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Re: PROPOSAL: Errata text to deal with the issue of predeclared 'xml' and 'xmlns' prefixes

From: Philip Taylor <pjt47@cam.ac.uk>
Date: Fri, 25 Sep 2009 15:09:30 +0100
Message-ID: <4ABCCF1A.7010004@cam.ac.uk>
To: Julian Reschke <julian.reschke@gmx.de>
CC: Shane McCarron <shane@aptest.com>, "public-rdf-in-xhtml-tf.w3.org" <public-rdf-in-xhtml-tf@w3.org>
Julian Reschke wrote:
> Clarifying... this is *really* academic, right? Or are you expecting 
> people to actually use things which are in the "xml" or "xmlns" namespaces?

It's not implausible that they *could* use them, e.g. you might use a 
RDDL extractor on http://www.w3.org/XML/1998/namespace and get a triple 
like:

   <http://www.w3.org/XML/1998/namespace>
     <http://www.rddl.org/purposes#normative-reference>
     <http://www.w3.org/TR/REC#sec-lang-tag> .

and then serialise it to RDFa (either manually, or with a tool that 
automatically CURIEfies URIs) using <p about="[xml:]" ...> if you think 
that prefix is predeclared.

(Fun fact: applying http://www.rddl.org/rddl2rdf.xsl to 
http://www.w3.org/XML/1998/namespace actually results in 
non-namespace-well-formed output, because it generates xmlns="$baseURI". 
XML Namespaces are hard.)

But I agree it's very rare and unlikely to affect real users in a 
noticeable way, so it's an academic point.

But I think it's an important academic point, for two reasons:

Firstly, given 
<http://lists.w3.org/Archives/Public/public-rdf-in-xhtml-tf/2009Sep/0293.html>, 
RDFa's specified behaviour here will determine whether it's possible or 
impossible to implement a conforming RDFa processor in XSLT, or in other 
technologies with similar constraints. (This is assuming you have a 
conforming XSLT processor - according to 
http://lists.w3.org/Archives/Public/public-rdf-in-xhtml-tf/2009Sep/0275.html 
it's already impossible to implement RDFa at all in one widely-deployed 
XSLT implementation.)

Secondly, it affects the conceptual relationship between RDFa and XML 
Namespaces. For example, 
http://lists.w3.org/Archives/Public/public-rdf-in-xhtml-tf/2009Sep/0253.html 
says:

> [Prefix] mappings could have been provided using the attribute @banana,
> containing syntax like "ex=http://example.org".
> 
> (And there is discussion about providing some additional mechanism to
> provide these mappings.)
> 
> But for now, the only mechanism available is that any attribute that
> conforms to the pattern described in [XMLNS], is interpreted as
> providing a prefix mapping.

http://lists.w3.org/Archives/Public/public-rdf-in-xhtml-tf/2009Sep/0209.html 
similarly says:

> I hope that we were very careful in the Recommendation to indicate that
> it is the *syntax* of the XML Namespace declarations that is used to
> define RDFa prefix mappings. [...] We don't use XML Namespaces.

If, in fact, the RDFa prefix mappings depend on XML Namespaces' own 
default bindings for xml/xmlns prefixes, then it is untrue that RDFa 
merely uses the Namespaces attribute syntax, and so this represents a 
shift in the view of the relationship between RDFa and Namespaces. So I 
think it's important to be clear about this and to understand the 
implications.

Also, it's an opportunity to demonstrate that XML Namespaces cause pain, 
which is always fun.

-- 
Philip Taylor
pjt47@cam.ac.uk
Received on Friday, 25 September 2009 14:10:07 GMT

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