Too often people think the problem between sync and async is a red/blue coloring within the programming language. The problem really is that every OS already has red/blue syscalls.

Everything else about “why can’t I mix these functions” is a direct result of that broken consistency exposed by the OS. As such I’m not sure there is a zero-cost abstraction (stackful coroutines is close) that any programming language can build to improve the situation.

Meanwhile, the trend is clear - every OS has adopted (and is adopting more) non-blocking syscalls because they are direly needed. The only benefit blocking syscalls offer is sugar to improve syscall ergonomics.

I think if more people talked about it this way it would become clear that adding a “blocking” tag to the syscalls and bubble that tag up the call stack is the right next step to depreciating those legacy OS APIs. I don’t mean to say we should accept poor ergonomics but adding the blocking tag is a great first step to 1. Reminding people of the problem. 2. Identifying areas where research is needed to improve ergonomics and replace the existing “blocking” tag with minimal downsides.