notes/pl/hs/ct/monads/monad-state.txt

https://www.fpcomplete.com/school/starting-with-haskell/basics-of-haskell/12-State-Monad

State Monad:

newtype State s a = State { runState :: (s -> (a,s)) }

instance Monad (State s) where
    return a        = State $ \s -> (a,s)
    (State x) >>= f = State $ \s -> let (v,s1) = x s in runState (f v) s1

class MonadState m s | m -> s where
    get :: m s
    put :: s -> m ()

instance MonadState (State s) s where
    get   = State $ \s -> (s,s)
    put s = State $ \_ -> ((),s)

instance MonadPlus (State s) where
    mzero = ...
    mplus = ...

http://stackoverflow.com/questions/5541828/haskell-state-monad-and-monadic-guard