[eulerfx234]

Also, to compare with other languages:

Async is similar to a Future (such as in Java), with the difference that Future produces a result once and caches it, whereas Async re-evaluates each time it is executed. It can be made to cache of course. A Future is more like a TPL Task, though IMO, Async provides a more predictable programming model.

Go has go-routines and channels. A go-routine is similar to Async. In essence, they are a notion of light-weigh thread. Note however that Hopac support for channels is far richer than that of Go.