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Re: Inheritance

From: Jeen Broekstra <jeen@aduna.biz>
Date: Wed, 06 Apr 2005 13:29:10 +0200
Message-ID: <4253C806.5080800@aduna.biz>
To: Hans Teijgeler <hans.teijgeler@quicknet.nl>
Cc: semantic-web@w3.org

Hans Teijgeler wrote:

> Thanks for your response! I was rather na´ve, huh?

Hardly. RDF is not trivial to grok if you come from a object-oriented 
background. It requires a bit of a shift in your thinking, that's all.

> When I understand this well, it is impossible to write something like a
> catalog with RDF/RDFS, such as for a Volkswagen Passat where one has the
> choice of engines, shift gear vs automatic, colors, etc, etc.

I depends a bit on what you expect RDF/RDFS to do for you. It's 
perfectly possible to create an RDF/RDFS ontology that describes such 
a catalogue. You can _describe_ your schema, and the different 
individuals perfectly. But checking that an instance is 'valid' 
against the schema (has the required number of seats, doors, engines, 
etc.) is something that a default RDF processor can not do for you: 
you will need to create a domain-specific application that does this 
for you (for example, a customized user interface through which 
individuals are added).

Also, because the lack of expressivity, you will have to 'spell out' 
quite a few things: you can not express in general that all cars have 
four wheels. Therefore, if this information is important to you, you 
will have to add this knowledge explicitly for each car. It's this bit 
where OWL can help.

> Would one need
> OWL for that, and can OWL do that? When I read the OWL documentation I doubt
> that.In the process industry we must model esoteric things like pump curves
> and multidimensional properties (our definition of property is totally
> different). But again, I am a newcomer in this world of the Semantic Web, so
> I may very well underestimate OWL.

OWL can do such things, yes. You can create a class in OWL and define 
that certain properties must be present on all instances of that 
class, and that these properties must have particular (types of) values.

> With respect to the range and domain matter I found the impossibility to
> define this locally very impractical to start with, with the results like
> you described below ("but will infer that your dinnerTables are
> PassengerVehicles").
> I have been working with XML Schema for the last four years. The nice thing
> about it is that you can create a schema as a template, and then generate an
> XML file from it (it needs population of the undefined attributes). So I had
> expected that RDF Schema could do the same, and produce skeleton RDF/XML
> files. Nope!

The problem here is that RDF Schema is a misnomer. The role RDF Schema 
has in relation to RDF is not the same as the role XML Schema has in 
relation to XML.

An XML document is valid wrt. the Schema if it follows the prescribed 
structure. XML Schema restricts/defines the structure of XML documents.

RDF Schema defines (but does not restrict!) the vocabulary of RDF 
documents. Different thing altogether: an RDF document uses the 
defined vocabulary but can not be said to be valid or non-valid with 
respect to an RDF Schema.

> As to the browser subject: I read, much to my surprise, in Appendix A of the
> Primer: "...RDF uses URIrefs only to identify things, while browsers also
> use URIrefs to retrieve things." I was surprised, because why on earth would
> you use a "Resource Description" Framework without being able to find that
> description? So, a browser should be able to find the resource at its URI,
> and then give me the information about that resource that I require. Again,
> too na´ve, I guess. 

Your scenario is quite well possible actually. But RDF does not 
require a URI to be 'navigable' for the simple reason that it is more 
generally applicable that way. All RDF uses URIs for is *identifying* 
things. This makes it very flexible: after all, you may want to 
describe an abstract concept rather than a particular web-page. The 
class 'myClass:Volkswagen' is referred to by a URI, but is there 
somewhere a webpage that embodies the general concept of a Volkswagen?

It's very well possible to have an RDF tool that tries to locate web 
resources based on their RDF description, but it is not guaranteed 
that this is possible in general, it depends on how it was modeled.

Jeen Broekstra          Aduna BV
Knowledge Engineer      Julianaplein 14b, 3817 CS Amersfoort
http://aduna.biz        The Netherlands
tel. +31(0)33 46599877  fax. +31(0)33 46599877
Received on Wednesday, 6 April 2005 11:27:22 UTC

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