The future/promise pattern is a clean way to represent work that finishes later. Instead of blocking a thread and waiting for an operation to complete, you return a Future (or Promise) immediately. That object acts as a handle to the eventual result: you can wait for it, attach callbacks, combine it with other futures, or propagate errors.

In this chapter, we'll explore how futures and promises work, implement them from scratch, and understand the composition patterns that make them powerful.

What is a Future/Promise?