darcs patch: forkChild, waitForChild, parIO, timeout

[Simon Marlow]

Taral wrote:
> On 11/9/06, Simon Marlow <simonmarhaskell at gmail.com> 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