Re: [reactive] problem with unamb -- doesn't kill enough threads

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 http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Timeout.ht... the docs are sadly missing the source links at the moment, I'm not sure why, but you can find the source in http://darcs.haskell.org/packages/base/System/Timeout.hs Cheers, Simon 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 mailto:andygill@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 http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice. 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) where 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

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