Coroutine

Coroutines are computer program components that generalize subroutines for nonpreemptive multitasking, by allowing multiple entry points for suspending and resuming execution at certain locations. Coroutines are well-suited for implementing more familiar program components such as cooperative tasks, exceptions, event loop, iterators, infinite lists and pipes.

According to Donald Knuth, the term coroutine was coined by Melvin Conway in 1958, after he applied it to construction of an assembly program. The first published explanation of the coroutine appeared later, in 1963.

"Subroutines are special cases of ... coroutines." –Donald Knuth.

When subroutines are invoked, execution begins at the start, and once a subroutine exits, it is finished; an instance of a subroutine only returns once, and does not hold state between invocations. By contrast, coroutines can exit by calling other coroutines, which may later return to the point where they were invoked in the original coroutine; from the coroutine's point of view, it is not exiting but calling another coroutine. Thus, a coroutine instance holds state, and varies between invocations; there can be multiple instances of a given coroutine at once. The difference between calling another coroutine by means of "yielding" to it and simply calling another routine (which then, also, would return to the original point), is that the latter is entered in the same continuous manner as the former. The relation between two coroutines which yield to each other is not that of caller-callee, but instead symmetric.

Any subroutine can be translated to a coroutine which does not call yield.

To implement a programming language with subroutines requires only a single stack that can be preallocated at the start of program execution. By contrast, coroutines, able to call on other coroutines as peers, are best implemented using continuations. Continuations may require allocation of additional stacks, and therefore are more commonly implemented in garbage-collected high-level languages. Coroutine creation can be done cheaply by preallocating stacks or caching previously allocated stacks.

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