This package provides a higher-level interface over threads, in which an
Async a is a concurrent thread that will eventually deliver a value of
type a. The package provides ways to create Async computations, wait for
their results, and cancel them.

Using Async is safer than using threads in two ways:

 - When waiting for a thread to return a result, if the thread dies with
   an exception then the caller must either re-throw the exception (wait)
   or handle it (waitCatch); the exception cannot be ignored.

 - The API makes it possible to build a tree of threads that are
   automatically killed when their parent dies (see withAsync).

WWW: https://github.com/simonmar/async