Simon Marlow wrote:
It's a hard problem. Trying to code up takeEitherMVar by hand always
seems to lead to solutions with race conditions - I can't see a way to
do it.
The following solution should be free from race conditions,
takeEitherMVar :: MVar a -> MVar b -> IO (Either a b)
takeEitherMVar mva mvb =
do ma <- tryTakeMVar mva
case ma of
Just a -> return (Left a)
Nothing -> do mb <- tryTakeMVar mvb
case mb of
Just b -> return (Right b)
Nothing -> takeEitherMVar mva mvb
but it has an efficiency problem, though...
Perhaps takeEitherMVar should be provided as a primitive, but I'm
concerned that the same problems will crop up if we tried to implement
takeEitherMVar in a multi-threaded runtime
Also, what if you wanted to choose between three variables?
So I think in order to do this you have to build another abstraction
around MVars. I've attached some code I just hacked up to do just that;
it's untested (but compiles), and has a couple of known deficiencies,
but it might be along the right lines.
Thanks, I'll see if I can understand that...
--
Thomas H
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