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   Re: Internal subset equivalent in new schema proposals?

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  • From: Paul Prescod <paul@prescod.net>
  • To: xml-dev@ic.ac.uk
  • Date: Sat, 28 Nov 1998 09:24:42 -0600

John Cowan wrote:
> Paul Prescod scripsit:
> > In fact, I'll go so far as to say that it does NOT make sense. SGML goes
> > *too far* in this direction: the ID/IDREF mechanism should be treated at a
> > separate level, like other integrity constraints.
> IMHO the purpose of ID/IDREF is to express non-hierarchical data using
> hierarchical notation.  Appropriate use of ID and IDREF attributes
> allows the representation of any directed graph, whether acyclic or
> cyclic.

That's fine, but it isn't clear why XML must be able to support directed
graphs at its most basic level. We know that we need multiple levels to
support multiple types of structures.

We also know that one of the requirements for supporting directed graphs
*properly* is the ability to do relative addressing (that's why we have
XPointer). But ID/IDREF can't do that. ID/IDREF also has a very inflexible
namespace mechanism. Strictly speaking it is "enough" in that every
element is addressable, but practically speaking it does not allow us to
express the structure explicitly enough.


This element has three unique identifiers, each unique in a different

In other words, ID/IDREF strikes me as the kind of half-solution likely to
be thought up by someone corrupted by too much exposure to SQL and not
enough to documents. :) :) :)

 Paul Prescod  - ISOGEN Consulting Engineer speaking for only himself.
Christmas shopping in a T-Shirt? Toto, I have a feeling we 
aren't in Canada anymore.

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