module NewType where

newtype ST s a = ST { unST :: s -> (a, s) }
-- newtype ST s a = ST (s -> (a, s))
-- unST (ST m) = m

unitST :: a -> ST s a
unitST a = ST (\ s -> (a, s))

bindST :: ST s a -> (a -> ST s b) -> ST s b
(ST m) `bindST` k = ST (\ s0 -> let (a, s1) = m s0 in unST (k a) s1)



instance Monad (ST s) where
  return = unitST
  (>>=)  = bindST