[Haskell-cafe] ReaderT and concurrency
tomasz.zielonka at gmail.com
Wed Nov 16 14:32:37 EST 2005
On Wed, Nov 16, 2005 at 10:53:17AM -0800, Andrew Pimlott wrote:
> On Wed, Nov 16, 2005 at 07:20:48PM +0100, Tomasz Zielonka wrote:
> > I think it wouldn't be possible using only methods in MonadIO.
> Here's what I had in mind for forkProcess. Recall that the fork syscall
> returns 0 to the child and a pid to the parent.
> forkProcess :: MonadIO m => m () -> m ProcessID
> forkProcess io = do pid <- liftIO forkSyscall
> if (pid == 0) then io >> exit
> else return pid
Well, yes, I haven't thought about it. I don't know forkSyscall
and I can't say if it's a good idea or not.
> But maybe the primitives used by forkIO do not allow this approach. I'm
> not sure.
I'm pretty sure they don't, but I may be wrong again. They could
probably be extended to allow this. However, I'm not sure I would like
this, because it would be too easy to break abstraction.
> > Besides,
> > what should be the semantics of forkIO for (StateT s IO)? I can't think
> > of anything reasonable.
> I don't see the problem--they would each get a copy of the state.
> Although I could be missing something, as I use System.Posix.Process
> more than Control.Concurrent.
Yes, but the changes made in the child thread would be ignored
in the parent.
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