• From: "stephengreenubl@g..." <stephengreenubl@g...>
• To: "rjelliffe@a..." <rjelliffe@a...> , "xml-dev" <xml-dev@l...>
• Date: Tue, 04 May 2010 06:41:37 +0000

Thanks Rick, thanks too everyone who has posted.

Interesting you say RDF where I have thought OWL. Unless we say that 'OWL isA RDF' :-)
I guess RDF has advantages like having a standardised query language. Then again OWL seems to me to be better when there are eumerations to deal with. It would be nice to have a standard query and predicate language for OWL. Then there could even be a way to bind it to assertions like Schematron (or TAML). Cos it seems to be a corollary of what all have agreed in this thread about structire vs semantics that XPath too might be limited to addressing structure-related, not semantic, assertions, even when it's type and schema-aware.

Best regards and thanks
Steve
Stephen D Green


-----Original Message-----
From: rjelliffe@a...
Sent:  04/05/2010 5:34:16 am
Subject:  Re:  'is-a' Relationships in XML?

>  Is there any way to express the
> other key relationship of 'is-a' in XML? Is this something a schema
> language can express?

Use RDF.

> Can we say that element (or even type) A 'is-an'
> element (or type) B? Any plans to add this 'feature' to the XML
> technologies if it isn't one already? It might be a key gap to fill. I
> might want to somehow imply that my <invoice/> is a <document/>. Are
> substitution groups (with, apparently, some inadequacies) the only way to
> express such a relationship in XML?

Schemas do not deal with semantics but values and occurrence
relationships. They are superficial.

Some people try to make schemas do more: for example the XBRL people try
to do semantic modeling of a kind using XML Schemas constructs. I don't
know that it is very prudent.

What substitution groups and so in in XML Schemas give us is not "X is a
Y" but "X has the same has-a's as Y". In other words, the superficial
properties of Y also are what we expect to see in X. Semantics is a
slippery word, so sometimes this superficial similarity between X and Y
may be enough; however, the fact that each element X and Y in the same
substitution group has a different name suggests that X and Y are not
semantically equivalent: if they were they would have the same name
(localization and legacy excepted.)

Cheers
Rick Jelliffe

P.S. I don't buy that XML and XML Schemas even expresses "has-a" on any
semantic level btw.


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