darcs patch: forkChild, waitForChild, parIO, timeout

[Simon Marlow]

Taral wrote:
> On 11/7/06, Simon Marlow <simonmarhaskell at gmail.com> wrote:
> 
>> So 'timeout N E' should behave exactly the same as E as long as E 
>> doesn't time
>> out.  Three difficulties with this:
>>
>>   1. if E raises an exception, you want the exception propagated to 
>> the parent.
>>
>>   2. if another thread throws an exception to this thread, E should 
>> receive the
>>      exception.
>>
>>   3. E should get the same result from myThreadId.
> 
> 
> I think 2 and 3 go together, and require an exception. Here's my try:
> 
> data Timeout = Timeout deriving Typeable
> 
> timeout n f
>    | n < 0 = fmap Just f
>    | n == 0 = return Nothing
>    | otherwise = do
>        pt <- myThreadId
>        bracket (forkIO (threadDelay n >> throwDynTo pt Timeout))
>                (killThread)
>                (fmap Just f) `catchDyn` \Timeout -> return Nothing

Nearly - but this one doesn't nest.  Also, it's possible for the Timeout 
exception to leak.

Cheers,
	Simon