won't  
> see
> conditionals for another week and a half.
>
> Alternatively, we could use a really large number, and just hope  
> nobody runs
> the animation for long enough that the radius gets up to that point  
> in the
> normal course of events.  And it has to be a small enough number  
> that a disk
> of that size COULD be drawn, or the draw handler will crash again as  
> above.
>
> Or I guess we could use a fraction.  Which feels really weird and  
> unnatural,
> and relies on the fact that the "circle" function rounds when you  
> give it a
> fractional radius.
>
> With an "end-of-time" function, it's trivial: the key handler  
> ignores its
> parameters and calls "end-of-time".
>
>
> Stephen Bloch
> sbloch at adelphi.edu
>
>
>
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