event-depending behaviors

gistfile1.hs

once :: Event t a -> Event t a
once e = let b = stepper True (False <$ e)
         in whenE b e

changes' :: Eq a => Event t (a, a) -> Event t a
changes' e = fst <$> filterE (uncurry (/=)) e

stepperP :: a -> Event t a -> (Event t (a, a), Behavior t a)
stepperP acc e = mapAccum acc (withPrev <$> e)
    where withPrev x acc = ((x, acc), x)

stepperC :: Eq a => a -> Event t a -> (Event t a, Behavior t a)
stepperC acc e = let (e', b) = stepperP acc e
                 in (changes e', b)