[Haskell-cafe] Re: Execution of external command

Duncan Coutts duncan.coutts at worc.ox.ac.uk
Thu Dec 13 13:08:44 EST 2007

On Thu, 2007-12-13 at 19:38 +0200, Yitzchak Gale wrote:

> Simon Marlow wrote:
> > It could deadlock if the script produces enough stderr to fill up its pipe
> > buffer
> If we need to worry about that, then what about this:
> (_,h,e,_) <- runInteractiveCommand "script params"
> forkIO (hGetContents e >>= evaluate . length)
> output <- hGetContents h
> It requires -threaded in the case of a huge amount

It does not require -threaded. GHC's single threaded rts has always been
able to cope with multiple haskell threads that want to do file/network

> of output to both stdout and stderr; maybe that isn't
> good for Bulat. If that is a problem, you can read a chunk
> at a time and call yield - but that is getting messier.

Calling yield ends up busy waiting if there is no output available from
either stdout or stderr. It also requires threads so it's not a portable

Something simple would be to allow attaching a pipe to just the stdout
and redirecting stderr elsewhere, or connecting both stdout and stderr
to the same output pipe. runProcess allows substituting any of
stdin/stdout/stderr for other Handles and runInteractiveProcess
substitutes them all for pipes. What we need is something in between
that allows substituting some for given Handles and connecting others to


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