Nov 18 notes

Brendan Eich brendan at mozilla.com
Sun Nov 21 19:36:31 PST 2010


On Nov 21, 2010, at 7:21 PM, Chris Sells wrote:

> I'm a huge fan of iterators. As far as index goes, I'm not a fan of the use of the colon. We should take our time, but something that I wanted to throw into the pot would be to build on the key-based nature of for-in with syntax like this:
> 
> forvals ( var x in xs ) {...}
> 
> Also, have we given any thought to making it easier to implement custom iterators ala the C# "yield return" operator?

We've prototyped and shipped generators based on Python 2.5 (with a simplification that avoids introducing GeneratorExit) for years, in SpiderMonkey and Rhino. The yield operator is a low-precedence expression prefix whose presence in a function makes it a generator. Wiki link:

http://wiki.ecmascript.org/doku.php?id=strawman:generators&s=generators

This strawman wil be revised based on specific feedback ("put something in the function preamble, not just yield expressions in the body, to make it clearly a generator", and the yield-in-try-with-finally argument), and prepared for discussion at the January 2011 TC39 meeting.

/be


> 
> 
> -----Original Message-----
> From: es-discuss-bounces at mozilla.org [mailto:es-discuss-bounces at mozilla.org] On Behalf Of Brendan Eich
> Sent: Sunday, November 21, 2010 7:05 PM
> To: Waldemar Horwat
> Cc: Mark S. Miller; es-discuss
> Subject: Re: Nov 18 notes
> 
> On Nov 18, 2010, at 4:08 PM, Waldemar Horwat wrote:
> 
>> Consensus that we should have iterators.
> 
> For this, after all these years (JS1.7 added meta-programmable for-in in 2006), I'm grateful, although I wanted to add something your notes did not report:
> 
> To get consensus, we made a tentative agreement to leave for-in as it was and not enable any meta-programmability of it, consigning it to the historically-underspecified and non-interoperable enumeration mystery-meat status.
> 
> Instead, we did the committee-plays-it-safe thing of inventing new syntax for meta-programmable iteration:
> 
> for (var i : x) ...
> 
> This is a break from Python and JS1.7+ in SpiderMonkey and Rhino -- although it matches very late Java and C++ extensions that are similar (but not the same), and really not relevant to JS.
> 
> Worse, the use of colon in this new for syntax is confusingly similar to long-standing future-proofing intentions around runtime type annotations (aka guards or contracts).
> 
> (BTW, I don't think :: is a good type annotation or guard punctuator, btw -- it's the C++ namespace qualification operator, also specified for namespacing by ECMA-357 (E4X) and used that way in ActionScript 3 (and in ES4, RIP). So I was surprised to see :: used for annotation-like syntax in http://wiki.ecmascript.org/doku.php?id=strawman:guards and http://wiki.ecmascript.org/doku.php?id=strawman:trademarks.)
> 
> for (var i : x) ...      // must be new iteration
> for (var i : T : x) ...  // iteration again, but parsed how?
> for (var i : T in x) ... // for-in with annotated var
> 
> Depending on what T might be, grammatically, this could get sticky for top-down parsers. It is confusing and ugly in any event, IMHO.
> 
> Probably we need to take our time and not rush into a meta-programming-here syntax variant of for-in. I'll not propose anything better right now.
> 
> /be
> _______________________________________________
> es-discuss mailing list
> es-discuss at mozilla.org
> https://mail.mozilla.org/listinfo/es-discuss
> 



More information about the es-discuss mailing list