Dan Sugalski's blog has become one of my favorites lately. His newest essay on coroutines is a perfect example of why.
Anyway, what is a coroutine? Well, a coroutine is a subroutine or function that can pause in the middle and return a value. When you call it again it picks up right where it was when you paused, environment intact, and keeps on going. Coroutines are almost always closures, though strictly speaking they don't have to be--if you had a language that didn't have closures, or lexical variables, you could still have coroutines. (Though generally speaking nobody does that. You could, though, if you were so inclined) Often coroutines are called iterators or generators, but that's not strictly correct--while you can make iterators and generators with coroutines, you don't have to. Nor are coroutines limited to being iterators or generators, as some systems use coroutines to implement threading, and coroutines can make really nifty callbacks.
From the outside you can't tell if a function is a coroutine any more than you can tell that a function is a closure. Your code calls it and gets back a value (or values) and that's all that's important. (Yay, encapsulation)
I understand coroutines better now after reading the article. Thanks Dan.
Thursday, May 1, 2003
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