[Haskell-cafe] Re: ANN: HSH 1.2.0

John Goerzen wrote:

On 2007-03-06, Simon Marlow wrote:

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John Goerzen wrote:

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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