W3C home > Mailing lists > Public > www-tag@w3.org > July 2003

Re: [metaDataInURI-31]: Initial draft finding for public review/ comme nt.

From: Jonathan Borden <jonathan@openhealth.org>
Date: Mon, 14 Jul 2003 13:44:29 -0400
Message-ID: <004501c34a2f$96d8ba10$b6f5d3ce@svhs.local>
To: "Williams, Stuart" <skw@hp.com>, "Roy T. Fielding" <fielding@apache.org>
Cc: <www-tag@w3.org>

Roy T. Fielding wrote:

> The resource referred to by the URI does not vary.  What varies is the
> target that is ultimately referred to by the "sentence" surrounding the
> URI referral.

While I generally agree with your stance on this issue (as far as I can
tell), I am slightly concerned with this introduction of the terms "target"
of a "sentence". If not a "resource", what is such a "target"? In RDF terms,
such targets of sentences are RDF resources, sometimes so called "anonymous
resources" or "b-nodes" (blank nodes) in RDF terms. If this is what you mean
then we have a readily available formalism to work out these issues.

>... For example, if I say
>     I want one of these cars: <http://www.vw.com/touareg/>.
> Then I have used that URI to identify a category of vehicles by
> reference to an HTTP resource identified by an http URI.  The URI
> is acting as an identifier for that VW brand of car, and it seems
> unlikely that www.vw.com will reuse that identifier for something
> else, even though it is clear that <http://www.vw.com/touareg/>
> on its own is a website for the vehicle brand and not the brand itself.

In trying to work through this issue bear with me, if you will, as I try to
translate your descriptions of these "sentences" into OWL/RDF
http://www.w3.org/TR/owl-ref statements. OWL does provide a logic for making
inferences about resources (owl:Thing's) based upon such "sentences".

In N3:

:I :WantCar <http://www.vw.com/touareg/> .

Now let's assume that the property :WantCar is constrained to target things
that are cars (sounds resonable :-) We can do this with an RDFS domain/range

:WantCar rdf:type rdf:Property .
:WantCar rdfs:domain :Person .
:WantCar rdfs:range :Car .

This means that when the :WantCar property is used in a statement
(sentence), the subject is constrained to be a :Person and the object is
constrained to be a :Car

given this we can conclude that:

:I rdf:type :Person .
<http://www.vw.com/touareg/> rdf:type :Car .

> In other words, context matters even when the URI itself is
> context-independent, and use within a given context is what defines
> the meaning of a reference.  That is why there is no conflict at all
> between the references <a href="http://example.com"> and
> <foo xmlns="http://example.com">; the context surrounding
> the reference defines meaning by its use, not by the URI scheme.

Whether there is or is not a conflict depends. For example suppose (edited
for better RDF clarity) and assuming that both "href" and "xmlns" can be
considered properties (xmlns is not normally so considered, but allow this

<a id="A1" href="http://example.com">
<foo id="FOO1" xmlns="http://example.com">

:A1 :href <http://example.com> .
:FOO1 :xmlns <http://example.com> .


:A1 rdf:type :a .
:FOO1 rdf:type :foo .
:href rdfs:range :WebSite .
:xmlns rdfs:range :XMLNamespace .

now I'd be able to conclude that

http://example.com rdf:type :WebSite .
http://example.com rdf:type :XMLNamespace .

which _might be_ a contradiction if I also say that web sites and namespaces
are _disjoint_ i.e.

:WebSite owl:disjointWith :XMLNamespace .

Bottom line: although this N3 might be painful for those of us who are not
RDF/OWL inclined, I believe it is important to try and define these things
in a precise fashion, otherwise there will be a danger of introducing terms
such as "sentence", "context" and "target" to the already muddled URI
discussions. I do think that the RDF/OWL treatment of such issues is
appropriate, and hope this will be considered in deciding these issues.

Received on Monday, 14 July 2003 13:44:42 UTC

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