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Re: The DOM XPath interface -- what is its status?

From: Robin Berjon <robin.berjon@expway.fr>
Date: Tue, 05 Apr 2005 00:18:37 +0200
Message-ID: <4251BD3D.8070503@expway.fr>
To: Philippe Le Hegaret <plh@w3.org>
Cc: www-dom@w3.org

Philippe Le Hegaret wrote:
> On Mon, 2005-04-04 at 21:36 +0200, Robin Berjon wrote:
>>We discussed it briefly in November and indeed we may have some. In 
>>particular, our users tend to think that it's overly complicated, which 
>>we have attributed (perhaps wrongly) to trying to be forward-compatible 
>>with XPath 2.0.
> I don't believe the current complexity should be attributed to XPath 2.0
> (which is a fine and compelling technology for XSLT 2.0 btw but this is
> not the place for XPath 2.0 advertisement :).

I didn't mean to say anything wrong about XP2, simply that adding 
support for it seemed *perhaps* a touch premature :)

> The only complexity added by XPath 2.0 is the result of the
> XPathEvaluator.evaluate method. It is a DOMObject instead of a
> XPathResult object.

Ok, that makes sense. It's an extra level of indirection but I guess it 
makes no difference in Ecmascript so our primary constituency won't ever 
care, or in fact notice. I'll note however that where our users use Java 
instead of Ecmascript, it tends to be on mobile devices and there 
generally try to avoid casting as it's an operation with measurable cost 
in those environments. We also tend to try to contain interface 
proliferation so as to be as implementable on limited devices as 
possible (see our work on the SVG uDOM for instance).

> The other added complexities were:
> - the type and result parameters on the XPathEvaluator.evaluate method;
> - the iterator vs snapshots in the XPathResult interface;
> - the XPathExpression object;
> - the support for XPath namespace nodes.
> The first three were done due to performance consideration. The last one
> was done for full XPath 1.0 support.

I'd forgotten about those, thanks, they came up too. Some serious 
concerns were raised during discussion that the iterator results were 
rarely available in existing XPath implementations that exposed an API. 
For the most part, SVG implementors are happy to add DOM 3 XPath if most 
of the work is in fact mostly glueing an existing implementation into 
theirs. If those are dropped, we can then drop the type parameter to 
XPathEvaluator.evaluate. This in turn drops a bunch of iterator-related 
fields, and the need for the implementation to maintain iterator validity.

We can then look at the result field, which is also something that isn't 
always available, and strikes me as the kind of optimization that users 
either ignore entirely or misuse.

The XPathExpression object is ok, though I'm not sure it's such a huge 
gain in performance. An implementation that doesn't want to recompile 
XPath expressions every time can cache them itself. XML::XPath for 
instance caches its own internal version of XPathExpression in a hash 
table keyed on the XPath expression it's being fed and check that before 
compiling. It's hardly rocket science, and it works much more reliably 
than putting the onus on the user to remember to pre-compile her expression.

The namespace bit is perfectly logical and not a big deal.

Hey, it looks like I could implement this atop XML::LibXML in a couple 
hours, docs included ;)

Now that thanks to you refreshing my memory I've had fun with my 
chainsaw, there's one thing that we found to be lacking in the 
interfaces: a way to register functions. Many implementations support 
that feature, and it's a very, very useful one (eg it would allow us to 
implement SVG's Extensions to XPath directly, as well as all the XForms 

>> Noise was made according to which it would be simpler to 
>>drop XP2 support entirely now and revisit it later, and add (optional) 
>>CSS support instead.
> I doubt you would simplify the current proposal by dropping XP2 and
> adding CSS, unless you reuse the XPathResult for CSS results as well.

Again, this is preliminary as I haven't had time to do full research 
into this yet but so far I have no reason to believe that the 
XPathResult couldn't be reused for CSS as well (though a different name 
may be an option :). The only exceptions I can think of are some of the 
pseudos, though an obvious mapping could be found. For first-letter and 
first-line a string could be returned, and for ::after and ::before the 
matching element with perhaps an additional field in XPathResult. But 
even the latter seems overkill and could simply be declared unsupported 
without anyone complaining about it (famous last words ;).

You would need some extra wording indicating that some queries won't 
ever return anything in some implementations (eg the dynamic ones such 
as :hover in non interactive agents and the rendered ones like 
::first-line in implementations that don't have a rendering) and also to 
provide the simple mapping between the two models (CSS doesn't talk 
about node sets even though they're there) but neither changes the 
actual implementation. This part would be optional and may be deferred 
to a later version depending on support for it (though bridging XPath 
and CSS is on my list of things I'd like to see more consensus about 
this year).

Robin Berjon
   Research Scientist
   Expway, http://expway.com/
Received on Monday, 4 April 2005 22:18:40 UTC

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