Make never propagate

[bergus]

never() claims to "consume virtually no resources". However, this is no more true if never is used in any combinator, especially merge/all/settle. Should we detect nevers, and futures that eventually resolve to a never there and set the result accordingly?

If we want to do that, any ideas about the how? Should Action implement an onNever handler that is called by Never::_runAction?

[briancavalier]

I believe it's mostly true: never itself does consume almost no resources, and near() will tend to collapse promise chains over time.

I'd thought about detecting never and short circuiting in merge/all/settle (I think I even wrote the code for it at some point), and I still think it's a pretty cool idea. I ended up believe that real world usages of never will be few and far between, and so I didn't feel like the extra code was worth it. I could definitely be convinced that it's worthwhile, tho, if the code to do it is extremely small and simple.

[bergus]

Yes, real-world usages of never seem to be rare. I wonder if anyone used it as a poor-mans cancellation, otherwise I can hardly imagine use cases.
If anything, it might have been used in the monoid instance together with concat, but that one already is short-circuited.

[briancavalier]

I wonder if anyone used it as a poor-mans cancellation

That's an interesting question. I hadn't even considered it :) Now I'm wondering what that would look like ... any chance you've tried it?

[bergus]

I mean something like

let isCancelled = false;
cancelButton.onclick = () => { isCancelled = true; … };
….then(r => {
    if (isCancelled) return never();
    …
}).then(r => {
    if (isCancelled) return never();
    …
})