Types and Context

[Jonathan Borden <jborden@mediaone.net>]

Gavin Thomas Nicol wrote:

> > <p:book>
> >   <title />
> >   <genre/>
> >   <author>
> >     <title/>
> >     <familyName/>
> >   </author>
> > </p:book>
> >
> > Then how do you distinguish between the two title elements?
>
> Context.

Exactly. And if you stop to think about this, XML Schema "datatypes"  are
hardly going to revolutionize XSLT/XPath 2.0.

Gasp! I hear cries of "no, no, no" echoing around the web...

In the Schema Algebra [1] statements [7-9] a "type" is the property of
belonging to a class. The predicate "typeOf(x, c)" tests a node "x" for
membership in the instance set of the class "c".

Think about XSLT templates, a template matches an XPath selector returning a
_set_ of nodes. This selector is seen directly as a _type_ which is defined
based on _context_. XML Schema may use rules other than context to classify
a node as a type, but this basic capability is one we already know and use
in XSLT/XPath.

Now let's think about one of the given examples:

"
<xs:schema xmlns:xs='http://www.w3.org/2001/XMLSchema'
           xmlns:this='urn:example.org.people'
           targetNamespace='urn:example.org.people' >

  <xs:complexType name='person' >
    <xs:sequence>
      <xs:element name='given' type='xs:string' />
      <xs:element name='family' type='xs:string' />
    </xs:sequence>
  </xs:complexType>

  <xs:element name='person' type='this:person' />

</xs:schema>
"

which would be used with a snippet such as:

<this:person xmlns:this='urn:example.org.people'>
  <family>Gudgin</family>
  <given>Martin</given>
</p:martin>

Now how might we refer to the _type_ of the <family> element. In XML Schema
terms the _type_ of the "this:person" element is "this:person" ... this is
also the XML 1.0 type as in XML 1.0 an element's type is its name, so far so
good.

What about the <family> element. In XML Schema, this element is declared in
the "this:person" complex type so you might use an XPath expression:

"this:person/family"

to refer to the "XML Schema datatype" in this new PSVI world (actually in
the XML Schema Formal Language it would be something like
"urn:example.org.people::type(person)::element(family) but assume the above
is shorthand)

... turns out that this is _exactly_ the same as the XPath used as an XSLT
selector. So I guess the real utility of XML Schema datatypes is the ability
to say: "use this QName as shorthand for this path selector".

Context.

-Jonathan


[1] http://www.rddl.org/SchemaAlgebra