I thought that killing a thread was basically done by throwing a ThreadKilled exception using throwTo. Can't these exception be caught? <div><div><br></div><div>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...</div>
<div><br><div class="gmail_quote">2008/12/18 Conal Elliott <span dir="ltr"><<a href="mailto:conal@conal.net">conal@conal.net</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
I realized in the shower this morning that there's a serious flaw in my unamb implementation as described in <a href="http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice" target="_blank">http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice</a>. I'm looking for ideas for fixing the flaw. Here's the code for racing computations:<br>
<br><span style="font-family:courier new,monospace"> race :: IO a -> IO a -> IO a</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> a `race` b = do v <- newEmptyMVar</span><br style="font-family:courier new,monospace">
<span style="font-family:courier new,monospace"> ta <- forkPut a v</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> tb <- forkPut b v</span><br style="font-family:courier new,monospace">
<span style="font-family:courier new,monospace"> x <- takeMVar v</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> killThread ta</span><br style="font-family:courier new,monospace">
<span style="font-family:courier new,monospace"> killThread tb</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> return x</span><br style="font-family:courier new,monospace">
<br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> forkPut :: IO a -> MVar a -> IO ThreadId</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> forkPut act v = forkIO ((act >>= putMVar v) `catch` uhandler `catch` bhandler)</span><br style="font-family:courier new,monospace">
<span style="font-family:courier new,monospace"> where</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> uhandler (ErrorCall "Prelude.undefined") = return ()</span><br style="font-family:courier new,monospace">
<span style="font-family:courier new,monospace"> uhandler err = throw err</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> bhandler BlockedOnDeadMVar = return ()</span><br>
<br>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.<br><br>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.<br>
<br>Or maybe there's a simple and dependable manual solution, enhancing the method above.<br><br>Any ideas?<br><font color="#888888"><br> - Conal<br><br><br>
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<br></blockquote></div><br></div></div>