> Stackless did not have a bright future because the stackless nature meant that you could not have interleaving Python -> C -> Python calls and suspend with them on the stack.

perl's Coro.pm (https://p3rl.org/Coro) includes a C-level coroutine implementation so if you call back into perl code from C code, it marks that underlying coroutine as occupied until it returns back through the C code into something that's perl all the way down.

(I got rapidly annoyed with it because I kept managing to segfault the damn thing, but I am the sort of programmer who seems to manage to break everything, and a bunch of other people whose abilities I respect have built substantial systems using it without suffering from the same; please either assume for the sake of argument that I was holding it wrong, or at least consider that the concept is extremely neat even if a tad on the tricky side to make fully robust)