[Haskell-beginners] Haskeline and forkIO

[Jeff C. Britton]

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