This program will steadily read lines from STDIN. When meet a line with "out", it will print out the line number. BUT. if there are many many many many lines before the "out", then the "out" will cause the program crash because of the lazy evaluation 'count + 1'. My solution is add a 'seq' to force the evaluation 'count + 1'.

gistfile1.hs

module T where

import Control.Concurrent.MVar

main = do
  countMVar <- newMVar 0 :: IO (MVar Int)

  let
    process :: IO ()
    process = do
      cmd <- getLine
      modifyMVar_ countMVar $ \ count -> return $ count + 1
      if cmd == "out"
        then withMVar countMVar $ \ count -> putStrLn $ show count
        else return ()
      process

  process