How to create an Event that contains the difference between the value at current tick and the value at the value at previous tick?

I'm using reactive-banana and sdl2 (using this glue library) for a game-like project. A Behavior is created for the "absolute mouse location", as well as a Behavior for the "relative mouse location" (a.k.a. the mouse movement). When not using FRP this works well, but with FRP the "relative mouse location" becomes a problem: it seems only a small amount of the data comes through. I suspect this happens because the underlying "SDL events" (that we represent with a Behavior) do not line up nicely with the tick Events.

So I want to calculate my own mouse movement, by simply comparing the mouse location at the current tick with the location at the previous tick. I'm not sure if this will solve my problems, but I have good hope :)

First of all I'm lost on how to approach it: the State monad, or an IORef, or does reactive-banana provide another means?

I'll give a small excerpt of the code I currently have:

makeNetwork :: GraphicsData -> SDLEventSource -> MomentIO ()
makeNetwork gd sdlEventSource = mdo
  tickE          <- tickEvent sdlEventSource
  mouseMovementB <- fromPoll SDL.getRelativeMouseLocation
  mousePositionB <- fromPoll SDL.getAbsoluteMouseLocation

  let mousePositionE = mousePositionB <@ tickE
      mouseMovementE = mouseMovementB <@ tickE  -- this yields flaky data

  -- ... the rest of the network description left out ...

As explained above I'd like to express mouseMovementE in terms of the mousePositionB at current tickE (known as mousePositionE) and the mousePositionE value at the previous tickE.

Any help is greatly appreciated!

Solution

You are looking for accumE which builds events from streams of events. I highly recommend reading the recursion section of the documentation which describes how it's implemented in terms of stepper and apply.

accumE :: MonadMoment m => a -> Event (a -> a) -> m (Event a)
--        starting value --^           |             |
--  stream of events that modify it  --^             |
--  resulting events                               --^

To compute the difference between two points with accumE we'll need to keep track of the previous point. We'll also keep track of the current point. This will keep a little two-item history of the most recent events.

  (Point V2 CInt , Point V2 CInt)
-- previous value, current value

edges :: MonadMoment m => a -> Event a -> m (Event (a, a))
edges initial later = accumE (initial, initial) (shift <$> later)
    where
        shift x2 (x0, x1) = (x1, x2)

To get the difference we'll subtract the previous one from the current one. This will give a complete network like

makeNetwork :: GraphicsData -> SDLEventSource -> MomentIO ()
makeNetwork gd sdlEventSource = mdo
  tickE          <- tickEvent sdlEventSource
  mousePositionB <- fromPoll SDL.getAbsoluteMouseLocation

  let mousePositionE = mousePositionB <@ tickE

  mouseHistoryE <- edges zero mousePositionE

  let mouseMovementE = (\(x0, x1) -> x1 ^-^ x0) <$> mouseHistoryE

  -- ...

zero and ^-^ come from Linear.Vector