Marking functions for side-effects would be a good thing but it isn't what function coloring means in this context.

An async function and a normal function are semantically the same, they just have different syntaxes and you can't easily call one from another.

They can be both be either blocking or non-blocking, especially if they take other functions as arguments.

That's precisely the coloring that makes async useful. Without it you need to explicitly protect all shared data with mutexes or other synchronization primitives. 40+ years of threaded programming has shown that programmers cannot generally be trusted to get this right, and this in an area ripe with bugs.

But they are not, AsyncIO and BlockingIO are different side effects, thus you have different types of computation, that's exactly what I'm talking about. In languages with monads or algebraic effects these would have different types.

You don't say that Lists and Arrays are semantically the same, despite being similar sequential collections, they still have separate types for a reason. Though it's good to be able to abstract over them.

And in languages with monads we can parametrize over various effect types by using tagless final approach, which allows us to write computations which could be interpreted in contexts of various effects (in this case, Async and Sync), just as we parametrize containers with types of content (in [1] there is an example of how we can parametrize computation over various async implementations), but still these are different effects.

[1] https://kubuszok.com/2019/io-monad-which-why-and-how/#typed-...

I do agree that parametrizing over the blocking strategy is a great idea, but languages that simply provide an async syntactic marker don't necessarily allow that, and if your language is powerful enough you do not need the annotation in the first place.

- async (or explicitly Future/Stream): external effects (I/O, interacts with synchronization, whatever)

- &mut: local effects

- otherwise: pure