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Earlier this week, email@example.com (Dave Beckett) wrote:
>As far as any XML schema language is appropriate for general RDF/XML
>with it's open use of XML Namespaces, RelaxNG is the one I'd suggest is
>most appropriate (compared to for example W3C XML Schemas which is more
>of a closed/complete DTD style).
This discussion has connected with a number of other conversations to
leave me wondering whether RELAX NG might prove to be more than a schema
language capable of of validating RDF/XML, but indeed a schema language
which eases the conflicts at the boundaries between RDF and XML.
RDF/XML has taken a lot of flak lately. On the one hand, it offers too
many options, so developers who want to work with RDF data using XML
tools face a pretty frustrating task, even before getting into the risks
of processing graphs with tree-oriented tools. On the other hand,
trying to make XML vocabularies RDF/XML compatible is not much fun
either. Some aspects of this  don't even seem like good markup
practice to me, especially things like "eschew mixed content", the use
of RDF-namespaced attributes in host vocabularies, and container issues.
We can struggle along with this, sure. RDF and XML seem stuck in a
lousy marriage at this point, each disappointing the other on a regular
basis. Mark Pilgrim's done a nice job  of delineating various ways
in which this conversation often flows, tying it to the
Pie/Echo/Atom/etc. project's concrete challenges. It seems like there
should be some way of at least separating those issues in practice.
It may be very naive of me to think this, but something keeps telling me
that RELAX NG's patterns and RDF's graphs may be able to talk to each
other in ways that go well beyond the rdf:parseType attribute or XSLT
transforms between attribute names in a local vocabulary and the rdf:ID,
rdf:about, etc. I suspect (though I'm still working it out, and don't
know nearly enough to be certain) that RELAX NG annotations could be
sufficient to provide a complete mapping from an unchanged XML
vocabulary to a set of RDF graphs.
In some ways, this feels perverse, as it uses something of a PSVI
approach to define a mapping between the XML and its RDF reading. At
the same time, however, RELAX NG patterns feel flexible enough to
support RDF's many possibilities and to express the different graphs
which may appear given different co-occurrence constraints. (I'm not
proposing RELAX NG as a general RDF schema language - I don't think that
would work. This is _just_ about mapping XML to RDF graphs.)
I don't yet have anything concrete to show, I'm afraid, so this is
pretty much playing a hunch. Anyone have the same hunch? Anyone have
reasons why this is obviously impossible? Possible?
 - http://www.xml.com/pub/a/2002/10/30/rdf-friendly.html
 - http://www.xml.com/pub/a/2003/08/20/dive.html
http://simonstl.com -- http://monasticxml.org