I realized that the previous timeout had problems when called in a masked thread. What happens is that the call to killThread will block because it can't throw the KillThread exception to the timeout thread because that thread is masked. I have to use unsafeUnmask to always unmask the timeout thread. Note that, for some reason, using forkIOUnmasked ... is much slower than using unsafeUnmask $ forkIO .... Any idea why? import GHC.IO (unsafeUnmask) imeout :: 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 <- unsafeUnmask $ forkIO $ threadDelay n >> throwTo myTid timeoutEx (restore (fmap Just f) >>= \mb -> killThread tid >> return mb) `catch` \e -> case fromException e of Just timeoutEx' | timeoutEx' == timeoutEx -> return Nothing _ -> killThread tid >> throwIO e For some reason this is slightly slower than the previous version which used restore instead of unsafeUnmask. However it's still 13 times faster than the original. The patch and benchmarks attached to the ticket are updated. Hopefully this is the last change I had to make so I can stop spamming. Regards, Bas