zipEvent :: forall a b. E.Event a -> E.Event b -> E.Event (Tuple a b)
zipEvent e1 e2 = E.makeEvent \o -> do
  latestA <- ST.liftST $ STArray.empty
  latestB <- ST.liftST $ STArray.empty
  emittedA <- Ref.new false
  emittedB <- Ref.new false
  c1 <- E.subscribe e1 \v1 -> do
    _ <- ST.liftST $ STArray.push v1 latestA
    Ref.write true emittedA
    tryEmit o emittedA emittedB latestA latestB
  c2 <- E.subscribe e2 \v2 -> do
    _ <- ST.liftST $ STArray.push v2 latestB
    Ref.write true emittedB
    tryEmit o emittedA emittedB latestA latestB
  pure (c1 *> c2)
    where
      tryEmit o emittedA emittedB latestA latestB = do
        ev1 <- Ref.read emittedA
        ev2 <- Ref.read emittedB
        if ev1 && ev2
          then do
            mv1 <- ST.liftST $ STArray.shift latestA
            mv2 <- ST.liftST $ STArray.shift latestB
            case mv1, mv2 of
              Just v1, Just v2 -> do
                o (Tuple v1 v2)
                Ref.write false emittedA
                Ref.write false emittedB
              _, _ -> pure unit
          else
            pure unit

eachFrame :: C.Context2D -> C.CanvasImageSource -> Effect Unit
eachFrame ctx img = E.subscribe event draw *> pure unit
  where
  event = EA.animationFrame *> updateScene
  draw d = drawCharacter ctx img d

withImage :: String -> (C.CanvasImageSource -> Effect Unit) -> Effect Unit
withImage path f = C.tryLoadImage path $ \mimg -> case mimg of
  Just img -> f img
  Nothing  -> throwException $ error ("Could not load image from path: " <> path)

initWithCanvas :: C.CanvasElement -> Effect Unit
initWithCanvas c = do
  ctx <- C.getContext2D c
  withImage character (eachFrame ctx)
  pure unit