Zombie.hs

newtype Zombie t a = Zombie { unZombie :: [Spine t (Zombie t) a] }

instance Functor (Zombie t) where
  fmap = liftM

instance Applicative (Zombie t) where
  pure = return
  (<*>) = ap

instance Alternative (Zombie t) where
  empty = Zombie []
  Zombie xs <|> Zombie ys = Zombie (xs ++ ys)

instance Monad (Zombie t) where
  return a = Zombie [Spine (Return a) id]
  Zombie xs >>= k = Zombie $ map (\(Spine t c) -> Spine t (c |> Kleisli k)) xs

deflesh :: Zombie t a -> Maybe (MonadView t (Zombie t) a)
deflesh (Zombie (Spine (Return a) s : ss)) = viewL s (Just $ Return a) $ \(Kleisli k) c -> case k a of
  Zombie ss' -> deflesh $ Zombie $ map (\(Spine h t) -> Spine h (c . t)) ss' ++ ss
deflesh (Zombie (Spine (t :>>= k) s : ss)) = Just $ t :>>= \a -> case k a of
  Zombie ss' -> Zombie $ map (\(Spine h c) -> Spine h (s . c)) ss' ++ ss
deflesh _ = Nothing