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Re: QSA, the problem with ":scope", and naming

From: Jonas Sicking <jonas@sicking.cc>
Date: Wed, 19 Oct 2011 19:07:39 -0700
Message-ID: <CA+c2ei8-xHZORr2hWJ63GR2Aa0gPPM3OKSs7sURGvwaLiTAP=w@mail.gmail.com>
To: Alex Russell <slightlyoff@google.com>
Cc: Webapps WG <public-webapps@w3.org>, Yehuda Katz <wycats@gmail.com>, John Resig <jeresig@gmail.com>, Paul Irish <paulirish@google.com>, Lachlan Hunt <lachlan.hunt@lachy.id.au>
On Tue, Oct 18, 2011 at 9:42 AM, Alex Russell <slightlyoff@google.com> wrote:
> Lachlan and I have been having an...um...*spirited* twitter discussion
> regarding querySelectorAll, the (deceased?) queryScopedSelectorAll,
> and ":scope". He asked me to continue here, so I'll try to keep it
> short:
>
> The rooted forms of "querySelector" and "querySelectorAll" are mis-designed.
>
> Discussions about a Scoped variant or ":scope" pseudo tacitly
> acknowledge this, and the JS libraries are proof in their own right:
> no major JS library exposes the QSA semantic, instead choosing to
> implement a rooted search.
>
> Related and equally important, that querySelector and querySelectorAll
> are often referred to by the abbreviation "QSA" suggests that its name
> is bloated and improved versions should have shorter names. APIs gain
> use both through naming and through use. On today's internet -- the
> one where 50% of all websites include jQuery -- you could even go with
> element.$("selector") and everyone would know what you mean: it's
> clearly a search rooted at the element on the left-hand side of the
> dot.
>
> Ceteris peribus, shorter is better. When there's a tie that needs to
> be broken, the more frequently used the API, the shorter the name it
> deserves -- i.e., the larger the component of its meaning it will gain
> through use and repetition and not naming and documentation.
>
> I know some on this list might disagree, but all of the above is
> incredibly non-controversial today. Even if there may have been
> debates about scoping or naming when QSA was originally designed,
> history has settled them. And QSA lost on both counts.
>
> I therefore believe that this group's current design for scoped
> selection could be improved significantly. If I understand the latest
> draft (http://www.w3.org/TR/selectors-api2/#the-scope-pseudo-class)
> correctly, a scoped search for multiple elements would be written as:
>
>   element.querySelectorAll(":scope > div > .thinger");
>
> Both then name and the need to specify ":scope" are punitive to
> readers and writers of this code. The selector is *obviously*
> happening in relationship to "element" somehow. The only sane
> relationship (from a modern JS hacker's perspective) is that it's
> where our selector starts from. I'd like to instead propose that we
> shorten all of this up and kill both stones by introducing a new API
> pair, "find" and "findAll", that are rooted as JS devs expect. The
> above becomes:
>
>   element.findAll("> div > .thinger");
>
> Out come the knives! You can't start a selector with a combinator!
>
> Ah, but we don't need to care what CSS thinks of our DOM-only API. We
> can live and let live by building on ":scope" and specifying find* as
> syntactic sugar, defined as:
>
>  HTMLDocument.prototype.find =
>  HTMLElement.prototype.find = function(rootedSelector) {
>     return this.querySelector(":scope " + rootedSelector);
>   }
>
>   HTMLDocument.prototype.findAll =
>   HTMLElement.prototype.findAll = function(rootedSelector) {
>     return this.querySelectorAll(":scope " + rootedSelector);
>   }
>
> Of course, ":scope" in this case is just a special case of the ID
> rooting hack, but if we're going to have it, we can kill both birds
> with it.
>
> Obvious follow up questions:
>
> Q.) Why do we need this at all? Don't the toolkits already just do
> this internally?
> A.) Are you saying everyone, everywhere, all the time should need to
> use a toolkit to get sane behavior from the DOM? If so, what are we
> doing here, exactly?
>
> Q.) Shorter names? Those are for weaklings!
> A.) And humans. Who still constitute most of our developers. Won't
> someone please think of the humans?
>
> Q.) You're just duplicating things!
> A.) If you ignore all of the things that are different, then that's
> true. If not, well, then no. This is a change. And a good one for the
> reasons listed above.
>
> Thoughts?

I like the general idea here. And since we're changing behavior, I
think it's a good opportunity to come up with shorter names. Naming is
really hard. The shorter names we use, the more likely it is that
we're going to break webpages which are messing around with the
prototype chain and it increases the risk that we'll regret it later
when we come up with even better functions which should use those
names. Say that we come up with an even better query language than
selectors, at that point .find will simply not be available to us.

However, it does seem like selectors are here to stay. And as much as
they have shortcomings, people seem to really like them for querying.

So with that out of the way, I agree that the CSS working group
shouldn't be what is holding us back. However we do need a precise
definition of what the new function does. Is prepending ":scope " and
then parsing as a normal selector always going to give the behavior we
want? This is actually what I think we got stuck on when the original
querySelector was designed.

So let's get into specifics about how things should work. According to
your proposal of simply prepending a conceptual ":scope" to each
selector group, for the following DOM:

<body id="3">
  <div id="context" foo=bar>
    <div id=1></div>
    <div class="class" id=2></div>
    <div class="withChildren" id=3><div class=child id=4></div></div>
  </div>
</body>

you'd get the following behavior:

.findAll("div")  // returns ids 1,2,3,4
.findAll("")      // returns the context node itself. This was
indicated undesirable
.findAll("body > :scope > div")  // returns nothing
.findAll("#3")  // returns id 3, but not the body node
.findAll("> div") // returns ids 1,2,3
.findAll("[foo=bar]") // returns nothing
.findAll("[id=1]") // returns id 1
.findAll(":first-child") // returns id 1

Is this desired behavior in all cases except the empty string? If so
this seems very doable to me. We can easily make an exception for the
case when the passed in string contains no selectors and make that an
error or some such.

I do however like the idea that if :scope appears in the selector,
then this removes the prepending of ":scope " to that selector group.
Is there a reason not to do that?

Additionally it seems to me that we could allow the same syntax for
<style scoped>. But maybe others disagree?

I think appropriate optimizations as well as extensible functions
should be out-of-scope for this thread. They are both big subjects on
their own and we're approaching 50 emails in this thread.

/ Jonas
Received on Thursday, 20 October 2011 02:08:46 GMT

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