[kristoff_it]

> If so, does that mean that every function in zig is a stackless coroutine?

No and yes.

If you're using Io.Threaded, then the concurrency model is multithreading and calling Future.await will block your thread on a OS futex.

If you're using Io.Evented, then the concurrency model is green threads / fibers and calling Future.await will suspend the current green thread by yielding (swapping CPU state with another fiber).

Zig currently does not support stackless coroutines so today you can't have that, but we used to have them (pre self-hosted compiler), and there's an accepted proposal to bring them back, in which case any function that calls await, or that otherwise has a suspension point, would have to be transformed into a stackless coroutine by the compiler, yes. The plan is for that to happen transparently without requiring an `async` annotation in the function signature, like we already did in the past.

This is an old post of mine that explains how that worked at a high level: https://kristoff.it/blog/zig-colorblind-async-await/

[tarruda]

> there's an accepted proposal to bring them back, in which case any function that calls await, or that otherwise has a suspension point, would have to be transformed into a stackless coroutine by the compiler, yes. The plan is for that to happen transparently without requiring an `async` annotation in the function signature, like we already did in the past.

If the compiler will treat functions that call a library function (Future.await) as special and change how the call site is compiled, why not just have an `await` keyword that when present will convert it into a state machine that can be suspended/resumed?

In other words: What is gained by not having a keyword that changes how a function is emitted if the compiler will change it anyway based on detection of a library call?

[kristoff_it]

It's not Future.await that is special per se, it's that it (Future.await) will have in it somewhere either in its body or in another function that it calls in turn, a use of `suspend` (using old Zig syntax).

`suspend` is the keyword that best fits your description (you can think of it as being closely related to `yield` in other languages), and it's kind of a lower-level primitive compared to async/await.

This also means that, according to our plans, Zig will have to propagate "stackless-ness" upwards in the call chain while analyzing the code (thus making Future.await not special per-se).

[tarruda]

> This also means that, according to our plans, Zig will have to propagate "stackless-ness" upwards in the call chain while analyzing the code (thus making Future.await not special per-se).

Very interesting, will be following Zig development more closely. Thanks for sharing!