W3C home > Mailing lists > Public > www-webont-wg@w3.org > January 2003

Re: OWL datatyping and example

From: Jeff Z. Pan <pan@cs.man.ac.uk>
Date: Fri, 24 Jan 2003 11:16:03 -0000
Message-ID: <006a01c2c39a$0d8adf20$6bc65882@cs.man.ac.uk>
To: <www-webont-wg@w3.org>
Cc: <mdean@bbn.com>

Mike,

>This messages cites the discussion of datatypes in the RDF Core WG
>last call candidate working drafts, and gives an example of their
>possible use in OWL.

The comments are mainly about using RDF datatypes in OWL.

[...]

>rdfs:Datatype should replace daml:Datatype in OWL.
>Since rdfs:Datatype is a subclass of rdfs:Class, its instances can be
>used in an rdfs:range statement to indicate the class associated with
> given property (as in DAML+OIL), however
>[[
>Because the schema describes the range of this property as an xsd:integer,
>the value of the property must be a typed literal of that datatype in order
>to match the range description (i.e., the range declaration does not
>"assign"
>a datatype to a plain literal)
>]] [3]
>rdfs:range specifies a global restriction.  Hopefully we haven't done
>anything in OWL to break the use of local datatype restrictions in
>DAML+OIL:
[...]

Up to now we haven't, and we have both global and local restriction on
datatypes. On the one hand, in some circumstances it is convenient to be
able to use rdfs:range to set a global restriction. On the other hand, a too
strict global restriction can leave too little room for local restrictions.
So one should be very careful when he uses global restrictions.

In fact, in DAML+OIL when we define a datatype property, we can also set its
range for it. E.g.

<daml:DatatypeProperty rdf:ID="age">
  <rdf:type
rdf:resource="http://www.daml.org/2001/03/daml+oil#UniqueProperty"/>
  <rdfs:range
rdf:resource="http://www.w3.org/2000/10/XMLSchema#nonNegativeInteger"/>
</daml:DatatypeProperty>


>My own preferred style is to use local Restrictions exclusively for
>both datatype and object properties rather than global rdfs:range
>constraints.

Me too.


>One of my colleagues, Michael Cook, has been marking up the Periodic
>Table in DAML+OIL.  Converting one of his instances to OWL with
>datatypes yields:
><table:Element rdf:ID="He">
>  <table:name rdf:datatype="&xsd;string">helium</table:name>
>  <table:symbol rdf:datatype="&xsd;string">He</table:symbol>
>  <table:atomicNumber rdf:datatype="&xsd;integer">2</table:atomicNumber>
[...]
></table:Element>

As far as we use typed literals, they are quite similar in DAML+OIL and OWL
(using RDF datatypes), e.g. in DAML+OIL the corresponding table:atomicNumber
property can be written as:

<table:atomicNumber><xsd:integer rdf:value="2"></table:atomicNumber>.

So the main difference is that in RDF datatypes, you can set the attribute
rdf:datatype as some datatype that is not an XMLS datatype, which may give
the users more choices than in DAML+OIL. Apart from that is the minor
difference of name space mentioned below - a more updated XMLS namespace is
used in RDF datatypes.


>assuming that xsd has been defined as an XML entity for
>http://www.w3.org/2001/XMLSchema# (note that this is different than
>the http://www.w3.org/2000/10/XMLSchema# namespace used in DAML+OIL).

>Note the use of the rdf:datatype attribute above to identify typed
>literals [5].  It has been suggested that simple literals (without
>rdf:datatype) could be used for the string values, but this precludes
>specifying rdfs:range or local restrictions on these properties
>(i.e. conveying the datatyping intentions of the ontology writer), so
>I don't find that good practice.

I agree - simple literals are redundant. It seems that the only use of them
are with rdfs:comment and rdfs:label etc. In RDFS interpretation,

rdfs:comment rdfs:range rdfs:Literal
rdfs:label rdfs:range rdfs:Literal.

It is very unusual to use typed literals as the range of rdfs:comment and
rdfs:label. And the reason that rdfs:comment and rdfs:label are included in
RDFS interpretation is because some constraints which apply to their use can
be stated using rdfs:range as above. [7] So the simplest solution might be
to abandon simple literals as datatypes, and keep rdfs:comment and
rdfs:label out of the interpretation (as that in DAML+OIL).


>Note that RDF provides for use of XML Schema datatypes, but also
>allows other type systems to be used:
>[[
>XML Schema datatypes have a "first among equals" status in RDF. They
>are treated no differently than any other datatype, but they are
>expected to be the most widely used, and therefore the most likely
>to be interoperable among different software. As a result, it is
>expected that many RDF processors will be programmed to recognize
>these datatypes. However, RDF software could be programmed to process
>other sets of datatypes as well.
>]] [5]
>DAML+OIL supported only XML Schema datatypes.

>Several additional notes:
>[[
>RDF provides no mechanism for defining new datatypes. XML Schema Datatypes
>[XML-SCHEMA2] provides an extensibility framework suitable for defining new
>datatypes for use in RDF.
>]] [1]

XML Schema Datatypes provides *an* extensibility framework suitable for
defining new datatypes for use in RDF. Other framework may also be used to
define new datatypes for use in RDF.

[...]
>Mike

Other strange things with RDF datatypes include:

1) ICEXT(I(rdfs:Datatype)) = D : the interpretation of rdfs:Datatype is
equal to D, which is a set of recognised datatypes. This means if two
ontologies use different sets of recognised datatypes, the interpretation of
rdfs:Datatype will be different. It will be much safer if we say D <=
ICEXT(I(rdfs:Datatype)), i.e. the interpretation of rdfs:Datatype is the set
of all possible datatypes (like that in DAML+OIL), any recognised datatypes
are sub-sets of it.

2) The built-in datatype rdf:XMLLiteral, exceptionally, allows *pairs* in
its lexical space. And the set of recognized datatypes always includes
rdf:XMLLiteral. [6]

Jeff


[1]
http://www.w3.org/2001/sw/RDFCore/TR/WD-rdf-concepts-20030117/#section-Datat
ypes

[2] http://www.w3.org/2001/sw/RDFCore/TR/WD-rdf-schema-20030117/#ch_datatype

[3] http://www.w3.org/2001/sw/RDFCore/TR/WD-rdf-primer-20030117/#properties

[4] http://www.w3.org/TR/daml+oil-reference#2

[5]
http://www.w3.org/2001/sw/RDFCore/TR/WD-rdf-primer-20030117/#typedliterals

[6] http://www.w3.org/2001/sw/RDFCore/TR/WD-rdf-mt-20030117/#dtype_interp

[7] http://www.w3.org/2001/sw/RDFCore/TR/WD-rdf-mt-20030117/#rdfs_interp
Received on Friday, 24 January 2003 06:14:46 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Monday, 7 December 2009 10:57:57 GMT