Re: darcs patch: forkChild, waitForChild, parIO, timeout

13 Nov 2006


      Taral wrote:
...
On 11/9/06, Simon Marlow  wrote:
...
If you can modify your version to nest properly, I think it's a 
winner.  The
ideal unique value to use is the ThreadId of the timeout thread.
You know, that's what I though, but then I realized that the timeout
thread's ThreadId isn't easily visible outside of the bracket, where
the catch is. So Unique it is:
module Timeout (timeout) where
import Control.Concurrent(forkIO, threadDelay, myThreadId, killThread)
import Control.Exception(handleJust, throwDynTo, dynExceptions, bracket)
import Control.Monad(guard)
import Data.Dynamic(Typeable, fromDynamic)
import Data.Unique(Unique, newUnique)
data Timeout = Timeout Unique deriving (Eq, Typeable)
timeout :: Int -> IO a -> IO (Maybe a)
timeout n f
   | n < 0 = fmap Just f
   | n == 0 = return Nothing
   | otherwise = do
       pt <- myThreadId
       ex <- fmap Timeout newUnique
       handleJust (\e -> dynExceptions e >>= fromDynamic >>= guard . (ex 
==))
                  (\_ -> return Nothing) $
                  bracket (forkIO (threadDelay n >> throwDynTo pt ex))
                          (killThread)
                          (\_ -> fmap Just f)
Yes, I think this version could go in.

Cheers,
	Simon