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

[Jason Dagit]

On Wed, Jul 6, 2011 at 2:20 AM, Simon Marlow <marlowsd at gmail.com> wrote:
> On 05/07/2011 20:38, Jason Dagit wrote:
>>
>> On Tue, Jul 5, 2011 at 12:11 PM, Simon Marlow<marlowsd at gmail.com>  wrote:
>>>
>>> On 04/07/11 06:02, Jason Dagit wrote:
>>>>
>>>> Hello,
>>>>
>>>> I'm trying to get some GUI code working on OSX and numerous forums
>>>> around the internet keep reiterating that on OSX to correctly handle
>>>> GUI events you need to use the original thread allocated to your
>>>> process to check for events and to call the Cocoa framework
>>>> functionality.  Specifically, using a secondary thread (even a bound
>>>> thread) is not sufficient with the Cocoa framework.
>>>>
>>>> I looked at the threading documentation in Control.Concurrent for GHC
>>>> and it's not clear to me if this is even possible with GHC without
>>>> restricting to the non-threaded RTS.  This means that using the GUI
>>>> library from GHCI is not an option and using multiple OS threads in
>>>> the final application is also not possible.  This means that some FFI
>>>> libraries will be unusable.
>>>
>>> In a compiled program, the main thread is a bound thread, bound to the
>>> main
>>> OS thread of the process (i.e. the GUI thread in your case).  So you can
>>> safely make Cocoa calls using the main thread of your compiled Haskell
>>> program, and from other threads if you add some way to forward operations
>>> to
>>> the main thread, like gtk2hs's postGUI.
>>
>> Is my understanding correct that this is only the case for the
>> non-threaded RTS?
>
> No - I'm talking about the threaded RTS here.  It's trivially true of the
> non-threaded RTS too, because there's only one OS thread.
>
>>  If so, what do you do when you need to use the
>> threaded RTS?  My test was to check if the main thread was bound when
>> compiling with -threaded.  I got the impression that I couldn't
>> guarantee that the code was running on the original thread.
>
> You do have that guarantee for the main thread.  Could you point out the
> docs that gave you the opposite impression - I'll see if we can improve
> them.

I did read this section:
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent.html#g:8

And I played with some of the functions there.  What I didn't take
away from this was the guarantee that main runs on the original OS
thread.  I had the impression that it was running from some bound
thread, possibly created via runInBoundThread.  I think that ghc and
ghci are quite different in this regard.  It would probably be good to
mention that distinction.

Jason