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://hackage.haskell.org/package/async
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