The importance of checking references.

Jeff Pollock has responded to Paul Gearon's post on Jeff's recent comments to the RDF DAWG list. In doing so he makes three points, I will address his other two points another time (if Paul dosn't beat me to it), but wanted to make a quick note here regarding the first.

Jeff's quote from the dawg charter didn't ring true to me in light of various comments on the dawg list, so I went and checked. Jeff wrote:

the DAWG charter says "It is a requirement that the query language be compatible with an XQuery context"

The charter reads:

There is a requirement for RDF data to be accessable within an XML Query context.

And in fact even this snipet looses signifigant, and important context. A true contextual quote reads:

At this stage, it is not clear to what extent XQuery technology is applicable to the task of querying RDF datasets. The RDF DAWG should aim to maximize W3C technology re-use, while also taking account of differences between the RDF graph data model and the XQuery data model. There is a requirement for RDF data to be accessable within an XML Query context. The working group should specify at least one mechanism for exposing RDF query facilities in an XQuery environment; that is, a way to take a piece of RDF Query abstract syntax and map it into a piece of XML Query using some form of extension to XQuery. ... While the data model of the query language of this protocol is dissimilar to that of XQuery, a non-XML concrete syntax might reuse syntactic elements from XQuery to aid learning time, even if XQuery is not chosen as the strawman.

Note that the sentence Jeff quoted is referring to an extension to XQuery to allow an RDF query to be embedded in, and accessed by, an XQuery. This is very different from a requirement for compatibility, and falls far short of requiring the use of an XQuery syntax for the rdf query language.

It is also worth noting here that the charter explictly recognises the difficulty in mapping XML-infoset semantics to rdf.

At this stage, it is not clear to what extent XQuery technology is applicable to the task of querying RDF datasets. ... While the data model of the query language of this protocol is dissimilar to that of XQuery, ... While RDF query addresses a different data domain (the RDF graph),

On the other hand, the charter also naturally recognises the value in cohesive standards that leverage off each other:

The RDF DAWG should aim to maximize W3C technology re-use, while also taking account of differences between the RDF graph data model and the XQuery data model. ... a non-XML concrete syntax might reuse syntactic elements from XQuery to aid learning time ... any query mechanisms defined by this group must leverage off, and, where possible, interoperate with, the related XML recommendations

In conclusion I will repeat what I said in my first post on this subject.

XML has a document centric, single-rooted, hierachial view of its data; and XQuery reflects that. RDF on the other hand is a open-world, non-rooted, cyclic directional graph-based view. It is not immediately obvious how to reconcile these two approaches; neither is it apparent that a syntax designed to query the former, will be appropriate for the latter.

I would have absolutely not problem with discovering that XQuery can be married with the semantics required by an RDF query language, in fact that would be great.

Update Andy Seabourne has posted a link to a paper by the jena team which nicely illustrates how BRDF can be made accessable to XQuery/XSLT.