Does XML Query Reinvent the Wheel?

Debates on the XML-DEV and XSL mailing lists over the last two weeks concern the futures of XSLT, XPath, and, the latest addition to the W3C XML toolkit, XML Query. There are no signs of these debates ending this week. Discussion on XML-DEV about the design of XML Query rages on.

Reinventing the Wheel

The focus of last week’s XML-Deviant was the concern expressed by several XML-DEV contributors that the interdependence of several W3C specifications may have exceeded the dictates of software reuse and become instead a tangled mess. Suggestions were floated for a refactoring of several standards in order to separate the component parts.

This debate has focused on XML Query in particular this week, following Evan Lenz’s claim that the overlap between XML Query and XSLT is so great that they are not really separate languages.

After reviewing the XQuery spec, I’m concluding that 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.

Lenz has fully documented his opinion in a paper he’ll be presenting at the XSLT-UK conference in April. The most obvious overlap between XML Query and XSLT is their shared use of XPath. Indeed, the XML Query and XSLT Working Groups are coordinating on the development of XPath 2.0. XPointer took a similar approach, layering itself on XPath 1.0. At first glance this seems a reasonable approach.

However, Lenz believes that the overlappings go deeper than sharing XPath.

The “navigation part” is only a small part of the overlap. The result construction mechanisms, the flow control mechanisms, the variable binding mechanisms — these are all virtually indistinguishable (other than syntax) from XSLT’s mechanisms for doing the same. I demonstrate all of this in my paper.

The introduction of datatypes is making its way not only into XQuery but the XPath 2.0 and XSLT 2.0 requirements. Regardless of whether datatypes are only part of query or are part of both query and transformations, there should be a common semantic and syntactic core for XSLT and XQuery, rather than an invention of an entirely new syntax.

Lenz characterized XML Query as a subset of XSLT (no template rules, no abbreviated XPath axes) with the addition of data typing, and he claimed that this should be the model upon which XML Query is developed. Noting concerns over the optimizability of XSLT, Lenz pointed out that the XSLT 2.0 requirements refer to an XPath subset that could be used to develop XML Query.

Read full story at XML.com http://www.xml.com/pub/a/2001/02/28/deviant.html