<amp-script>: should AMP.setState() return a Promise?

In ``, `AMP.getState()` returns a Promise. But `AMP.setState()` doesn't. This... is confusing, since I'd assume that both of these depend on sending a message to the main thread. In either case, I think it's confusing that the interface is different for two very similar methods.

I might think it would be simplest if both `getState()` and `setState()` were simply synchronous. I'd guess there aren't a lot of use cases where your script would want to request the contents of a state variable, then go on with other activities until the contents return. Even if this makes any sense, I imagine it's too late to change this now.

On the other hand, what if `setState()` did return a Promise too?

Of course, what would that mean?

What if the Promise was fulfilled when any binding was completed and any resulting DOM mutations had occurred? Then we could still allow the current usage, and make sense out of the Promise version too.

@samouri , I'm sure you can improve this proposal...