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A coroutine or (co-routine) is similar to a thread (in the sense of multithreading): a line of execution, with its own stack, its own local variables, and its own instruction pointer; but sharing global variables and mostly anything else with other coroutines. The main difference between threads and coroutines is that, conceptually (or literally, in a multiprocessor machine), a program with threads runs several threads concurrently. Coroutines, on the other hand, are collaborative: A program with coroutines is, at any given time, running only one of its coroutines and this running coroutine only suspends its execution when it explicitly requests to be suspended.
Coroutine is a powerful concept. As such, several of its main uses are complex. Do not worry if you do not understand some of the examples in this chapter on your first reading. You can read the rest of the book and come back here later. But please come back. It will be time well spent.