W3C home > Mailing lists > Public > uri@w3.org > September 2000

Re: URIs-Resource relationships

From: Graham Klyne <gk-lists@dial.pipex.com>
Date: Mon, 11 Sep 2000 18:12:47 +0100
Message-Id: <4.3.2.7.2.20000911175249.00bbdd90@pop.dial.pipex.com>
To: liberte@crystaliz.com
Cc: uri@w3.org
Returning to an earlier message...

At 03:01 PM 9/7/00 -0400, liberte@crystaliz.com wrote:
>There are a bunch of interesting philosphical issues that tend to come up,
>but just as philosophers try to make mathematically clear statements out of
>otherwise fuzzy concepts, I believe we can do a similar job here.
>Avoiding the issues up until now was fine partly because there has been
>a lot of basic infrastructure to get out there before anything more
>complex mattered.  It is starting to matter more and more.

Yes.

But which should come first?  Infrastructure or a sound underlying 
philosophy (and accompanying architecture)?

>There is a sense in which URIs can be considered equivalent regardless
>of the resources to which they might refer.  This is looking at the
>characters of the URIs themselves and the low-level semantics of
>the rules that constrain the namespace.  E.g. treating a/b/../c as
>equivalent to a/c.
>
>We can talk about the equivalence of resources independent of the
>URIs that refer to them, but the tricky bit is that you can't even
>talk *about* the resources without referring to them in some way.
>Nevertheless, we need to talk about resources that may have no
>URIs (we would do so indirectly, still by reference, but not by URIs).

I think these ideas can be extended to several layers of equivalence; e.g.
    string-equivalence
    scheme-equivalence
    resource-equivalence
    entity-equivalence

And, in general, one of these equivalences does not imply any other 
equivalence.  (There are cherished exceptions, such as 
string-equivalence=>resource-equivalence, but I'm not sure if they are 
important.)

>Saying that two URIs refer to equivalent resources also says that
>any other URIs that we previously knew to be equivalent to either
>of those URIs (really the resources to which they refer) are now
>all equivalent to each other.  So it is saying something about the
>URIs, it seems, and not just the resources.

Does this help?  It seems to me to be just stating transitivity of equivalence.

> > However, it still seems to be official that the relation between
> > URIs and resources is an isomorphism.
>
>Some people might believe in that isomorphism, and one could probably
>build a self-consistent system around it, but I don't think it is
>useful to do so because it denies the reality that we (people and
>applications) use multiple identifiers to refer to the same thing.
>I've got a page of elaborations if anyone is interested.

Interesting point.  I don't believe the isomorphism denies the reality you 
describe, just doesn't fully reflect it.  (By which I mean that I think a 
self-consistent model can be build that accommodates the reality by 
introducing some new steps.)

> > > * URIs that are mapped to no internet representation but still
> > >   somehow identify an abstract resource.
> >
> > No special situation:  urn:isbn:1565921496 refers to the 2nd edition
> > of the Camel Book, which has no Internet representation.
>
>Yes, this happens, but how does an application know about it?

I'd say it doesn't (in general).  If there are different URIs and one has 
no Internet representation, there's no general way for an application to 
determine if they're equivalent or not at any higher level.

>So you say, but let me see if I can mix it up a bit.
>
>One might define relationships between URIs such that
>you can tell just by looking at the URI (and a declaration about
>the relationships that should hold for the URI) that it's resource
>is contained in the collection of another resource identified by a
>URI computed from the first.    We already do this to a limited extent,
>by the way, with hierarchical http URIs.  And it's not a bad thing,
>in my opinion, it's a good thing.

Hmmm... I think this could be a dangerous territory.  (c.f. TimBL's 
comments about opacity of URIs, except when dereferencing to retrieve a 
resource entity:  http://www.w3.org/DesignIssues/Axioms.)


>Actually, the point of my posting was that
>we need to talk about not only relationships between URIs
>and resources, but also relationships and properties of URIs
>and relationships and properties of resources.  We don't have a
>good language for talking about any of that yet, and it is basic
>for building anything more complex in a way that does not resort
>to application-specific hackery.  Various XML specs are starting to
>hack away at the larger problem, so to speak, and RDF has some promise
>of being able to address it, but we are only at the pre-beginning,
>recognizing the problem.

Yes!  But my intuition is that RDF is not, of itself, a solution here, but 
another application seeking a solution.  Once we fully know and understand 
these relationships then RDF might be used to express and model them.

#g

------------------------------------------------------------
Graham Klyne                       Content Technologies Ltd.
Strategic Research              <http://www.mimesweeper.com>
<Graham.Klyne@mimesweeper.com>
------------------------------------------------------------
Received on Monday, 11 September 2000 14:50:40 UTC

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