[Haskell-cafe] Faster timeout but is it correct?

[Simon Marlow]

On 16/02/2011 08:39, Bas van Dijk wrote:

> 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.

uninterruptibleMask is quite unsavoury, I don't think we should use it 
here.  I can see why you used it though: the killThread in the main 
thread will always win over the throwTo in the timeout thread, and that 
lets you avoid the outer exception handler.

Hmm, it makes me uncomfortable, but I can't find any actual bugs.  At 
the very least it needs some careful commentary to explain how it works.

Cheers,
	Simon