Haskell random generator demonstrating state monad.

HaskellRandom.hs

import System.Random
newtype State s a = State {
        runState :: s -> (a, 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') 

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

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

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

getAny :: (Random a) => State StdGen a
getAny = do
                g <- getSt
                (x, g') <- return $ random g
                putSt g'
                return x

getOne :: (Random a) => (a, a) -> State StdGen a
getOne bounds = do g <- getSt
                   (x, g') <- return $ randomR bounds g
                   putSt g'
                   return x

makeRandomValue :: StdGen -> (Int, StdGen)
makeRandomValue = runState (do 
                                n <- getOne (1, 100)
                                return n)

main = do
        g <- getStdGen
        print $ fst $ makeRandomValue g
        return ()