Anonymous

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Control.Monad.ST
import Control.Monad.State
import Control.Monad
import Data.STRef

type TheState = Int

newtype M a = M { runM :: State (TheState) a }
  deriving (Monad, MonadState TheState)

newtype N s a = N { runN :: StateT (TheState) (ST s) a }
  deriving (Monad, MonadState TheState)


nAndStCode = do
  call mCode
  ref <- new_ref "hi"
  return ref
  

new_ref :: a -> N s (STRef s a)
new_ref = N . lift . newSTRef

call :: M a -> N s a
call (M m) = N $ do
  state <- get
  let (r, st') = runState m state
  put st'
  return r

mCode :: M String
mCode = do
  modify (+1)
  return "hi"