[Haskell-cafe] File handles and pipes

[Thomas Hartman]

Hackage has the pipe library and the shell-pipe library. The package
page for shell-pipe says it's probably to use system.Process now, but
pipe was recently hackagized, by another person on this thread
actually, I believe (Matti).

Once you solve this problem, perhaps your solution could be patched
into the Pipe package.


2008/10/19 Stephen Hicks <sdh33 at cornell.edu>:
> Hi,
>
> I'm trying to understand how to get pipes working in Haskell, in
> particular with the revamped System.Process (though I've tried similar
> experiments with System.Posix.IO and Control.Concurrent).
> Specifically I'm trying to concatenate the output of two system calls
> into the input of a third.  The following code does not get the job
> done:
>
>> import Data.Maybe ( fromJust )
>> import System.IO
>> import System.Process
>
>> sink :: String -> IO (Handle,ProcessHandle)
>> sink s = do let p = (shell s) { std_in = CreatePipe }
>>             (mh, _, _, ph) <- createProcess p
>>             return (fromJust mh,ph)
>
>> source :: String -> Handle -> IO ()
>> source s h = do let p = (shell s) { std_out = UseHandle h,
>>                                     close_fds = False }
>>                 (_, _, _, ph) <- createProcess p
>>                 waitForProcess ph
>>                 return ()
>
>> main :: IO ()
>> main = do (h,p) <- sink "sort"
>>           source "df" h
>>           source "df -h" h
>>           waitForProcess p
>>           return ()
>
> When I run this, I see that the filehandle h is closed after the first
> df, so the output only has one of the two df's included in it.  I
> tried also System.Posix.IO (createPipe, fdToHandle, dup) to make this
> work, but that gave a "read failed" error.  Is there any way to get
> this done...?  In general, given a handful of system calls (and/or
> read/write handles), is there a way to combine them all together?  (I
> also tried using forkIO on my own function pipe :: [Handle] -> Handle
> -> IO (), which does what you'd expect given multiple read handles and
> a single write handle, but couldn't get any output from that at
> all...)
>
> Any help is appreciated,
> steve
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