import Control.Monad

data Result a e = Ok a | Error e

newtype StateError s e a = StateError { runStateError :: s -> (Result a e, s) }

instance Functor (StateError s e) where
  fmap f x = x >>= (pure . f)

instance Applicative (StateError s e) where
  pure x = StateError $ \s -> (Ok x, s)
  x <*> y = ap x y
  
instance Monad (StateError s e) where
  return x = pure x

  m >>= f = StateError $
    \s -> case runStateError m s of
            (Ok x, s1) -> runStateError (f x) s1
            (Error e, s1) -> (Error e, s1)

get = StateError $ \s -> ((Ok s), s)

put s = StateError $ \_ -> ((Ok ()), s)

main = return ()