[ moved to glasgow-haskell-users@haskell.org... ]
I need to compress the output of my Hakell program. To avoid the creation of huge files, I want to compress before writing by means of gzip or bzip2. However, this seems to be quite involved.
I decided to run the compressor with
runProcess :: FilePath -- Command -> [String] -- Arguments -> Maybe [(String, String)] -- Environment(Nothing -> Inherited) -> Maybe FilePath -- Working directory (Nothing -> inherited) -> Maybe Handle -- stdin (Nothing -> inherited) -> Maybe Handle -- stdout (Nothing -> inherited) -> Maybe Handle -- stderr (Nothing -> inherited) -> IO ()
First I wanted to use Posix.createPipe to connect to the compressor but Posix.createPipe returns a Fd. So this does not fit.
Aha, you want the (unadvertised) function PosixIO.fdToHandle :: Fd -> Handle
I ended up with using Posix.createNamedPipe. This creates a named pipe that can be openend with IO.openFile (which again yields the Handle needed for runProcess).
At this point, it turned out that the pipe has to be opened for reading before it can be opened for writing (ghc-4.08.1). This seems to be a bug. (In a shell, the order does not matter; the processes are suspended as expected.)
We've had some problems with named pipes - check the archives of glasgow-haskell-bugs. It seems certain operating systems disagree on the semantics of a non-blocking open of a named pipe. I don't recommend using named pipes if you want portable code.
Then, as expected, the problem occured that writing to the pipe and compressing cannot be sequenced. If the producer is started first, the producer blocks because there is no consumer. If the consumer is started first, the consumer blocks because there is no input.
So I played around with threads. If both consumer and producer are executed as threads, the program terminates immediately without any output. There seems to be no need to start executing any of the threads.
In GHC, the main program terminates as soon as the main thread terminates. It doesn't wait for any child threads to terminate - if you want this behaviour, you can program it using MVars. We'll be happy to look at any of the other problems you mentioned: please some code demonstrating the problem to glasgow-haskell-bugs@haskell.org. Cheers, Simon