• From: rjelliffe@a...
• To: "xml-dev" <xml-dev@l...>
• Date: Tue, 4 May 2010 14:34:16 +1000

>  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.

• References:
  • 'is-a' Relationships in XML?
    • From: "stephengreenubl@g..." <stephengreenubl@g...>