[Haskell-cafe] Detecting unused read handles? (was: File handles and pipes)

Thomas Hartman tphyahoo at gmail.com
Thu Oct 23 15:38:00 EDT 2008

>> 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

Can the left-fold enumerator described by oleg in


and with more references at http://okmij.org/ftp/Haskell/

be adapted for reading the output from arbitrary system calls? I.e.,
to deal with pipes? My intuition says yes but I'm still trying to wrap
my head around the whole left fold enumerator approach to io.

The text around "We can now test reading from two streams with
interleaving:" in the first reference seemed particularly promising to
> 2008/10/19 Stephen Hicks <sdh33 at cornell.edu>:
>> On Sun, Oct 19, 2008 at 1:44 AM, Brandon S. Allbery KF8NH
>> <allbery at ece.cmu.edu> wrote:
>>> On 2008 Oct 19, at 1:37, Stephen Hicks wrote:
>>>> 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:
>>> Pipes are perhaps a bit misnamed:  if you want to combine the output of two
>>> pipes and funnel it into a third you can't simply "plumb them together", you
>>> need to provide code which reads from the output pipes and writes into the
>>> input pipe.  If you don't care about order, forkIO a thread for each output
>>> pipe which reads from the output pipe it's passed and writes to the input
>>> pipe.  If order is significant, use mapM/forM to run the output-to-input
>>> code on each handle in turn.
>> Thanks a lot - that seems to work very well, and even scales to large
>> amounts of data nicely (and quickly, with Lazy ByteStrings).
>> I've got one more question now.  Suppose I want to do the same thing
>> on the other side, with two processes *receiving* the data.  Is there
>> a way to tell whether the first process wants input, and if not, wait
>> for the second process to do anything?
>> That is, suppose I have something like
>>> do (Just inh1, _, _, p1) <- createProcess (shell "echo 1") { std_in = CreatePipe }
>>>    -- wait for p1, possibly feeding it some input?
>>>    (Just inh2, _, _, p2) <- createProcess (shell "cat") { std_in = CreatePipe }
>> Is there a way to figure out that the "echo 1" process never wanted
>> any input, and therefore not give it any?  I looked through all of
>> System.IO and everything seemed to indicate that inh1 was open, even
>> after the process ended.  The only indication otherwise was that
>> "hflush inh1" failed with "resource vanished".  I guess what I'm
>> asking is I want to "wait until the process p1 is waiting for input
>> and/or terminates", and if it's the latter, move on to the next
>> process in line, before actually doing any hPutStr into the read
>> handles.  Is that possible?
>> Thanks,
>> steve
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