Hello Haskellers, Having been pretty much impressed by Don Stewart's "Practical Haskell" (http://donsbot.wordpress.com/2010/08/17/practical-haskell/), I started to write a Haskell script to run maven jobs (yes, I know...). In the course of undertaking this fantastic endeavour, I started to use the System.Process.readProcessWithExitCode function, but following the advice in the comment for this function, I rolled my own stuff and ended up writing the following:

doRunMvnInIO pom args filters e = do (Just inh, Just outh, Just errh, pid) <-          createProcess (proc (maven e)  (["-f", pom] ++ args)) { std_in  = CreatePipe,                                                                  std_out = CreatePipe,                                                                  std_err = CreatePipe }        waitQ <- newEmptyMVar

       mapM (printAndWait waitQ)  [outh, errh]

       hClose inh        -- wait on the process        waitForProcess pid          where            printAndWait waitQ hdl = do out <- hGetContents hdl                                        forkIO (mapM (putStrLn) (filter filters $ lines out) >> putMVar waitQ ())                                        takeMVar waitQ                                        hClose hdl

This is actually a rewrite of the following function:

doRunMvnInIO' pom args filters e = do (Just inh, Just outh, Just errh, pid) <- createProcess (proc (maven e) (["-f", pom] ++ args)) { std_in = CreatePipe, std_out = CreatePipe, std_err = CreatePipe } waitQ <- newEmptyMVar

mapM (printAndWait waitQ) [outh, errh] >>= mapM (\_ -> takeMVar waitQ)

hClose inh hClose outh hClose errh -- wait on the process waitForProcess pid where printAndWait waitQ hdl = do out <- hGetContents hdl forkIO (mapM (putStrLn) (filter filters $ lines out) >> putMVar waitQ ())

What surprised me is that I would expect the behaviour of the two functions to be different: - in doRunMvnInIO, I would expect stdout's content to be printed before stderr, ie. the 2 threads are ordered because I call takeMVar in between calls to forkIO - in doRunMvnInIO', this is not true and both theads run concurrently. but actually there does not seem to be a difference: printing is still interleaved in both functions, AFAICT. I would welcome any help on this. Best regards, Arnaud