[Haskell-cafe] Re: Emulating bash pipe/ process lib

Simon Marlow simonmarhaskell at gmail.com
Thu Feb 9 06:47:27 EST 2006

Marc Weber Marc Weber wrote:
> Hi. I want to write a little haskell program executing about 4 programs
> passing data via pipes. As my python script seems to be slower than a
> bash script I want to try a ghc executable now.
> It should invoke different parts of a text to speech chain. This way I
> have one interface then.
> Talar und #haskell told me that I might use runProcess and pass handles
> for stdin and out created by createPipe and fdToHandle.
> So my simple test looks like this:
> module Main where
> import System.IO
> import System.Posix.IO
> main = do
>   (fdIn,fdOut) <- createPipe
>   let (iohIn, iohOut) = (fdToHandle fdIn, fdToHandle fdOut)
>   hIn <- iohIn
>   hOut <- iohOut
>   hPutStr hIn "test"
>   line <- hGetLine hOut
>   print line -- should now print test having been piped through my pipe
> but I get the error:
> pipe2: <file descriptor: 3>: hPutStr: illegal operation (handle is not
> open for writing)
> And in current CVS docs in base.System.Process.hs it is said that
> createPipe is no longer exported ?

If you want to communicate with external programs via pipes, then 
System.Process should provide everything you need.  Take a look at 
runInteractiveProcess in particular.


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