XSchema Spec, Sections 2.0 and 2.1 (Draft 1)

Chris Maden crism at oreilly.com
Mon Jun 8 22:18:20 BST 1998

[John Cowan]
> I concede that many, perhaps most, general entities are document
> specific, but not all.  Consider the MIXED element from the my early
> drafts of XSchema itself.  The form
> 	<MIXED> <REF .../> <REF .../> ... </MIXED>
> declared #PCDATA-and-element content, whereas "<MIXED/>" declared
> #PCDATA-only content as a degenerate case.  The DTD could have
> declared (though actually it didn't) <!ENTITY PCDATA "<MIXED/>"> to
> allow people to write "&PCDATA;" in XSchemas as a better-documented
> version of "<MIXED/>".
> Surely this entity would be worthy of declaration in the XSchema
> XSchema?

There are certainly cases where an entity is useful for a class of
documents, but generally if an entity is of interest to more than one
document, it's also of interest to more than one class of documents:
for instance, the ISO character entity sets.  I think the problem of
defining entity replacements is fundamentally different from defining
element structures, and should be addressed separately.  Now,
certainly a class of documents could be defined by a combination of
element structure restrictions and an initial set of defined entities,
but the two things should not be conflated as DTDs conflate them.

> No, not at all.  There is no XSchema equivalent of parameter
> entities.

That's unfortunate.  We tried to kill them in XML, but couldn't
because they're so useful.  The processing concerns they introduced in
XML go away if you treat them as ID'd objects and references thereto,
so there's no reason to avoid them and very good reasons to include
them.  Especially since the syntax is going to be more verbose, the
gain in reusing pieces is larger.

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