Ruby methods are colorless

Then I'd say the vast majority of Ruby libraries are buggy.

> Not having documentation just removes the ability to determine if what the code _does_ is what the code is _intended to do_.

But usually there isn't any intent when it comes to async-or-not. Library authors usually just write a method that results in the right value; whether that method can yield or not isn't even something they thought about, much less had a specific design in mind for.

BulletML is not even Turing complete and still has a wait function that does the exact thing mentioned.

Unfortunately, we do not live in that parallel universe, and JavaScript had to evolve the way it did. When callbacks have already been established as the standard concurrency mechanism, promises were a big improvement. When promises became standard, async/await was the natural evolution rather than breaking existing programs by introducing stackful coroutines.

Other languages which chose async/await (Rust, C#, Kotlin) had their own reasons. In all of the cases above, the language designers wanted to give the user more control over the scheduler (Colorless Stackful coroutines require a global scheduler).

In Rust's case, it's async/await state machines are just not wasting memory and unnecessarily copying it like stackful coroutines do (not to mention that Rust doesn't have a garbage collector and pointer rewrites, so it cannot just copy stacks around when they grow). In fact, Rust did have garbage collection and green threads, but they were both removed early on, as the language changed its focus to became a language where you can expect performance that are _at least_ on par with C or C++.

In case of Kotlin, this all mostly had to do with finding an abstraction that can run on pre-Loom JVMs, JavaScript targets and native targets. Kotlin does not own the runtime.

Saying everybody should follow Go here is silly. Not all languages share the same background and design constraints. Go-style Stackful coroutines require garbage collection, full control of the runtime and generally a language that is designed from scratch to have them - or a 6-year project to adapt your language and runtime to have them, like Project project Loom did.

Async/await imposes some costs, but the benefits (compatibility, performance, flexibility) often outweigh these. After working with both async functions and stackful coroutines in multiple languages, I cannot say I ever felt concurrency in Go to be more ergonomic than Kotlin. If anything, it was the reverse for me in that particular case (owing to Go's lack of reification for goroutines and verbose syntax). Deciding in advance what color is my function (i.e. whether it needs to deal with I/O), is pretty simple and it rarely changes during the lifetime of a program (and even when it does, refactoring is not hard unless your design is bad).

The only case where I did feel some pain using async/await, was Rust, and this has more to do with Pin, competing async runtimes (for a while), lack of support for async traits (for a while) and other areas where Async Rust still needs some work.

Me neither, however, it is generally the most flexible approach and I can see why a library takes it. If you want to communicate it via the return, you also have to impose a restriction that the tasks all return the same type. I think it makes sense for a library to work this way because you can easily add this around the library call yourself, but it's more difficult to go the other way. (Not impossible, just more difficult.)

"I don't think async functions in JS can be cancelled though."

Poking around, it looks pretty hard.

That said, cancelling in generally is very difficult in imperative languages, so even as someone who finds async in JS quite distasteful I can't dock too many points. Go basically just reifies the JS solution into a standard community practice, which is definitely an improvement since you can largely rely on it being supported everywhere, but one could reasonably debate how good it is. It is occasionally a problem that if you want to cancel an ongoing computation you may have to have your code manually check a context every so often, because there's no "real" connection between a context and a goroutine.