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

[Peter Verswyvelen]

I thought that killing a thread was basically done by throwing a
ThreadKilled exception using throwTo. Can't these exception be caught?
In C#/F# I usually use a similar technique: catch the exception that kills
the thread, and perform cleanup. I have no experience with Haskell in that
regard so most likely I'm missing something here...

2008/12/18 Conal Elliott <conal at conal.net>

> 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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