I am trying to modify an example in RealWorldHaskell from Chapter 24. The example is the first code snippet labeled -- file: ch24/Compressor.hs I am trying to replace the use of Readline with Haskeline. In my code the forkIO thread does not run. I guessed that since the result of the worker thread was thrown away that perhaps laziness was the problem. So, I attempted to use `seq`, but that does not work either. I am able to run the RealWorldHaskell example. I am using GHC 7.8.3. I have tried runhaskell with and without the -threaded option and on both Linux and Windows 7. import Control.Concurrent (forkIO) import Control.Exception import qualified Data.ByteString.Lazy as L import System.Console.Haskeline hiding (handle) -- Provided by the 'zlib' package on http://hackage.haskell.org/ import Codec.Compression.GZip (compress) -- Read the file, compress the data, write the compressed data worker :: FilePath -> IO () worker path = L.readFile path >>= L.writeFile (path ++ ".gz") . compress -- Run the worker on a new thread runWorker :: FilePath -> IO() runWorker path = handle (print :: SomeException -> IO ()) $ do forkIO (worker path) return () loop :: InputT IO () loop = do maybeLine <- getInputLine "Enter a file to compress> " case maybeLine of Nothing -> return () -- user entered EOF Just "" -> return () -- treat no name as "want to quit" Just path -> let f = runWorker path in f `seq` do return f loop main = runInputT defaultSettings loop