[derriz]

Indeed. Async runtimes/sytles are attempts to provide a more readable/useable syntax for CPS[1]. CPS originally had nothing to do with blocking/non-blocking or multi-threading but arose as a technique to structure compiler code.

Its attraction for non-blocking coding is that it allows hiding the multi-threaded event dispatching loop. But as the parent comment suggests, this abstraction is extremely leaky. And in addition, CPS in non-functional languages or without syntactic sugar has poor readability. Improving the readability requires compiler changes in the host language - so many languages have added compiler support to further hide the CPS underpinnings of their async model.

I've always felt this was a big mistake in our industry - all this effort not only in compilers but also in debuggers/IDE - building on a leaky abstraction. Adding more layers of leaky abstractions has only made the issue worse. Async code, at first glance, looks simple but is a minefield for inexperienced/non-professional software engineers.

It's annoying that Rust switched to async style - the abstraction leakiness immediately hits you, as the "hidden event dispatching loop" remains a real dependency even if it's not explicit in the code. Thus libraries using asycn cannot generally be used together although last time i looked, tokio seems to have become the de-facto standard.

[1] https://en.wikipedia.org/wiki/Continuation-passing_style

[kaba0]

I absolutely agree that the virtual/green thread style is much better, more ergonomic, less likely to be correct, etc, but I can’t fault Rust’s choice, given it being a low-level language without a fat runtime, making it possible to be called into from other runtimes. What the JVM does is simply not possible that way.