Lenz's implication is that the functionality overlap in XQuery is unnecessary:

...the overlap between XQuery and XSLT is far too great for the W3C to reasonably recommend them both as separate languages. If XSLT (or XSLT 2.0) isn't considered adequate as an XML query language by itself, then the development of an XML query language should still build from the same semantic and syntactic base as XSLT

He goes on to provide a simple formula describing what he sees to be the relationship between the two languages:

XQuery = XSLT - templateRules - nonAbbreviatedXPathAxes

And he elaborates in a paper which he indicates is adapted from a presentation he will give at the upcoming XSLT-UK conference.

In a response that's worth reading in full, Jonathan Robie (one of the editors of the XQuery Working Draft, as well as several related documents) begins by pointing out that the list of XQuery editors includes "a former member of the XSL Working Group,... one of the inventors of SQL, one of the inventors of XML-QL, and one of the inventors of XQL, a precursor of XPath." He goes on to explain:

XQuery and XSLT will share a common expression language, including path expressions. XSLT is really two languages, an XML-based language used to write the templates, and XPath, an expression language used for patterns. Both XQuery and XSLT will use XPath 2.0, and the two Working Groups are working closely together on this. So the two languages will share a great deal.

That said, he gives three areas that he believes to justify creating XQuery as a language separate from XSLT: And he goes on to provide details about each area.

The XQuery debate follows an earlier, more general discussion and followup on xml-dev (kicked off by a posting, titled intertwined specs, from Simon St.Laurent) regarding movement "toward a jungle of intertwined specs, with complexity increasing despite/because of reuse".

That earlier discussion is summarized and elucidated by Leigh Dodds in his February 21st XML-Deviant column.

(Dodds and St.Laurent are both xmlhack contributing editors.)

Recent related stories: