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

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
restriction.

: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
example).

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

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

and

: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.

Jonathan

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