[Haskell-cafe] Question about concurrency, threads and GC

[Joey Adams]

On Fri, Mar 2, 2012 at 3:04 PM, Alexander V Vershilov
<alexander.vershilov at gmail.com> wrote:
> Hello, Paul.
>
> It seems you should not use 3 threads, but run in a data-driven behaviour with
> one thread per client.

I don't think this will work for Paul's situation.  He needs to be
able to send notifications to clients.  This means sending to the
client even when the client has nothing to say at the moment.

I've been grappling with the same problem.  See:

    http://www.haskell.org/pipermail/haskell-cafe/2012-January/098495.html

The discussion was recently summed up in Parallel Haskell Digest 8
(scroll down to "How to make asynchronous I/O composable and safe?"):

    http://www.well-typed.com/blog/64

To try to address this problem, I wrote a module called stm-channelize:

    http://hackage.haskell.org/package/stm-channelize

However, I did not end up using it.  I would point you to the source
of my channelize function, but I think it's more complicated than it
needs to be.

I, for one, would recommend a three-thread approach.  That is, for
each client, you basically have:

 * A thread for receiving

 * A thread for sending

 * A main thread that waits for the other two threads, and kills them
when one of them finishes or dies.

I'll try to put together a simple chat server example, like the one I
wrote for stm-channelize.

-Joey