I'm skimming your link trying to understand the design. It sounds like there is a global flag for whether the whole program is in evented or blocking mode?

> during compile-time, it’s possible to inspect if the overall program is in evented mode or not, and properly designed code might decide to move to a threaded model when in blocking mode, for example.

Yes, it's a top level application decision, not a library decision. In fact, this is exactly how allocation and unwinding is handled in Rust.

So, it's like the choice of a Tokio executor? That's a top-level default, too.

Sounds like a horribly complex special case. I'd far rather just have higher-kinded types and be able to write sometimes-async code using normal polymorphism (like I do in Scala all the time).