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Re: [xml-dev] XPath 2.0 Best Practice Issue: Graceful Degradation

Philippe Poulard writes:

> Schema technologies were primarily designed to express constraints 
> on XML document classes.

To add a bit more detail, the following is the text of the section of the 
Schema Recommendation that describes the purpose of a schema [1]

"The purpose of XML Schema: Structures is to define the nature of XML 
schemas and their component parts, provide an inventory of XML markup 
constructs with which to represent schemas, and define the application of 
schemas to XML documents.

The purpose of an XML Schema: Structures schema is to define and describe 
a class of XML documents by using schema components to constrain and 
document the meaning, usage and relationships of their constituent parts: 
datatypes, elements and their content and attributes and their values. 
Schemas may also provide for the specification of additional document 
information, such as normalization and defaulting of attribute and element 
values. Schemas have facilities for self-documentation. Thus, XML Schema: 
Structures can be used to define, describe and catalogue XML vocabularies 
for classes of XML documents.

Any application that consumes well-formed XML can use the XML Schema: 
Structures formalism to express syntactic, structural and value 
constraints applicable to its document instances. The XML Schema: 
Structures formalism allows a useful level of constraint checking to be 
described and implemented for a wide spectrum of XML applications. 
However, the language defined by this specification does not attempt to 
provide all the facilities that might be needed by any application. Some 
applications may require constraint capabilities not expressible in this 
language, and so may need to perform their own additional validations."

> Unfortunately, schema technologies are still trailing behind. (One 
> of) the missing feature(s) is the support of semantics data types.

See those last two sentences in the "purpose" section.   It's not at all 
clear to me that Schema structures is the right place to go after these 
semantic types, except insofar as the existing ability to define named 
types, and inheritance hierarchies in which both the intensional 
refinement of a type (I.e. it's base type chain) as well as its 
extensional refinement (a restriction allows no more than its base) 
matter.  To make that last bit clearer, what I'm trying to say is, you can 
have two vacuous restrictions of integer, one named "employeeAge" and one 
named "partNumber".  They both accept the same numbers, but you can't put 
xsi:type="partNumber" on an element that's expecting an "employeeAge". 
That's what I meant by intensional as well as extensional, and it takes 
you just a bit of the way toward semantic typing.


[1] http://www.w3.org/TR/2004/PER-xmlschema-1-20040318/#intro-purpose

Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142

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