Proposal: Control.Concurrent.Async

[Simon Marlow]

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:

http://community.haskell.org/~simonmar/async-stm/Control-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