In the JS example, a synchronous function cannot poll the result of a Promise. This is meaningfully different when implementing loops and streams. Ex, game loop, an animation frame, polling a stream.

A great example is React Suspense. To suspend a component, the render function throws a Promise. To trigger a parent Error Boundary, the render function throws an error. To resume a component, the render function returns a result. React never made the suspense API public because it's a footgun.

If a JS Promise were inspectable, a synchronous render function could poll its result, and suspended components would not need to use throw to try and extend the language.

.NET has promises that you can poll synchronously. The problem with them is that if you have a single thread, then by definition while your synchronous code is running, none of the async callbacks can be running. So if you poll a Task and it's not complete yet, there's nothing you can do to wait for its completion.

Well, technically you can run a nested event loop, I guess. But that's such a heavy sync-wrapping-async solution that it's rarely used other than as a temporary hack in legacy code.

I see. I guess JS is the only language with the coloring problem, then, which is strange because it's one of the few with a built-in event loop.

This Io business is isomorphic to async/await in Rust or Python [1]. Go also has a built-in "event loop"-type thing, but decidedly does not have a coloring problem. I can't think of any languages besides JS that do.

[1]: https://news.ycombinator.com/item?id=46126310