Proposal: Control.Concurrent.Async

Sjoerd Visscher sjoerd at
Fri Jun 15 15:43:24 CEST 2012

> > An alternative, equivalent interface for waitEither and friends would be
> >
> >    waitEither :: Async a ->  Async a ->  IO (Either SomeException a)
> >    waitEitherThrow :: Async a ->  Async a ->  IO a
> >
> > This might be conceptually simpler, leaving it to the user to decide
> > whether to mark the results with Left/Right, or something else, or not. 
> > Similarly the Async now returned by waitAny and friends could be left
> > to the user.  That would leave the two sets of functions more in line. 
> I liked this suggestion at first, but later concluded that it isn't 
> as nice.  The problem is that to convert an 'Async a' to an 'Async 
> (Either a b)' has to be done when you create the Async in the first 
> place, or else you have to write some STM code to lift the result.  
> If you're writing some STM code anyway, then there's little point in 
> using waitEither. 

What's the problem with converting when you create the Async?

> withAsync (Left  <$> getUrl url1) $ a1 -> do
> withAsync (Right <$> getUrl url2) $ a2 -> do
>   waitEither a1 a2

This seems more explicit to me. And if you really don't care which of 
the results you want, they'll probably have the same type anyway. 


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