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2012-12-24T21:56:56.117
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You can use <code>pipes</code> to concurrently merge two input streams. The first trick is to read from two streams concurrently, which you can do using the <code>stm</code> package: <pre><code>import Control.Applicative import Control.Proxy import Control.Concurrent import Control.Concurrent.STM import System.Process toTMVarC :: (Proxy p) => TMVar a -> () -> Consumer p a IO r toTMVarC tmvar () = runIdentityP $ forever $ do a <- request () lift $ atomically $ putTMVar tmvar a fromTMVarS :: (Proxy p) => TMVar a -> () -> Producer p a IO r fromTMVarS tmvar () = runIdentityP $ forever $ do a <- lift $ atomically $ takeTMVar tmvar respond a </code></pre> I will soon provide the above primitives in a <code>pipes-stm</code> package, but use the above for now. Then you just feed each <code>Handle</code> to a separate <code>MVar</code> and read from both concurrently: <pre><code>main = do (_, mStdout, mStderr, _) <- createProcess (proc "ls" []) case (,) <$> mStdout <*> mStderr of Nothing -> return () Just (stdout, stderr) -> do out <- newEmptyTMVarIO err <- newEmptyTMVarIO forkIO $ runProxy $ hGetLineS stdout >-> toTMVarC out forkIO $ runProxy $ hGetLineS stderr >-> toTMVarC err let combine () = runIdentityP $ forever $ do str <- lift $ atomically $ takeTMVar out `orElse` takeTMVar err respond str runProxy $ combine >-> putStrLnD </code></pre> Just change out <code>putStrLnD</code> with however you want to process the input. To learn more about the <a href="http://hackage.haskell.org/package/pipes"><code>pipes</code> package</a>, just read <a href="http://hackage.haskell.org/packages/archive/pipes/3.0.0/doc/html/Control-Proxy-Tutorial.html">Control.Proxy.Tutorial</a>.
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1. POHaskell: System.Process merge stdout and stderr
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1. USGabriel Gonzalez
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