Concurrency and interactive input

[Sigbjorn Finne]

Hi,

I suspect you're using ghc on a win32 platform. ghc-win32 doesn't
presently support non-blocking I/O, hence the behaviour you're
seeing.

--sigbjorn

----- Original Message -----
From: Walker Duhon
To: glasgow-haskell-users@haskell.org
Sent: Wednesday, October 30, 2002 18:02
Subject: Concurrency and interactive input



Hello,
            I am playing around with concurrency using ghc 5.02.2.  What I'd
like to do is have a bunch of threads do something indefinitely, while one
thread monitors stdin for user input (the user input affects the behavior of
the other threads in some way).  From the documentation I had assumed that
standard Haskell I/O functions (like getline) would not block other running
threads, but this doesn't seem to be the case as illustrated here:

conTry3 = forkIO (loop 'a') >> quitYet
          where
            loop ch = hPutChar stdout ch >> loop ch
            quitYet = do
                x <- getLine
                if x == "quit" then return ()
                               else quitYet

Output: (stuff in between {} is a comment, and not part of the output)
rere
ttt
fff
dfsd
fgfg
fgfg
aaaaaaaaaaaaaaaaaa { and on and on }
dfdf
dfd
fdfd
fgfg

fgf
aaaaaaaaaaaaaaaa { and on and on }
quit
aaaaaaa { and on and on }
{program ends}

The behavior I wanted was for 'a' to be printed (almost) continuously, echo
the relatively infrequent input, and quit upon receiving "quit".
            I am aware (through a web search) that there was a mail thread
addressing a similar issue a while back (1999) entitled:
                           I thought concurrent haskell was _preemptive_!
            That thread made it seem like the issue would be cleared up
soon.

In any case, I would still like to be able to have an interactive input
thread that doesn't block everything in my program.
Is there any way to do this?

Thanks in advance,
            Walker