frp.hs

-- I'm currently trying to translate this code https://github.com/bernstein/breakout/blob/master/src/ReactiveUtils.hs
-- to use linear package instead of vector-space, and I'm not sure if I've understood diffE completely.

import Linear.Vector
import Reactive.Banana

type Time = Double

integral :: (Additive f, Num a) => Event t Time -> Behavior t (f a)
                                                -> Behavior t (f a)
integral t b = sumB $ (\v dt -> dt *^ v) <$> b <@> diffE t

sumB :: (Additive f, Num a) => Event t (f a) -> Behavior t (f a)
sumB = accumB zero . fmap (^+^)

diffE :: (Additive f, Num a) => Event t (f a) -> Event t (f a)
diffE = withPrevEWith (^-^)

withPrevEWith :: (a -> a -> b) -> Event t a -> Event t b 
withPrevEWith f e = filterJust . fst . mapAccum Nothing $ g <$> e
    where
    g y Nothing = (Nothing , Just y)
    g y (Just x) = (Just (f y x), Just y)

-- When I try to compile this I get this type error:
{-
game.hs:37:58:
    Couldn't match type `Double' with `f0 Integer'
    Expected type: Event t (f0 Integer)
      Actual type: Event t Time
    In the first argument of `diffE', namely `t'
    In the second argument of `(<@>)', namely `diffE t'
    In the second argument of `($)', namely
      `(\ v dt -> dt *^ v) <$> b <@> diffE t'
-}
-- Where line 37 is the line where linear is defined