Глобальное состояние в Haskell

{-# LANGUAGE MultiParamTypeClasses #-}

import Data.IORef
import Control.Monad.State

data MyHandler = MyHandler
  { handler :: Int -> Int
  }

newtype MyHandlerT m a = MyHandlerT { fromMyHandlerT :: IORef MyHandler -> m a }

instance MonadTrans MyHandlerT where
  lift = MyHandlerT . const

instance MonadIO m => Monad (MyHandlerT m) where
  return = MyHandlerT . const . return
  MyHandlerT m >>= k = MyHandlerT $ 
    \r -> do
      x <- m r
      let MyHandlerT m' = k x in m' r
  MyHandlerT m >> MyHandlerT m' = MyHandlerT $ \r -> m r >> m' r
  fail = error

instance MonadIO m => MonadState MyHandler (MyHandlerT m) where
  get = MyHandlerT $ liftIO . readIORef
  put s = MyHandlerT $ liftIO . flip writeIORef s

myDefaultHandler :: MyHandler
myDefaultHandler = MyHandler 
  { handler = (+ 1)
  }

runMyHandlerT :: MonadIO m => MyHandlerT m a -> m a
runMyHandlerT m = liftIO (newIORef myDefaultHandler) >>= fromMyHandlerT m

test :: MyHandlerT IO ()
test = do
  h <- gets handler
  lift $ print $ h 0
  put $ myDefaultHandler { handler = (+ 2) }
  h <- gets handler
  lift $ print $ h 0

main :: IO ()
main = runMyHandlerT test

  modify $ \oldHandler -> oldHandler { handler = ... }