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RE: 'is-a' Relationships in XML?

  • From: "Michael Kay" <mike@saxonica.com>
  • To: <stephengreenubl@gmail.com>
  • Date: Mon, 3 May 2010 20:37:10 +0100

RE:  'is-a' Relationships in XML?
> Can we then say that XSD is the primary way we add semantics 
> to XML? 

Oh dear, semantics. I hate the word. A schema may well hint at the semantics
of the data, but I don't think it really does anything other than constrain
the space of valid documents. It's quite possible and common to write
schemas that impose constraints but tell you nothing about the semantics of
the data - for example that the type (content model) for a table cell is a
restriction of the type (content model) for a paragraph.

The fact that you can derive by restriction or by extension exacerbates
this: if <employee> allows all the child elements of <person> and then some
extra fields, then you can derive employee by extension from person, or you
can derive person by restriction from employee. In the latter case, if you
inferred person is-a employee ("A person is an employee with no salary"),
you would be wrong. It's a syntactic/structural relationship not a semantic


Michael Kay

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