Futures and promises

Futures and promises are terms that are used somewhat interchangeably to refer to abstractions representing the idea of a proxy to a value that will be known at some future time. In other words, futures and promises allow you to create objects that encapsulate asynchronous tasks and their result, and use them without waiting for the asynchronous task to finish, as long as you do not need to access the result of that task. When you need to access the task result, either the task has already completed, in which case you can use it without delay; otherwise, you will need to delay the code that wants to use that result until it becomes available. Strictly speaking, a way to differentiate between futures and promises is to say that a future represents a read-only value that may be available or not, while a promise represents the function that is responsible...