[plt-scheme] some more frtime questions

Hi Dave,

I think the easiest approach would be to modify your second example so
that the test in the IF expression uses a behavior indicating whether
the object has ever collided:

   (if (not (hold (when-e (< (vdist pos player-pos) 2)) false))
      ...)

'when-e' returns an event stream that fires a 'true' value whenever
its argument changes from false to true (i.e., non-false).

Cheers,
Greg

On Feb 16, 2008 12:13 PM, Dave Griffiths <dave at pawfal.org> wrote:
>
> Hi Greg, me again ;)
>
> Bit of a sticky problem, with I'm sure a simple answer I'm not seeing -
> predictably, I want to be able to delete objects when they collide with
> other objects.
>
> This code gives me a moving line of cubes, one appearing every second.
> When the player moves inside the collision sphere they get removed by
> the filter:
>
> (define objects
>   (collect-b
>    (metro 1) '()
>    (lambda (e lst)
>      (cons
>       (object
>        #:shape 'cube
>        #:colour (vec3 1 0 0)
>        #:translate (vsub (vec3-integral (vec3 0 0.002 0)) (vec3 0 3 0)))
>       (filter
>        (lambda (ob)
>          (> (vdist (value-now (object-struct-translate (value-now ob)))
>                    (value-now player-pos)) 2))
>        lst)))))
>
> ('metro' makes an event source which fires every second btw)
>
> The problem is that my cubes only get checked for collision when a new
> one is created, once a second. I can't figure out how to get at and
> filter this list from another place - or is there a way of filtering a
> behaviour? I can also do something like this:
>
> (define objects
>   (collect-b
>    (metro 1) '()
>    (lambda (e lst)
>      (let ((pos (vsub (vec3-integral (vec3 0 0.002 0)) (vec3 0 3 0))))
>        (cons
>         (if (> (vdist pos player-pos) 2)
>             (object
>              #:shape 'cube
>              #:colour (vec3 1 0 0)
>              #:translate pos))
>         lst)))))
>
> Which feels more on the right track, but of course, they pop back into
> existence when they move out of the collision volume... Is there some
> way of 'locking' them off when they have collided?
>
> As usual, for the full context, my code is here:
> http://www.pawfal.org/flotsam/frisbee/
>
> The script in question is:
> http://www.pawfal.org/flotsam/frisbee/examples/delete.scm
>
> Many thanks.
>
> dave
>
> http://www.pawfal.org/dave
>
>