Dear all, I wrote a faster implementation for System.Timeout.timeout but wonder whether it's correct. It would be great if someone can review the code. The implementation comes with a tiny Criterion benchmark: darcs get http://bifunctor.homelinux.net/~bas/bench_timeouts/ On ghc-7.0.1 with -O2 the following benchmark, which always times out, doesn't show a difference with the current timeout: timeout 1 (threadDelay 1000000) However the following benchmark, which never times out, shows nice speedups for both a non-threaded and a threaded version: timeout 1000000 (return ()) non-threaded: 3.6 faster threaded: 10.8 faster I suspect the reason why my timeout is faster is that I use only one 'catch' while the original uses two: handleJust and bracket. I have to admit that my implementation is more complex and more fragile than the original. That's why I'm not sure of it's correctness yet: newtype Timeout = Timeout Unique deriving (Eq, Typeable) instance Show Timeout where show _ = "<<timeout>>" instance Exception Timeout timeout :: Int -> IO a -> IO (Maybe a) timeout n f | n < 0 = fmap Just f | n == 0 = return Nothing | otherwise = do myTid <- myThreadId timeoutEx <- fmap Timeout newUnique uninterruptibleMask $ \restore -> do tid <- restore $ forkIO $ threadDelay n >> throwTo myTid timeoutEx let handle e = case fromException (e :: SomeException) of Just timeoutEx' | timeoutEx' == timeoutEx -> return Nothing _ -> killThread tid >> throwIO e mb <- restore (fmap Just f) `catch` handle killThread tid return mb If nobody proves it incorrect I will make a patch for the base library. Regards, Bas