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I've been trying to understand just *why* the Schema WG is working on the
"XML Schema: Formal Description" document. I remember James Clark
criticized  WXS for the lack of integrating this formal description in
the spec from the start:
More than a year after the publication of the W3C XML Schema
Recommendation, "XML Schema: Formal Description"  is still a work
in progress and is still far from being a complete and correct
description of the semantics of XML Schema; moreover, it cannot be
relied on as it has no normative force.
The RELAX NG formalism has a solid basis in tree automata theory. W3C
XML Schema has no such basis.
The role of a schema in a specification is to serve as a formalism.
How good is a formalism if that formalism itself lacks a proper formal
Still, I have seen enough successful standards based on DTDs, and does DTDs
have a formal description? Well, I think the description is formal enough,
but I guess there's must be something lacking, because the RNG spec and the
formal description for WXS looks all different.
If it's so important to have a formal description, why hasn't the document
made any progress since 2001? I found this quote  from Jonathan Robie,
that suggests that if it was completed today, it would define a slightly
Personally, I would like to see us finish the Formal Description, make
it normative, and make XML Schema agree with it.
Finally, was the idea of a formal description for WXS inspired by Relax NG
or is it obvious that any schema language needs it?