That's one where we got lucky. We made the wrong decision, but it didn't matter because the distinction was invisible when using the higher-level abstractions (like coroutines) and combinators.

My opinion now is that JavaScript does it right: `.then` should _always_ defer the callback. BUT, it does come at a real performance cost. And for code that is performance-sensitive and can handle the callback running immediately, the API should support a `.thenImmediate` or something that will run immediately if possible.