[Haskell-cafe] How to ensure code executes in the context of a specific OS thread?

[Simon Marlow]

On 06/07/2011 07:37, Jason Dagit wrote:
>
> On Jul 5, 2011 1:04 PM, "Jason Dagit" <dagitj at gmail.com
> <mailto:dagitj at gmail.com>> wrote:
>  >
>  > On Tue, Jul 5, 2011 at 12:33 PM, Ian Lynagh <igloo at earth.li
> <mailto:igloo at earth.li>> wrote:
>  > > On Tue, Jul 05, 2011 at 08:11:21PM +0100, Simon Marlow wrote:
>  > >>
>  > >> In GHCi it's a different matter, because the main thread is running
>  > >> GHCi itself, and all the expressions/statements typed at the prompt
>  > >> are run in forkIO'd threads (a new one for each statement, in fact).
>  > >> If you want a way to run command-line operations in the main thread,
>  > >> please submit a feature request.  I'm not sure it can be done, but
>  > >> I'll look into it.
>  > >
>  > > We already have a way: -fno-ghci-sandbox
>  >
>  > I've removed all my explicit attempts to forkIO/forkOS and passed the
>  > command line flag you mention.  I just tried this but it doesn't
>  > change the behavior in my example.
>
> I tried it again and discovered that due to an argument parsing bug in
> cabal-dev that the flag was not passed correctly. I explicitly passed it
> and verified that it works. Thanks for the workaround. By the way, I did
> look at the user guide for options like this and didn't see it. Which
> part of the manual is it in?
>
> Can I still make a feature request for a function to make code run on
> the original thread? My reasoning is that the code which needs to run on
> the main thread may appear in a library in which case the developer has
> no control over how ghc is invoked.

I'm not sure how that would work.  The programmer is in control of what 
the main thread does, not GHC.  So in order to implement some mechanism 
to run code on the main thread, we would need some cooperation from the 
main thread itself.  For example, in gtk2hs the main thread runs an 
event handler loop which occasionally checks a queue for requests from 
other threads (at least, I think that's how it works).

Cheers,
	Simon


> The alternative appears to be context specific workarounds. I think that
> a general solution, if possible, will make gui libraries easier to
> develop for Ghc.  I'm hooping it's as simple as exposing a pthreads call.