Netwire 5 - A box cannot bounce - collision

Question

I am trying to convert challenge 3 ( https://ocharles.org.uk/blog/posts/2013-08-01-getting-started-with-netwire-and-sdl.html ) from netwire 4.0 to netwire 5.0 using OpenGL. Unfortunately, the box cannot bounce. The entire code is following. It seems to me that the function velocity does not work. When the box collides with a wall, it does not bounce but stops. How do I correct my program? Thanks in advance.
{-# LANGUAGE Arrows #-}
import Graphics.Rendering.OpenGL
import Graphics.UI.GLFW
import Data.IORef
import Prelude hiding ((.))
import Control.Monad.Fix (MonadFix)
import Control.Wire
import FRP.Netwire
isKeyDown :: (Enum k, Monoid e) => k -> Wire s e IO a e
isKeyDown k = mkGen_ $ \_ -> do
s <- getKey k
return $ case s of
Press -> Right mempty
Release -> Left mempty
acceleration :: (Monoid e) => Wire s e IO a Double
acceleration = pure ( 0) . isKeyDown (CharKey 'A') . isKeyDown (CharKey 'D')
<|> pure (-0.5) . isKeyDown (CharKey 'A')
<|> pure ( 0.5) . isKeyDown (CharKey 'D')
<|> pure ( 0)
velocity :: (Monad m, HasTime t s, Monoid e) => Wire s e m (Double, Bool) Double
velocity = integralWith bounce 0
where bounce c v
| c = (-v)
| otherwise = v
collided :: (Ord a, Fractional a) => (a, a) -> a -> (a, Bool)
collided (a, b) x
| x < a = (a, True)
| x > b = (b, True)
| otherwise = (x, False)
position' :: (Monad m, HasTime t s) => Wire s e m Double (Double, Bool)
position' = integral 0 >>> (arr $ collided (-0.8, 0.8))
challenge3 :: (HasTime t s) => Wire s () IO a Double
challenge3 = proc _ -> do
rec a <- acceleration -< ()
v <- velocity -< (a, c)
(p, c) <- position' -< v
returnA -< p
s :: Double
s = 0.05
y :: Double
y = 0.0
renderPoint :: (Double, Double) -> IO ()
renderPoint (x, y) = vertex $ Vertex2 (realToFrac x :: GLfloat) (realToFrac y :: GLfloat)
generatePoints :: Double -> Double -> Double -> [(Double, Double)]
generatePoints x y s =
[ (x - s, y - s)
, (x + s, y - s)
, (x + s, y + s)
, (x - s, y + s) ]
runNetwork :: (HasTime t s) => IORef Bool -> Session IO s -> Wire s e IO a Double -> IO ()
runNetwork closedRef session wire = do
pollEvents
closed <- readIORef closedRef
if closed
then return ()
else do
(st , session') <- stepSession session
(wt', wire' ) <- stepWire wire st $ Right undefined
case wt' of
Left _ -> return ()
Right x -> do
clear [ColorBuffer]
renderPrimitive Quads $
mapM_ renderPoint $ generatePoints x y s
swapBuffers
runNetwork closedRef session' wire'
main :: IO ()
main = do
initialize
openWindow (Size 1024 512) [DisplayRGBBits 8 8 8, DisplayAlphaBits 8, DisplayDepthBits 24] Window
closedRef <- newIORef False
windowCloseCallback $= do
writeIORef closedRef True
return True
runNetwork closedRef clockSession_ challenge3
closeWindow

Answer 1

By experience, I think the trick here is the fact that you technically have to bounce a few pixels before the actual collision, because if you detect it when it happens, then the inertia put your square a little bit "in" the wall, and so velocity is constantly reversed, causing your square to be blocked.
Ocharles actually nods to it in the blog post :
If this position falls outside the world bounds, we adjust the square (with a small epsilon to stop it getting stuck in the wall) and return the collision information.
Good luck with Netwire 5, I'm playing with it too, and I just begin to like it. ;)