ATs

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, TypeFamilies #-}
module AT where

{- --- ORIGINAL --- -}

data Step s a = Done
              | Skip !s
              | Yield !a !s

{- --- WITH ATs --- -}

class AStep s a where
    data Stepper s a
    done :: Stepper s a
    skip :: s -> Stepper s a
    yield :: a -> s -> Stepper s a

instance (AStep s) Char where
    data Stepper s Char = CDone
                        | CSkip !s
                        | CYield {-# UNPACK #-} !Char !s
    done = CDone
    {-# INLINE done #-}
    skip = CSkip
    {-# INLINE skip #-}
    yield = CYield
    {-# INLINE yield #-}