Stopping progress propagation

[briancavalier]

See #1

Progress handlers should be allowed to explicitly stop progress propagation. The trend in #1 seems to be toward allowing handlers to throw a duck-typed "stop propagation" error.

What should this error look like? One option is to follow builtin Error convention and have it be an Error whose .name == "<some specific name>", since we can't rely on instanceof across implementations.

Two related questions:

1. Would it be better to return such an error rather than throw it?
2. Would it be ok to allow returning such an error in addition to throwing it?

My instinct is that we should only support throwing the stop propagation error.

[domenic]

My instinct is that we should only support throwing the stop propagation error.

Mine as well. No strong feelings, but that seems like the right path.

[briancavalier]

Yeah, it seems like if our options are only return or throw, implementations have to to "corrupt" one of those code paths with logic that checks for the stop propagation value. If we allow both, implementations have to execute that logic in both paths. I'd rather corrupt the exception path :)

It's also a decent analog to throwing StopIteration.

[domenic]

It's also a decent analog to throwing StopIteration.

The analogy is a bit strained since StopIteration is actually not an Error (which is weird). But yeah.

[briancavalier]

Yeah, more that the approach of using throw is the same. And as @ForbesLindesay pointed out in #1, eventually the stop propagation value could be upgraded to something on which we can use instanceof, like StopIteration. So, yeah, it might be a better analogy in the future :)

Hmmm, that does make me think, though, that maybe an Error with a .name may not be best--i.e. since StopIteration isn't an Error. Another thought is that promises are duck-typed using the presence/type of the then property. Should we consider a similar approach here? For example: an object with a truthy stopProgressPropagation property?

To my eyes:

if(result && result.stopProgressPropagation)

is nicer than

if(result && result.name == 'StopProgressPropagation')

Thoughts?

[domenic]

Meh, I weakly prefer the name option. I wish StopIteration was specced somewhere. We can ask es-discuss I guess?

[ForbesLindesay]

I prefer .name. I also think it should be a real error object, so I'd be inclined to have the check be:

if(result instanceof Error && result.name == 'StopProgressPropagation')