gistfile1.txt

    children :: MVar [MVar ()]
    children = unsafePerformIO (newMVar [])
    
    waitForChildren :: IO ()
    waitForChildren = do
      cs <- takeMVar children
      case cs of
        []   -> return ()
        m:ms -> do
           putMVar children ms
           takeMVar m
           waitForChildren

    forkChild :: IO () -> IO ThreadId
    forkChild io = do
        mvar <- newEmptyMVar
        childs <- takeMVar children
        putMVar children (mvar:childs)
        forkIO (io `finally` putMVar mvar ())

     main =
       later waitForChildren $