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   Re: Equivalence classes

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  • From: ht@cogsci.ed.ac.uk (Henry S. Thompson)
  • To: xml-dev@xml.org
  • Date: 16 Feb 2000 09:03:14 +0000

Philip Wadler <wadler@research.bell-labs.com> writes:

> A request for clarification about equivalence classes.
> If all elements are declared via top-level `element' declarations,
> then I think that I understand how equivalence classes work.  Can
> someone please explain how equivalence classes work in the presence
> of element declarations nested within type declarations?

'equivClass' is incoherent on a nested (or 'local') element
declaration, because of the asymmetry pointed out below.  This should
be made clear in the spec. -- I agree it is not clear now.

> Also, I gather that equivalence classes are not really symmetric.
> That is, declaring A with equivalence class B is not at all the
> same as declaring B with equivalence class A.  I think this is the
> right choice, but the schema specification needs to be clarified
> and it would help to choose a name that did not imply symmetry.
> In particular, I suggest exploiting the analogy with object-oriented
> languages, and replacing `equivClass' with `super'.

I agree the name is misleading, and the asymmetry needs to be clearly
spelled out in the spec.  Whether we'll change it before Last Call is
another matter.  We've studiously avoided _all_ use of 'sub' and
'super', so far, for what I think is a good reason:  documents are not 
programs, there are important but subtle differences between our type
system and that of an OO language, accordingly encouraging false
parallels blurs important distinctions.

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/
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