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