I am trying to make objects bounce off the walls, using mutually dependent velocity and location wires. Simple one-dimentional example looks like this:
{-# LANGUAGE Arrows #-}
import Prelude hiding ((.), id)
import Control.Wire
import FRP.Netwire
import Control.Monad.IO.Class
-- location -> (corrected-location, did-bounce)
-- "reflect" location back behind the border and indicate the bounce
checkW :: (HasTime t s) => Wire s () IO Double (Double, Bool)
checkW = mkSF_ check
where
check loc
| loc < 0 = (-loc, True)
| loc > 1 = (2-loc, True)
| otherwise = (loc, False)
-- did-bounce -> velocity, initial velocity in the argument
velW :: Double -> Wire s () IO Bool Double
velW iv = mkSFN $ \bounce -> (iv, velW (if bounce then -iv else iv))
-- produce tuple (location, velocity)
locvelW :: (HasTime t s) => Wire s () IO a (Double, Double)
locvelW = proc _ -> do
rec (loc, bounce) <- (checkW . integral 0.5) -< vel
vel <- (velW 0.3) -< bounce
returnA -< (loc, vel)
main :: IO ()
main = testWireM liftIO clockSession_ locvelW
If I run this, after the first bounce velocity starts flipping between negative and positive values every step.
I know that I can "fix" it by signalling the velocity wire to make velocity negative or positive depending on which border I bounced off. It works. But I want to understand why I see this behaviour, I know that flip should only happen once, as I explicitly push the object to the other side of the border. I suspect that laziness plays role here, and maybe a strategically placed seq would make it work "as intended".
I'd like to have an explanation, and suggestion how to fix it without resorting to "brute force" solution.
This behavior is due to a simple logical error in velW. velW isn't changing the velocity when it bounces, it's only changing the initial velocity for the next time the velocity is calculated; they should both change. Here's a correct version.
-- did-bounce -> velocity, initial velocity in the argument
velW :: Double -> Wire s e m Bool Double
velW iv = mkSFN $ \bounce -> let vel = if bounce then -iv else iv in (vel, velW vel)
Laziness can't play a role here. Laziness affects when a computation happens, but it doesn't affect what the computation means. A pure computation is unaffected by side-effects, so its result doesn't depend on when it is evaluated.