I'm writing a program that will be using multiple threads to handle network activity on multiple ports in a responsive way. The treads will all need access to some shared data, so I'm using an MVar. So far so good. The problem is that passing the MVar around everywhere is kind of a pain, so I was hoping to use a ReaderT monad on top of the IO monad to handle that for me. I have that working, but one piece seemed a bit out of place so I wondered if there was a better way. Below is a small test program that presents my question. My program uses forkIO to create the separate threads (Set A), and some of *those* threads will need to create threads (Set B). In order for the ReaderT to handle the environment of the threads in Set B, do I have to perform another runReaderT when forking? Or is there a way to get the ReaderT environment automatically carried over to the newly created Set B thread? See the "NOTE" comment in the code below for the particular spot I'm asking about.
import Control.Monad.Reader import Control.Concurrent
type MyReader a = ReaderT (MVar Integer) IO a
main = do mv <- newMVar 1 forkIO $ runReaderT (printAndFork "one" 5) mv forkIO $ runReaderT (printAndFork "two" 5) mv getChar -- Pause so the threads can print before program exit
printAndFork :: String -> Integer -> MyReader () printAndFork _ 0 = return () printAndFork str n = do mv <- ask lift $ do modifyMVar_ mv $ \i -> do print $ str ++ show i return (i + 1) -- NOTE: Is this runReaderT necessary to carry the -- environment to the new thread? Is there a better way? forkIO $ runReaderT (justPrint $ "inner " ++ str) mv printAndFork str (n - 1)
justPrint :: String -> MyReader () justPrint str = do mv <- ask lift $ do modifyMVar_ mv $ \i -> do print $ str ++ show i return (i + 1) return ()
Kurt Hutchinson