> whenever you call a function, it might or might not spawn some background task. The function seemed to return, but is it still running in the background? There's no way to know without reading all its source code, transitively. When will it finish? Hard to say.

This reminds of me "colored functions" (red vs blue) where it becomes imperative to know if a function you are calling returns a value or a Future/Promise.

Some languages allow annotating a function to indicate as such so the IDE can help. His particular solution he presents actually doesn't address this question: Is your function sync or async? You still have to know when calling a function if it's async and needs to be in a nursery or not.

Should a programming language abstract away whether a function is async or not? async/await is a step forward (C#/JS) but it still requires knowing if the child function is async or not.