Re: XML Schema WG comments on RDF documents

One of the comments from XML Schema WG that RDF Core is still considering is
the following:

[[[
2.2. Values without lexical forms (schema-related, important)

    In [22]http://www.w3.org/TR/rdf-concepts/#section-Datatypes:

      [22] http://www.w3.org/TR/rdf-concepts/#section-Datatypes

      * Each member of the value space may be paired with any number
        (including zero) of members of the lexical space (lexical
        representations for that value).

    The provision for values without corresponding lexical forms
    contradicts an assumption to which the XML Schema spec appeals from
    time to time. The lexical space of any simple datatype in XML Schema
    is the domain of the type's lexical mapping; the value space is its
    domain. There are no meaningless lexical forms in the lexical space of
    the type, nor are there ineffable values in the value space. By
    eliminating values from the value space (e.g. by setting minimal and
    maximal values), the type definer may indirectly also eliminate
    lexical forms from the lexical space; conversely, by eliminating some
    items from the lexical space (e.g. by setting a pattern), the type
    definer may eliminate items from the value space.
    Are there crucial aspects of RDF which will break if the list item
    quoted above is changed to read "paired with one or more members of
    the lexical space"?
]]]

As far as I remember, the motivation for the "(including zero)" is
because of our understanding of XML Schema Union datatypes.
Hence I am seeking clarification.

An XML Schema example:
=====================

<xsd:element name="foo">
  <xsd:simpleType>
    <xsd:union memberTypes="xsd:string xsd:int"/>
  </xsd:simpleType>
</xsd:element>

The lexical space of the first type masks the lexical space of the second.
Thus no integers have a corresponding lexical form.

Then the lexical mapping maps no values to the integer 2, but it is in the
value space since
   <foo xsi:type="xsd:int">2</foo>

conforms with this type, and accesses the hidden part of the value space.

(My XML Schema is weak - I am sure I have made a number of errors, but the
objective should be clear enough)

Please help me understand where I have gone wrong.

Jeremy

Received on Monday, 28 April 2003 05:30:14 UTC