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"XML Schema specifies just syntax" versus "XML Schema specifies semantics"

From: Costello, Roger L. <costello@mitre.org>
Date: Wed, 9 Nov 2011 19:10:17 +0000
To: "xmlschema-dev@w3.org" <xmlschema-dev@w3.org>
Message-ID: <B5FEE00B53CF054AA8439027E8FE177501D7CE@IMCMBX04.MITRE.ORG>
Hi Folks,

Sally says, "XML Schema specifies just syntax."

John says, "XML Schema specifies semantics."

Who is correct?

First, we need to be clear on what is meant by "semantics."

I will define it as such: 

    The semantics of a thing is its relationships to other things
    and its properties. 

In this discussion I focus exclusively on "relationships." 

Here are examples of expressing relationships:

    - Book is an Object.
    - Person is an Object.
    - author is a property.
    - title is a property.
    - name is a property.

The examples show a "kind-of" relationship between Book/Person to Object, and author/title/name to property.

"kind-of" is only one type of relationship. There are many others, such as "same-as."

So the question resolves, at least in part, to this:

      Does XML Schema allow relationships to be expressed?

With XML Schema you have the ability to create a complexType and then do derive-by-restriction or derive-by-extension on it.  For example, you may create a Book complexType that extends a Publishing complexType. Isn't that an example of XML Schemas expressing a relationship? If yes, then isn't it demonstrating that "XML Schema specifies semantics"?

No, not really.

The derive-by-extension and derive-by-restriction capability specifies a complexType by reusing another complexType. That is quite a different thing from expressing a relationship for the purpose of informing.

Here's what Michael Kay said regarding the use of derive-by-restriction and derive-by-extension to specify semantics:

    I think it's probably a mistake to try and use the concept 
    of 'type' to represent an ontological distinction ...

Even if we grant that derive-by-restriction and derive-by-extension specifies a legitimate ontological relationship, it can only specify one type of relationship (namely, subclass). Typically, a much more varied set of relationships is needed to fully inform.

Here's what Eliot Kimber said:

    There is no sense in which they [XML Schemas] can be 
     anything more than a very weak reflection of some deeper 

Conclusion: XML Schemas specifies syntax only.

What do you think?

Received on Wednesday, 9 November 2011 19:10:52 UTC

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