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Re: resource equivalence was: Re: fragment identifiers

From: Jonathan Borden <jonathan@openhealth.org>
Date: Wed, 24 Jul 2002 11:16:34 -0400
Message-ID: <058201c23325$1a61aa90$0a2e249b@nemc.org>
To: "Michael Mealling" <michael@neonym.net>
Cc: <www-tag@w3.org>

Michael Mealing wrote:

On Wed, Jul 24, 2002 at 09:40:23AM -0400, Jonathan Borden wrote:
> BTW: I've suggested equating resources by the equality of their
> representation sets as axioms [10,11] in
> http://www.openhealth.org/RDDL/SchemaAlgebra
> [[
> [10] equivalent(URIa,URIb) := Entities(URIa) = Entities(URIb) and
> cardinality(Entities(URIa)) > 0
> Two URIs are equivalent when they map to the same set of entities.
> [11] equivalent(A,B) <=> exists URIa such that A = resource(URIa) and
> URIb such that B = resource(URIb) and equivalent(URIa,URIb)
> Two resources a and b are equivalent if the set of entities given the URIa
> and URIb are equal where URIa identifies a and URIb identifies b.
> ]]
> where Entities(URI) is the set of entities which the URI maps to across
> media-types, time, and other e.g. HTTP request parameters.

Wrong. Two URIs are equivalent when they match each other according to
the canonicalization rules in RFC 2396.

The terms "equal" and "equivalent" are subtly different, and I did not use
the phrase "URI equivalence" randomly, nor did I intend it to be conflated
with "URI equality"

There is often more than one way to compare things, e.g.

The only uniform statement you can make about the equality of two or more
Resources is whether or not the URIs in question are syntactically
equal according to RFC 2396's normalization rules. Period.

Yes, well this definition of "URI _equality_" appears a priori true given a
1:1 relationship between URI and resource. I am intending to capture the
situation where one may desire a number of URIs to refer to the same
resource, and so while the resources identified by such a set of URIs are
not exactly _equal_ we can say they are _equivalent_ in some sense of this

If there were no need to consider such issues the DAML/OWL terms:
isEquivalentTo, sameClassAs, samePropertyAs wouldn't be necessary, but they
are actually quite useful when trying to have a logical description of
various things.

Received on Wednesday, 24 July 2002 11:22:36 UTC

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