Chapter 4.  Asynchronous Programming with Futures and Promises

	Programming in a functional style makes the state presented to your code explicit, which makes it much easier to reason about, and, in a completely pure system, makes thread race conditions impossible.
	--John Carmack

In the examples of the previous chapters, we often dealt with blocking computations. We have seen that blocking synchronization can have negative effects: it can cause deadlocks, starve thread pools, or break lazy value initialization. While, in some cases, blocking is the right tool for the job, in many cases we can avoid it. Asynchronous programming refers to the programming style in which executions occur independently of the main program flow. Asynchronous programming helps you to eliminate blocking instead of suspending the thread whenever a resource is not available; a separate computation is scheduled to proceed once the resource becomes available.

In a way, many of the concurrency...