Coroutine - Comparison With Generators

Comparison With Generators

Generators are also a generalisation of subroutines, but are more limited than coroutines. Specifically, while both of these can yield multiple times, suspending their execution and allowing re-entry at multiple entry points, they differ in that coroutines can control where execution continues after they yield, while generators cannot, instead transferring control back to the generator's caller. That is, since generators are primarily used to simplify the writing of iterators, the yield statement in a generator does not specify a coroutine to jump to, but rather passes a value back to a parent routine.

However, it is still possible to implement coroutines on top of a generator facility, with the aid of a top-level dispatcher routine (a trampoline, essentially) that passes control explicitly to child generators identified by tokens passed back from the generators:

var q := new queue generator produce loop while q is not full create some new items add the items to q yield consume generator consume loop while q is not empty remove some items from q use the items yield produce subroutine dispatcher var d := new dictionary(generator → iterator) d := start produce d := start consume var current := produce loop current := next d

A number of implementations of coroutines for languages with generator support but no native coroutines (e.g. Python before 2.5) use this or a similar model.