MonadState instance for IO (not sound!)

IoState.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ExtendedDefaultRules #-}

module IoState where

import Control.Monad.State.Class

import Data.IORef
import System.IO.Unsafe

type IoState s = IO

stRef :: IORef a
stRef = unsafePerformIO (newIORef undefined)
{-# NOINLINE stRef #-}

instance MonadState s (IoState s) where
     get = readIORef stRef
     put st = writeIORef stRef st

runIoState :: s -> IoState s a -> IO a
runIoState st action = do
    writeIORef stRef st
    action

test1 :: IO ()
test1 = runIoState (2 :: Int) (get >>= print)
-- prints "()"

test2 :: IO ()
test2 = runIoState (2 :: Int) (get >>= print . (+ (1::Int)))
-- prints "3"

test3 :: IO ()
test3 = runIoState (2 :: Int) (get >>= print . (+ (1 :: Double)))
-- prints "1.0"