Promises

Axel Rauschmayer axel at rauschma.de
Tue Nov 6 11:07:40 PST 2012


That’s at a weird intersection between HTML5 and ECMAScript, but it would be great to have!

I recently played a little more with IndexedDB and found it complicated to use:

1. In contrast to the Web SQL database API and the MongoDB API when it comes to functionality

2. In contrast to Node.js callbacks (where input and output are clearly separated) and promises when it comes to invocation mechanics

It might just be me “not getting it”, but there must be ways to make things more elegant (on both accounts).

Axel


On Nov 6, 2012, at 19:47 , David Bruant <bruant.d at gmail.com> wrote:

> Hi,
> 
> In a post to public-script-coord yesterday, Alex Russel wrote the following [1]:
> "[Web]IDL *is handy. *More to the point, it's the language of the specs we have now, and the default mode for writing new ones is "copy/paste some IDL from another spec that looks close to what I need and then hack away until it's close". This M.O. is exacerbated by the reality that most of the folks writing these specs are C++ hackers, not JS developers. For many, WebIDL becomes a safety blanket that keeps them from having to ever think about the operational JS semantics or be confronted with the mismatches."
> 
> I wasn't aware of this and then read through about a dozen WebAPIs [2] between yesterday and today and... discovered it's the case. In my opinion, one of the most absurd example is the DOMRequest thing which looks like:
> {
>   readonly attribute DOMString readyState; // "processing" or "done"
>   readonly attribute DOMError? error;
>   attribute EventListener      onsuccess;
>   attribute EventListener      onerror;
>   attribute readonly any?      result;
> };
> 
> Read it carefully and you'll realize this is actually a promise... but it has this absurd thing that it has to have both an error and result field while only one is actually field at any given point.
> Also, these APIs and JavaScript as it they are won't support promise chainability and the excellent error forwarding that comes with it off-the-shelf. Also, the lack of a built-in Q.all really doesn't promote good code when it comes to event synchronization.
> Oh yes, of course, you can always build a promise library on top of the current APIs, blablabla... and waste battery with these libraries [3].
> 
> I'm coming with the following plan:
> 1) get promises in ECMAScript
> 2) get WebIDL to support ECMAScript promises
> 3) get browser APIs to use WebIDL promises
> 
> About the first step, there is a strawman [4] that contains promises and it requires to define the event loop, so that's probably too much for ES6. Yet, it doesn't prevent to agree on a promise API that will be adopted in ES7.
> Besides the strawman, promises have run a long way from CommonJS [5] to jQuery [6] to Q [7] to Irakli's excellent post [8] to Domenic's recent rant [9] and I've missed a lot of other examples probably. The JS community is ready for promises. The idea has been used a lot.
> Different libraries have different APIs and I have no preference. The only things I really care about is chaining (with error forwarding) and a promise-joining function à la Q.all. I'll let people who care about naming fight.
> 
> I'm sure TC39 can come to an agreement *before* ES7 standardization, agreement that can be used by WebIDL and browser APIs (why not implemented long before ES7 work has even started).
> If you're a JS dev and care about promises, please show some support to this proposal :-)
> 
> David
> 
> [1] http://lists.w3.org/Archives/Public/public-script-coord/2012OctDec/0122.html
> [2] https://wiki.mozilla.org/WebAPI#APIs
> [3] http://assets.en.oreilly.com/1/event/79/Who%20Killed%20My%20Battery_%20Analyzing%20Mobile%20Browser%20Energy%20Consumption%20Presentation.pdf
> [4] http://wiki.ecmascript.org/doku.php?id=strawman:concurrency
> [5] http://wiki.commonjs.org/wiki/Promises
> [6] http://api.jquery.com/category/deferred-object/
> [7] https://github.com/kriskowal/q
> [8] http://jeditoolkit.com/2012/04/26/code-logic-not-mechanics.html
> [9] https://gist.github.com/3889970
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-- 
Dr. Axel Rauschmayer
axel at rauschma.de

home: rauschma.de
twitter: twitter.com/rauschma
blog: 2ality.com

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