The RDF and OWL Recommendations use the simple types from XML Schema. This document discusses two questions left unanswered by these Recommendations: Which values of which simple types are the same? And What URIref should be used to refer to a user defined type?
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This document is intended for public discussion: it poses two questions to
do with the use of XML Schema simple types within the Semantic Web, and
sketches multiple answers. None of these suggestions have
any recorded consensus around them.
After public feedback, possibly including
additional answers to these questions, the WG, in co-ordination with other
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This will then form the basis of a second publication indicating a
We particularly seek feedback reporting on implementation experience on these issues.
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@@@TODO: intro to datatyping in RDF and OWL, links to relevant sections of Recs.
What URI should be used within RDF and OWL for user defined XML Schema Datatypes.
@@@TODO exoand this, give an example, to be used throughout section.
>From Peter F. Patel-Schneider:
OWL can use XML Schema non-list simple types defined at the top level of an XML Schema document and given a name, by using the URI reference constructed from the URI of the document and the local name of the simple type. That is, if U is the URI of an XML Schema document that contains,
<xsd:schema ...> <xsd:simpleType name="foo"> <xsd:restriction base="integer"> <xsd:minInclusive value="1700"> </xsd:restriction> </xsd:simpleType> ... </xsd:schema>
then the URI reference
U#foo will be that datatype.
Implementations of OWL may choose to ignore the facets such a type.
See the definition of Senior in the example file from DAML+OIL.
This is a non-standard approach to fragIDs, and not in conformance with RFC 2396 and RFC XMLMIMETYPE @@@@which.
XML Schema Component Designators WD
the same example would have URI ref
XSCD also defines URIrefs for unnamed simple types within
This is still at WD stage. It depends on XPointer which is a W3C rec, but whose relationship with RFC XMLMIMETYPE is not yet fully secured. XPointer has less than full support from the IETF community.
The resulting URIrefs cannot be used with the qname abbreviation used in N3 and RDF/A, because they end in a ")" which is not an NCNameChar.
XSCD refers to such URIrefs as being to:
The canonical schema component designator for this simple type definition
i.e. referring to the definition rather than to the type defined.
@@@ Todo Example
Following xpointer an id attribute within an XML document can be used as a fragment to identify an XML element. This is only partially endorsed by RFC XMLMIMETYPE.
We could thus modify the example schema to:
<xsd:schema ...> <xsd:simpleType id="foo" name="foo"> <xsd:restriction base="integer"> <xsd:minInclusive value="1700"> </xsd:restriction> </xsd:simpleType> ... </xsd:schema>
Then use the URIref
U#foo as before.
This suffers from the same defect as the XSCD solution, that what is referred to is a syntactic object (the XML element, or the datatype definition) rather than the datatype itself.
The DAML+OIL solution is non-standard, and suffers from problems such as the non-uniqueness of names within some XML Schema.
The other two solutions are conformant with XPointer, but this is not fully endorsed by the IETF who control the XML mimetype, which RFC 2396 defers to concerning fragID semantics.
The XSCD solution is still at WD stage.
Both the XSCD and the id solutions have the problem that the proposed URIrefs refer to a syntactic thing, rather than the datatype itself. This could be seen as a property of the relevant definitions (XSCD and XPointer) being concerned about representations of resources rather than resources themselves. The URIref potentially can be seen as denoting (in the sense of RDF Semantics) the datatype, and represented by the XML element describing the datatype.
Possible advice might be to use id where possible, and once XSCD goes to Rec to also use that, particularly for XML Schema that are not under the control of the RDF or OWL author.
What is the relationship between the value spaces of the various XML Schema built-in simple types when used within RDF and OWL ?
It is clear that when two simple types are derived from the same
primitive type then the two values may be the same, (in the sense of
The issue concerns whether floats might be the same as decimals, or string sthe same as anyURIs, etc.
@@@ todo two or three test cases
The simplest solution is to agree that all primitive XML Schema Datatypes have disjoint value spaces.
XPath 2.0 provides an
eq operation that
will make true comparisons between some values with different primitive datatypes.
e.g. 0 as a float and 0 as a decimal compare true under
Most pairs of primitive types are incomparable under
and given the strong typing in XPath 2.0 such comparisons are errors.
Strings do not compare
eq to anything other than strings.
@@@ todo datetime stuff
hexEncodeBinary and base64EncodedBinary do not compare under
Numeric comparisons are provided with detailed implementation instructions to allow for rounding errors etc. This results in non-transitive behaviour. @@@todo example.
As a special case INF^^xsd:float is not eq to itself.
In XML Schema Datatypes in RDF Carroll argues that the numeric types should be compared with exact numeric semantics and that string and anyURI have overlapping value space, and that hexBinary and base64Binary have the same value space.
The XPath 2.0 position is a compromise between the two theoretically sound positions and may well be the best solution on an 80/20 rule. However, it is problematic that eq is not an equivalence relation because of the corner cases. Implementor feedback is needed as to how significant this problem is for DL reasoners.