On 09/04/2010 09:40, Bertram Felgenhauer wrote:
Simon Marlow wrote:
but they are needlessly complicated, in my opinion. This offers the same functionality:
mask :: ((IO a -> IO a) -> IO b) -> IO b mask io = do b<- blocked if b then io id else block $ io unblock
How does forkIO fit into the picture? That's one point where reasonable code may want to unblock all exceptions unconditionally - for example to allow the thread to be killed later.
Sure, and it works exactly as before in that the new thread inherits the masking state of its parent thread. To unmask exceptions in the child thread you need to use the restore operator passed to the argument of mask. This does mean that if you fork a thread inside mask and don't pass it the restore operation, then it has no way to ever unmask exceptions. At worst, this means you have to pass a restore value around where you didn't previously.
timeout t io = block $ do result<- newEmptyMVar tid<- forkIO $ unblock (io>>= putMVar result) threadDelay t `onException` killThread tid killThread tid tryTakeMVar result
This would be written
timeout t io = mask $ \restore -> do result<- newEmptyMVar tid<- forkIO $ restore (io>>= putMVar result) threadDelay t `onException` killThread tid killThread tid tryTakeMVar result
though the version of timeout in System.Timeout is better for various reasons. Cheers, Simon