Hi Greg.

I'd use a couple of integrals and a snapshot, and the result would indeed be recursive.  I carefully designed the semantics of reactivity to make this sort of thing possible.  In particular, switcher switches immediately after event occurrences.  Not "one tick" or some such (since Reactive is based on continuous time), but really immediately.  Thus snapshotting a behavior at the moment it switches gets the pre-event-occurrence value.  This sort of formulation worked fine in Fran and was great fun.

Now here's the temporary bad news:  Currently, there's a bug in the Reactive implementation, and this sort of (recursive) example locks up. :(  Creighton is looking into it.  http://trac.haskell.org/reactive/ticket/1 .

  - Conal

On Thu, Nov 20, 2008 at 5:35 PM, Greg Fitzgerald <garious@gmail.com> wrote:
How do I make a Behavior of a bouncing motion?  I want to reverse the
velocity when the object goes beyond a certain position, but since the
position depends on the velocity, I have a loop.  Is there a way to
make recursive behaviors, or am I going about this all wrong?

bouncingPosition :: Behavior Double
bouncingPosition = ???

bouncingVelocity :: Double -> Behavior TimeT -> Behavior Double
bouncingVelocity v0 t = velocityB v0 t `switcher` ???

collision :: Event a -> Behavior Double -> Event Double
collision e = once . filterMP (< -2) . snapshot_ e

positionB :: Double -> Behavior Double -> Behavior Double -> Behavior Double
positionB x0 v t = (x0 +) <$> liftA2 (*) v t

velocityB :: Double -> Behavior Double -> Behavior Double
velocityB v0 t = velocity v0 <$> t

velocity :: Double -> TimeT -> Double
velocity v0 t = v0 - 9.8 * t

Thanks,
Greg
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