http://www.stephendiehl.com/posts/haskell_web.html

diehl-repl.hs

import Control.Monad.State

type Interact a = StateT Int IO a

repl :: Interact ()
repl = forever $ do
  val   <- liftIO readLn
  state <- get
  liftIO $ putStr "Total: "
  liftIO $ print (val + state)
  put val

main = runStateT repl 0

--

import Data.IORef
import System.IO
import Control.Monad

hReadInt:: Handle -> IO Int
hReadInt h = do val <- hGetLine h
                return $ read val

repl handle ioref = forever $ do 
  val   <- hReadInt handle
  state <- readIORef ioref
  putStr "Total:"
  print (val+state)
  writeIORef ioref val

main = do handle <- openFile "input.txt" ReadMode
          newIORef 5 >>= repl handle