Nov 18 notes

Brendan Eich brendan at mozilla.com
Sun Nov 21 19:05:12 PST 2010


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


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