module EST where

import GHC.Base

newtype EST s a = EST (s -> Maybe (a, s))

unEST (EST m) = m

instance Functor (EST s) where
  fmap f x = x >>= \ a -> return (f a)

instance Applicative (EST s) where
  pure a = EST (\ s -> Just (a, s))
  f <*> x = f >>= \ g -> x >>= \ a -> return (g a)

instance Monad (EST s) where
  (EST m) >>= k = EST (\ s0 -> case m s0 of
                                 Just (a, s1) -> unEST (k a) s1
                                 Nothing      -> Nothing)


instance Alternative (EST s) where
  empty = mzero
  (<|>) = mplus

instance MonadPlus (EST s) where
  mzero = EST (\ _ -> Nothing)
  (EST m) `mplus` (EST n) = EST (\ s -> m s `mplus` n s)

evalEST st s = case unEST st s of { Just (a, _) -> Just a; _ -> Nothing }