[Haskell-iPhone] Thread model questions

[David Pollak]

Dan and Greg,

Thanks for the most excellent advice and pointers, and in near-real-time,
too!

Rock on.

David

On Fri, Jun 10, 2011 at 11:32 AM, Daniel Peebles <pumpkingod at gmail.com>wrote:

> I haven't looked specifically at ghc-iphone, but as it's just using the GHC
> RTS, I'd assume the situation isn't very different from regular GHC. The
> deal there is that the GHC runtime gives you green threads, and
> forkIO/forkOS start new green threads. These have tiny overhead and are
> scheduled cooperatively by the RTS (every time you go to allocate, which is
> all the time in your average haskell program, the RTS checks to see if it
> needs to switch). IO and stuff is using select behind the scenes (ghc-iphone
> is built on an older version of GHC that still used select, iirc). The RTS
> uses a thread pool that you specify by passing arguments like +RTS -N3 (for
> three threads) to your program.
>
> You can't actually have "bound threads" in GHC, but forkOS does give you
> the guarantee that any foreign calls you make from the green thread will be
> made from the same OS thread. And that should not be observably different
> from ensuring all your code runs on the same OS thread.
>
> So to address your question more directly, the way I'd probably approach
> what it seems you want to do: create a new Chan for events you want to
> happen from a single thread, then forkOS a reader that reads forever (the
> Control.Monad.forever) and runs your foreign functions, then all your other
> green haskell threads (forkIO is good enough for them) can write to the Chan
> (it's designed to be safe for concurrent access and will block if necessary)
> and will thus be talking to the foreign-talking thread.
>
> Hope this helps,
> Dan
>
> On Fri, Jun 10, 2011 at 2:12 PM, David Pollak <
> feeder.of.the.bears at gmail.com> wrote:
>
>> Howdy,
>>
>> I've got a question about threading and thread models in ghc-iphone.
>>
>> My understanding is that Haskell has its own threading model that under
>> the covers uses OS threads, but allows for significantly more active
>> threads.  Is this a correct understanding?  If so, how does the Haskell
>> threading model work on the iPhone?  Is it that iOS is close enough to OS
>> X/Darwin that it "just works"?
>>
>> If it "just works", then how does one marshal an event back onto the iOS
>> "UI thread" so that the display or draw or whatever command can take place
>> on the UI thread?  Is there a convention or recipe for doing so?  Would it
>> be possible to put an event on the UI event queue that applies an Haskell
>> closure such that the Haskell code that contains the computed value (I'd
>> force the computation before marshaling the event to minimize the actual
>> delay on the UI thread) then mutates the UI based on the computed value?
>>
>> Given the 5 second rule about an iOS app getting shut down... if there are
>> Haskell threads doing Haskell-y kind of work, is there a way of notifying
>> those threads en masse about the shutdown?
>>
>> Thanks,
>>
>> David
>>
>> --
>> Lift, the simply functional web framework http://liftweb.net
>> Simply Lift http://simply.liftweb.net
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>> Blog: http://goodstuff.im
>>
>>
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>>
>>
>


-- 
Lift, the simply functional web framework http://liftweb.net
Simply Lift http://simply.liftweb.net
Follow me: http://twitter.com/dpp
Blog: http://goodstuff.im
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