[Haskell-cafe] Re: [reactive] problem with unamb -- doesn't kill
marlowsd at gmail.com
Fri Dec 19 04:48:33 EST 2008
Sounds like you should use an exception handler so that when the parent
dies it also kills its children. Be very careful with race conditions ;-)
For a good example of how to do this sort of thing, see
the docs are sadly missing the source links at the moment, I'm not sure
why, but you can find the source in
Conal Elliott wrote:
> (I'm broadening the discussion to include haskell-cafe.)
> Andy -- What do you mean by "handling all thread forking locally"?
> - Conal
> On Thu, Dec 18, 2008 at 1:57 PM, Andy Gill <andygill at ku.edu
> <mailto:andygill at ku.edu>> wrote:
> Conal, et. al,
> I was looking for exactly this about 6~9 months ago. I got the
> suggestion to pose it as a challenge
> to the community by Duncan Coutts. What you need is thread groups,
> where for a ThreadId, you can send a signal
> to all its children, even missing generations if needed.
> I know of no way to fix this at the Haskell level without handling
> all thread forking locally.
> Perhaps a ICFP paper about the pending implementation :-) but I'm
> not sure about the research content here.
> Again, there is something deep about values with lifetimes.
> Andy Gill
> On Dec 18, 2008, at 3:43 PM, Conal Elliott wrote:
>> I realized in the shower this morning that there's a serious flaw
>> in my unamb implementation as described in
>> I'm looking for ideas for fixing the flaw. Here's the code for
>> racing computations:
>> race :: IO a -> IO a -> IO a
>> a `race` b = do v <- newEmptyMVar
>> ta <- forkPut a v
>> tb <- forkPut b v
>> x <- takeMVar v
>> killThread ta
>> killThread tb
>> return x
>> forkPut :: IO a -> MVar a -> IO ThreadId
>> forkPut act v = forkIO ((act >>= putMVar v) `catch` uhandler
>> `catch` bhandler)
>> uhandler (ErrorCall "Prelude.undefined") = return ()
>> uhandler err = throw err
>> bhandler BlockedOnDeadMVar = return ()
>> The problem is that each of the threads ta and tb may have spawned
>> other threads, directly or indirectly. When I kill them, they
>> don't get a chance to kill their sub-threads.
>> Perhaps I want some form of garbage collection of threads, perhaps
>> akin to Henry Baker's paper "The Incremental Garbage Collection of
>> Processes". As with memory GC, dropping one consumer would
>> sometimes result is cascading de-allocations. That cascade is
>> missing from my implementation.
>> Or maybe there's a simple and dependable manual solution,
>> enhancing the method above.
>> Any ideas?
>> - Conal
>> Reactive mailing list
>> Reactive at haskell.org <mailto:Reactive at haskell.org>
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
More information about the Haskell-Cafe