Re: [xml-dev] XML spec and XSD

["G. Ken Holman" <gkholman@CraneSoftwrights.com>]

At 2009-11-14 21:55 -0500, I wrote:
>There may be other "core" faults, but that is one that has been a 
>problem for me.

Of course I thought of another one right after I posted the above, a 
fault that is a deficiency rather than something broken (though you 
could consider the specification broken without the functionality):

W3C Schema validation semantics cannot express the constraints of 
XHTML's "head" element which in DTD-speak is:

<!ENTITY % head.misc "SCRIPT|STYLE|META|LINK|OBJECT" -- repeatable 
head elements -->
<!ENTITY % head.content "TITLE & BASE?">
<!ELEMENT HEAD O O (%head.content;) +(%head.misc;) -- document head -->

The problem is with the limited cardinality of "title" and "base" and 
the arbitrary order of elements preventing the use of alternation 
because of the ambiguous content models.

In RELAX-NG the interleave concept addresses this.

And yet another fault of 1.0 that will be addressed with 1.1 is 
mutually-exclusive attributes, which is needed if one wanted to 
express W3C XSL-FO using a schema (for <basic-link> one has either 
internal-destination= or external-destination= but not both and not 
neither).  And I believe 1.1 doesn't address this with a semantic 
expressing the exclusivity as an operator with operands, but with an 
assertion after the fact that the presence of both is unacceptable.

In RELAX-NG attributes and elements can both be operands in the 
constraint semantics.

So these are but two examples of W3C specifications of XML 
vocabularies that existed before the W3C Schema specification was 
finalized, where each one has document constraints that cannot be 
expressed by W3C Schema semantics.

Other posters in this debate have cited new W3C specifications that 
are using RELAX-NG instead of W3C Schema, and I doubt the reason they 
used it was because of the size and complexity of W3C Schema.  If 
they are XML experts, they can make their way through specifications 
of any size to look for what they need.  I suspect the decisions were 
based on deficiencies in the validation semantics required for their needs.

Again, I hope this is considered helpful to the debate.

. . . . . . . . . Ken

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