[c-cube]

I'm curious. Intuitively, a non-recursive coroutine would yield a state machine that only goes "forward". If you add tail recursion into the mix, you could have cycles in the state machines (going back to the begin state), correct? Of course non-tail recursion would not work within a single frame.

[hctaw]

Yes, a tail recursive coroutine could reuse its previous frame context across yields.

With a state machine transform the `resume()` method on a coroutine is a state transformation, it doesn't necessarily know what is "forward" or "backward" in the control flow graph. There are some tricky bits though, since tail recursive functions can have multiple exits but single entries. A recursive coroutine might have multiple exits and multiple entries, so it's not always clear what is "forward" and what is "backward."