Reactogon/Auxiliary/Auxiliary.hs

   1 {-# LANGUAGE Arrows #-}
   2
   3 module Reactogon.Auxiliary.Auxiliary where
   4
   5 import Data.Maybe
   6 import FRP.Yampa
   7
   8 -- stepBack contains its previous argument as its output. Because it's
   9 -- hard to define it at time 0, it's wrapped up in a Maybe.
  10 stepBack :: SF a (Maybe a)
  11 stepBack = sscan f (Nothing, Nothing) >>^ snd
  12   where f :: (Maybe a, Maybe a) -> a -> (Maybe a, Maybe a)
  13         f (Nothing,Nothing) x' = (Just x', Nothing)
  14         f (Just x, _) x' = (Just x', Just x)
  15
  16 -- Throws an Event when the incoming signal change. The Event is
  17 -- tagged with the new value.
  18 onChange :: (Eq a) => SF a (Event a)
  19 onChange = proc x -> do
  20   x' <- stepBack -< x
  21   let makeEvent x x'
  22         | isNothing x' = NoEvent
  23         | isJust x' = let x'' = fromJust x' in
  24             if x'' == x then NoEvent else Event x
  25   returnA -< makeEvent x x'