State Haskell

state.hs

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

test :: Int -> State Int () -> State Int () -> State Int Bool
test n f g = (f `bindState` (\_ -> g) `bindState` (\_ -> State $ (\s -> ((s > n), s)))) 

runState (State t) s = t s

returnState :: a -> State s a 
returnState a = State $ (\s -> (a, s))

bindState :: State s a -> (a -> State s b) -> State s b
bindState m k = State $ (\s -> let (a, s') =  runState m s
					in runState (k a) s')

get :: State s s
get = State $ (\s -> (s, s))

set :: s -> State s ()
set s = State $ (\_ -> ((), s))

instance Monad (State s) where  
	return = returnState
	(>>=) = bindState

testMonad :: Int -> State Int () -> State Int () -> State Int Bool
testMonad n f g = ( f >> g >> (State $ (\s -> ((s>n), s))))

testMonad2 :: Int -> State Int () -> State Int () -> State Int Bool
testMonad2 n f g = do
			f
			g
			State $ (\s-> ((s>n), s))

f :: State Int ()
f  = State $ (\s -> case s of
			1 -> ((), (s+1))
			2 -> ((), (s+2))
			_ -> ((), (s))
			) 

g :: State Int ()
g = State $ (\_ -> ((), 11))