Proposal: Control.Concurrent.Async

Iavor Diatchki iavor.diatchki at
Sat Jun 9 18:50:44 CEST 2012

A nice library, I've had to re-implement similar functionality, so this
would be useful!

Some questions about the waiting combinators:

1. I was expecting that `waitAny` and friends, would also return the result
of the first computation that completed.  Is there a reason not to do so?
2. Is `waitAny` waiting for one of the Asyncs to complete _successfully_ or
is throwing an exception considered as completion?  I am guessing the
former, with `waitAnyThrow` providing the latter functionality?
3. As far as I understand, `waitAny` does not stop the other asyncs (it'd
be nice to add this note to its documentation, explicitly).  If that's the
case, perhaps it's type should be:  `waitAny :: [Async a] -> IO (a,[Async
a])`, so that we can keep on waiting for more asyncs, if needed?


On Fri, Jun 8, 2012 at 1:37 AM, Simon Marlow <marlowsd at> wrote:

> I'd like to add a higher-level concurrency API to the base package. This
> API has grown while I've been working on examples for my book, and I'm
> convinced that we should have something like this in the standard
> libraries.  Here's the API I propose:
> Concurrent-Async.html<>
> In fact it already exists in at least two packages on Hackage: 'async' and
> 'threads'.  My version is a superset of both of these, except for a few
> small differences (which are up for discussion, of course).
> One good reason to have this package is that it avoids some common
> pitfalls, such as forgetting to handle exceptions in your child threads.
>  In the Async API, you get to choose whether to (a) receive the result as
> 'Either SomeException a', or (b) have the exception re-thrown in the
> waiting thread.  You don't get to ignore it altogether.
> Another common pitfall avoided by this library is accidentally leaving
> threads running in the background.  This is handled by withAsync:
>  withAsync :: IO a -> (Async a -> IO b) -> IO b
> the child thread is always cancelled (i.e. killed) when the second
> argument returns or throws an exception, if it hasn't finished already.
> I'm opening this up for discussion so that we can tidy up any inconsistent
> details and add any functions that people feel are missing.  Naming is
> obviously up for discussion too.
> Cheers,
>        Simon
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