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
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