John Goerzen wrote:
On 2007-03-06, Simon Marlow <[EMAIL PROTECTED]> wrote:
John Goerzen wrote:
possible to create a pipe going directly from program A to program B.
You certainly can pipe directly from one process to another:
That only works for 2 processes. What if I have 4 processes, and want
to pipe from one to the next, all along, as in
ls -l /tmp | tr a-zA-Z | sort | md5sum
to give a contrived example
Before I answer the question, if you don't mind I'll quote from my own message:
> However, I'll accept that there are certain arrangements of pipes that
> aren't possible, or will construct too many pipes and you'll need extra
> threads to connect them together, because the only way to create pipes
> is with runInteractive* and that creates pipes for all of
> stdin/stdout/stderr.
>
> Of course we're open to suggestions for improving System.Process.
> Perhaps there should be a version that lets you specify a Handle *or*
> create a pipe, for each standard file descriptor.
So ideally we'd have a version of runInteractiveProcess that didn't create all
three pipes, and then it would be easy to program your example. Unfortunately
we don't have such a function, but it's still possible to code up your example -
you just end up with too many pipes and you'd need some Haskell threads to act
as dummy "processes" between the extra pipe ends. I'm aware this isn't a
satisfactory solution, for various reasons.
Cheers,
Simon
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