[Haskell-cafe] Re: Waiting for thread to finish
haskell at list.mightyreason.com
Tue Nov 27 12:59:59 EST 2007
> After I have spawned a thread with
> 'forkIO', how can I check if that
> thread work has finished already?
> Or wait for it?
The best way to do this is using Control.Exception.finally:
> myFork :: IO () -> IO (ThreadId,MVar ())
> myFork todo =
> m <- newEmptyMVar
> tid <- forkIO (finally todo (tryPutMVar m ()))
> return (tid,m)
No other part of the program should write to the MVar except
the finally clause.
The rest of the program can check (isEmptyMVar m) as a non-blocking
way to see if the thread is still running. Or use (swapMVar m ()) as
a way to block until the MVar has been filled as a way of blocking
until the thread is finished.
These techniques are needed because forkIO is a very lightweight threading
mechanism. Adding precisely the features you need makes for good performance
control, as seen in the great computer language shootout benchmarks.
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