Draft A

[ForbesLindesay]

This has lots of holes in it, I'm putting it here though, because I'm hoping that we'll start to see where the holes are and fill some of them in a little.

Progress/A+

This proposal specifies how progress is triggered and propagated in a PromisesA+ promise library.

Requirements

The then method

The then method is the same as that specified in the promises-spec except that it also takes a third argument. We'll call that third argument the onProgress.

The onProgress

1. If the onProgress is not a function
   1. It is ignored
2. If the onProgress is a function
   1. When progress is emitted, the onProgress is called with the ProgressValue as the first argument.
   2. If the onProgress throws an exception with a .name property equal to 'StopProgressPropagation' then the error is silenced and progress is not propagated. In all other cases, the result of the function is used as the progress value to propagate.
   3. onProgress is never called once a promise has already been fulfilled or rejected.

The progress method

The resolver has a .progress(value) method. This triggers all the onProgress methods. It returns a promise which is fulfilled with undefined once all progress methods are complete or is rejected with the first (non-StopProgressPropagation) exception thrown by the handlers, if any.

[novemberborn]

What is supposed to happen when progress is propagated, and a propagated handler throws? Is the promise returned by the first resolver dependent on the results of all propagations, or only of the handlers it directly knows about?

[novemberborn]

This draft doesn't mention anything about emitting progress after the resolver has been fulfilled/rejected. AFAIK current implementations stop emitting progress at that point.

[novemberborn]

WIP implementation at https://github.com/novemberborn/legendary/compare/progress. See examples.

[domenic]

Some minor nits:

• For consistency with Promises/A+, call it onProgress, not ProgressHandler.
• The fulfillment value of the progress method should be specified to be undefined, and the rejection reaso specified to be the rejection reason of the first progress handler to throw a non-StopProgressPropagation exception.
• It might be nice to have some terminology for non-StopProgressPropagation exceptions. Maybe that's the new definition of "reason"?

This spec doesn't include any of our discussion on propagation.

[ForbesLindesay]

I'm inclined to say it should depend on all the propagated progress, not just that first one, or we have to find another way to display those errors.

[ForbesLindesay]

@domenic I know this is pretty seriously lacking, I just thought a draft (even if it is a bit vague/unfinished) might help the discussion move forwards, I'll try and improve it when I get time.

[novemberborn]

I've now implemented this draft in Legendary 0.2.0, including tests. I did reword the draft a bit, and it's hard to collaborate on that unless we can create a wiki page for it. Anyway, I'm including my version below.

---

Progress/A+

This proposal specifies how progress is triggered and propagated in a PromisesA+ promise library.

Requirements

The then method

The then method is the same as that specified in the promises-spec except that it also takes a third argument. We'll call that third argument the onProgress.

1. onProgress is an optional argument:
   1. If onProgress is not a function, it must be ignored
2. If onProgress is a function:
   1. It must be called after progress is emitted, with the progress value as its first argument.
   2. Unless the onProgress callback throws an exception with a .name property equal to 'StopProgressPropagation', the result of the function is used as the progress value to propagate.
   3. If the onProgress callback throws an exception with a .name property equal to 'StopProgressPropagation', then the error is silenced.
   4. onProgress is never called once a promise has already been fulfilled or rejected.

The progress method

1. The resolver has a .progress(value) method.
2. This triggers all onProgress callbacks.
3. It returns a promise which is fulfilled with undefined once all progress callbacks are complete,
4. or is rejected with the first (non-StopProgressPropagation) exception thrown by the callbacks, if any.

[ForbesLindesay]

I think we need to make it clear that onProgress callbacks can be asynchronous too. This means that the result could include rejected promises, as well as thrown exceptions.

[novemberborn]

I think we need to make it clear that onProgress callbacks can be asynchronous too. This means that the result could include rejected promises, as well as thrown exceptions.

You mean when they're propagated? I suppose that makes sense. It'd also mean that propagation waits until the returned promise fulfills.

[novemberborn]

If onProgress callbacks can return promises, and the progress is only propagated when those promises resolve, then what happens if the progress value itself is a promise? If that exact same value is returned by the callback I think it should be propagated as-is.

[ForbesLindesay]

I wasn't really thinking we'd support a promise as the progress value, if we are then we need to think about how that's done. Is promises as progress values something anyone's using in the wild? @domenic ?

[novemberborn]

I'd be fine with not supporting that, but then we'd have to decide whether it's silently ignored or if it's an error.

[ForbesLindesay]

Neither, I'd say it get's fully resolved. i.e. you can call resolver.progress(promise) but it's equivalent to calling:

promise.then(resolver.progress);

[novemberborn]

Works for me.

[novemberborn]

Implemented those changes in Legendary 0.3.0, with an updated spec at https://gist.github.com/4400252.