zigcoro VS craftos2-lua

> Do you have examples of any async code returning values without “objects” or compiler magic?

Lua coroutines represent a yieldable computation, but given a coroutine object, you can't await it or retrieve a return value from it. The only things you can do with a coroutine are check its status and resume it. So how do asynchronous computations return values in Lua?

It's simple: don't create an extra coroutine around it. If you're already in a yieldable coroutine, then you can call the yielding function directly. Nested yields will bubble up, and from the caller's perspective you just call a function and it returns a value.

Doing this from Lua is a bit of compiler/interpreter magic, but some modified Lua interpreters support yieldable C functions that can also perform yieldable calls [0]. This requires some due diligence on the part of the caller, making sure to bubble up yields from the inner function and also pass down resumes until it completes (you need to add a state for "this function call yielded"). But it uses no promises, futures or channels.

[0]: https://github.com/MCJack123/craftos2-lua/blob/89dcba94c28be... (lua_getctx returns whether you're being resumed from a yield, and expects you to have pushed your own state to the Lua stack if you need any.)