On Wed, Sep 28, 2011 at 12:52 AM, Brendan Eich <[email protected]> wrote:
> On Sep 27, 2011, at 4:02 PM, Dean Landolt wrote: > > On Tue, Sep 27, 2011 at 4:57 PM, Brendan Eich <[email protected]> wrote: > >> On Sep 27, 2011, at 1:21 PM, Dean Landolt wrote: >> >> > Out of curiosity is there any reason to keep holes the holes around in >> ObjectLiteral and ArrayLiteral? >> >> No holes in ObjectLiteral. >> > > > Apologies -- I was thinking of the trailing comma, but that's not at issue. > > > Right, one (1) comma at end of either object or array literal is a > terminator, for uniform maintenance when appending vs. inserting. Very big > deal for those of us burned by ANSI C banning trailing single comma in C > enum! > > > What bugs have you seen due to holes in literas? > > > > Copy/paste errors, mainly. I'd think the same rationale to justify the > trailing comma applies against comma holes in array literals. > > > Hmm, I think I see what you mean, but the hole case is different enough and > anyway it has been in the language for 12 years. > True enough, but wouldn't you say it's still commonly misunderstood? > I've made this mistake more than once. This kind of bug tends to fail fast > so it's not as easy to slip into production, but it's still a runtime error. > > > Ouch. And apologies. > > > What's worse, the problem is exacerbated by the fact that tools that lean > on Array.prototype.forEach hide the hole, while those that use for(;;) treat > it as undefined. > > > That is a pain. I've heard independently that some people want forEach etc. > to fill holes. Can you confirm? > It'd be less confusing to the novice, that's for sure. For instance the array extras in at least one lib (dojo) still fill the holes (though I wonder what their plans are around 2.0 when they can break back-compat?). But there are real perf opportunities on sparse arrays when implemented natively, so they really shouldn't go. Plus, these cases may be tricky to catch with static analysis, so I doubt they even *could* go. But it'd be nice to take some bullets out of the chamber of this inevitable footgun -- for starters, by making them more explicit. > I understand that this is a problem with holes in general, and not with > ArrayLiteral syntax in particular, but try a console.log on a holy array in, > for instance, node and then chrome -- you'll get two VERY different-looking > results. The way that console.log in node completely papers over array > holes, compounded by the popularity of the comma-first style in that > community (which makes a double-comma harder to spot), > > > I find comma first just ugly on aesthetic grounds. > As do I, and I'm still no convert, but I have to say that after toying with it for a bit this is the only substantive criticism I can make of it :) could be a pretty nasty trap to fall into, even for a js veteran. > > > Adjacent comma does scream HOLE to me, but that's just me. > Same here, but in comma-first it's not always adjacent: var middleware = [ context, , csrf , xsite , head , considtional , error ] You've got to squint a little to see that hole, right? So, if we still have holes, is it really worth getting rid of ArrayLiteral >> support for them? I think not. >> > > > But are they of any real use for ArrayLiterals? IMHO a hole in a small > array (i.e. an ArrayLiteral) is almost always a mistake, > > > Or a testcase for the otherwise unwanted feature: > > > http://codesearch.google.com/#search/&q=%22,%20,%22%20lang:^javascript$&type=cs<http://codesearch.google.com/#search/&q=%22,%20,%22%20lang:%5Ejavascript$&type=cs> > > Some of those look intentional, and non-testy, though. > But what does it say that the so many are just explicitly testing this edge case? And an entry from wtfjs, right there on the first page :) Another wart that springs to mind is the fact that a hole === void 0 but is, in fact, not void 0. I guess technically this is correct -- reifying the index gets you void 0 -- but it's still odd. If there were a way to indexOf for the first hole, for instance, this would at least allow them to be exploited in userland code for more efficiently spare array looping constructs -- that alone would be a win. But really, my real beef is with the variance in behaviors for the array extras. IMHO the best bet would be for these APIs to grow an opt-in option to skip holes, but that ship probably sailed: an opt-in would be breaking and not statically analyzable; an opt-out would defeat the purpose -- if you're aware of holes you'd almost always want to skip over them, right? So one reasonable alternative would be to promote holes into a real language construct we can both *visualize *and *exploit* from userland explicitly, along with fixing as much of the API surface as is practical that creates them implicitly -- starting with holes in array literals.
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