In last week's W3C Technical Plenary session in Boston, some attendees
were far from happy about the state of W3C XML Schema, including James
Clark.
Clark, the technical lead for XML 1.0 and author of influential
XML
open source software, noted that he was reluctant to hurt the
feelings of the XML Schema Working Group, but then expressed his view
that the XML Schema effort was "little short of a disaster."
Echoing concerns expressed commonly in the XML community, Clark
commented on the length of time the specification had taken to create,
and noted that few people seemed to like it. He drew especial attention
to XML Schemas Part 1 as being overly complex, yet lacking in expressive
power.
Clark recently created TREX, an alternative schema language, due to his
dissatisfaction with the W3C's XML Schema.
It is clear that W3C XML Schema in its current form will not be
acceptable to a large part of the XML developer world (as well as
several members of its own Working Group). Difficulties with XML
Schema's lack of an underlying mathematical
model recently led XML Query WG members
Jonathan Robie, Phillip Wadler and colleagues to develop Model
Schema Language (PDF), essentially an attempt to retrofit a
mathematical model to W3C XML Schema.
Many developers
would be prepared to ignore W3C XML Schema and use an alternative, were it not for
the proposals to drive Schemas into specifications such as XSLT and
XPath, accompanied by the prospect of increasingly W3C XML
Schema-centric programming.
Update: (2001/03/12) Jonathan Robie writes to clarify his role in MSL and the relationship between MSL and XML Schema:
First off, a great deal of the work on MSL was done by Matthew Fuchs of
Commerce One and Allen Brown of Microsoft, and the original name was
"Matt's Schema Language", though we decided to make it sound a bit more
formal. So I felt that it was just a unfair to reduce Matthew and Allen to
the phrase "and colleagues".
Second, of the four MSL authors, three are members of the XML Schema
Working Group, and three are members of the XML Query Working Group. One of
the big reasons for MSL was to provide a simpler, formal basis for a query
language type system, which was a major motivation for those of us
interested in query languages. The response of the XML Schema Working Group
was to accept formalization as an exit criterion for their Candidate
Recommendation phase, which will provide XML Schema with a mathematical
model. I think that the XML Schema Working Group should be commended for
agreeing to this. The ongoing work in XML Schema formalization is being
done in the XML Schema Working Group itself.
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