module Main (main) where

import Control.Monad (liftM)
import Control.Monad.Trans (lift)
import Control.Monad.State (StateT, evalStateT, State, evalState, get, put)

main :: IO ()
main = do xs <- readFile "data"
          ys <- readFile "data"
          print (evalState readChunks xs == ys)

---

type InnerMonad = State String

readChunks :: InnerMonad String
readChunks = do xs <- get
                if null xs then return []
                           else do ys <- foo get put
                                   rest <- readChunks
                                   return (ys ++ rest)

---

data St m = St { get_inp :: m String, put_inp :: String -> m () }
type OuterMonad m = StateT (St m) m

foo :: Monad m => m String -> (String -> m ()) -> m String
foo getter putter = evalStateT bar (St getter putter)

bar :: Monad m => OuterMonad m String
bar = do st <- get
         inp <- lift $ get_inp st
         case inp of
             [] -> return []
             x:xs -> do lift $ put_inp st xs
                        liftM (x:) bar