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RE: RNG vs. XSD : is the use of abstract types andpolymorphis

  • From: "Costello, Roger L." <costello@mitre.org>
  • To: "xml-dev@lists.xml.org" <xml-dev@lists.xml.org>
  • Date: Tue, 13 Mar 2012 11:29:52 +0000

RE:  RNG vs. XSD :  is the use of abstract types andpolymorphis
Hi Folks,

I have been thinking about the issue of "inheritance" in schema languages for XML.

Recall that James Clark says (paraphrasing) that it is not the role of a schema language to model conceptual or semantic relationships such as inheritance. Such relationships are best modeled elsewhere.

That makes sense to me. Separation of concerns is a good thing. Use a schema language to define a template for syntactic organization. Using my chocolates example, use a schema language to show the organization of boxes (elements) and what chocolates (data) goes into each box. 

Use other technologies for expressing relationships and meaning -- use ontologies, data specifications, UML, etc.

That's a nice, clean separation of concerns. That yields more productivity and better results. (Adam Smith, Wealth of Nations)

I wonder why XML Schemas ever introduced inheritance machinery (derive-by-extension, derive-by-restriction, element substitution) into the language?  The inheritance machinery muddies things up.  It results in XML Schema trying to be both a language of expressing syntactic template and a poor man's pseudo UMLish relationship ontology language. 

This muddiness has created enormous confusion over the years. 

"XML Schema is just syntax."   

"No, XML Schema is semantics, just look at the meaning in this inheritance tree."

It seems that the prudent path is to avoid all inheritance machinery in XML Schema. 

Don't use derive-by-extension, derive-by-restriction, and element substitution . Use XML Schema just for expressing templates of elements and attributes. 

Use ontologies, data specifications, UML, etc. for expressing relationships and meaning.



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