mstate.hs

import Control.Monad
import Data.Monoid

data MState s a = MState s a

instance Monoid s => Functor (MState s) where
    fmap f (MState st x) = MState st (f x)

instance Monoid s => Applicative (MState s) where
    pure = return
    (<*>) = ap

instance Monoid s => Monad (MState s) where
    return x = MState mempty x

    (>>=) (MState st x) f = MState (mappend st st') x'
        where (MState st' x') = f x