state.hs

{-# LANGUAGE FlexibleInstances, FunctionalDependencies, UndecidableInstances #-}

newtype State s a = State (s -> (a, s))

instance Monad (State s) where

    return a = State $ \s -> (a, s)
    
    State st >>= f = State $ \s ->
        let
            (a, s') = st s
            State st' = f a
        in
            st' s'
            
class Monad m => StateMonad m s | m -> s where
    
    update :: (s -> s) -> m s
    
    set :: s -> m s
    set = update . const
    
    fetch :: m s
    fetch = update id
    
instance StateMonad (State s) s where

    update f = State $ \s -> (s, f s)