One of the more widely deployed ones is Go. Code looks like with synchronous IO, but everything runs in green threads that can be suspended when IO would block. So if you do a File.Read, after it returns, you might be running on a different OS thread because you were suspended mid-Read and later woken up on a different thread. It's generally nice to work with, unless you need to interface with a non-Go library that expects you to use a single thread throughout (e.g. for callbacks), in which case suddenly everything turns into a royal mess.

> everything runs in green threads that can be suspended when IO would block

AFAIK the main objection to this model is that you have to write your code so that it can handle being suspended at any point. OTOH when using async-await, the only potential suspension points are the places marked with `await`.

Rust was originally built with green threads. The RFC that proposed removing them has an extremely detailed explanation of why the change was made pre-1.0: https://github.com/rust-lang/rfcs/blob/0806be4f282144cfcd55b...

I think this has proven itself to be the right decision. It's one of the main reasons that Rust works so well alongside other languages—there's very little default Rust runtime that complicates interop. There are also lots of programming scenarios for which green threads are not the right tool, and Rust accommodates those.

I don't think that's a very strong guarantee though.

Trivially you can call any function that can block. Less trivially, any function that you can can resume any other suspended coroutine. So in practice there isn't much you can rely on. In general you can not rely on state that has escaped your function to be unmodified across function calls (although here of course rust has tools to prevent that, but they would work even without the sync/async distinction).

It is true that other code can only run at function call boundaries, but that's true for any cooperatively scheduled runtime, even with stackfull coroutines.

IMHO there are non!

Through there are mono colored languages.

E.g. go would be mono red.

Normally mono blue languages allow you to "emulate" red contexts using callback's, listeners and similar. So some would argue there are no mono blue languages.

Many of the points (like red being harder to use) are 100% language impl. dependent.

The only think which is generally true about coloring is that mixing colors is a bit harder and sometimes can be sub-optimal for performance. But how much harder depends fully on your programming language and use-case might be negligible. Same is true for performance.

Also the two big problem of mono red languages is that while red has less overhead when a lot of parallelism is involved it has more overhaed if little parallelism is involved. And that interacting with other programs using a FFI conceptually harder. Which can lead to a varying degree of complexity and potentially performance drawbacks. But again how much this matters is again very language and use-case dependent.

All kind of tooling (including type checking) can reduce the drawbacks of either model (but introduces new drawbacks like longer compilation times).

As a side note: Communicating between red and blue functions is only potentially hard when they call each other. In the same way mono-red languages have only problems with external programs if they communicating over a FFI. If things like futures, streams, channels or similar are used (assuming proper implementation) red and blue code can normally communicate just fine, even if run in the same process.

> Normally mono blue languages allow you to "emulate" red contexts using callback's, listeners and similar. So some would argue there are no mono blue languages.

well, that's exactly what is done in traditional C, C++, Java...