- From: Guus Schreiber <guus.schreiber@vu.nl>
- Date: Wed, 2 Oct 2013 13:23:45 +0200
- To: David Booth <david@dbooth.org>
- CC: public-rdf-comments <public-rdf-comments@w3.org>
Dear David, Thanks for your comment. We have raised an issue for tracking your comment [1]. We will get back to you on this. Best, Guus, on behalf of the RDF WG [1] https://www.w3.org/2011/rdf-wg/track/issues/148 On 02-10-13 07:05, David Booth wrote: > In https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html > I see this statement: > > "IRIs have global scope: Two different appearances of an IRI > denote the same resource." > > This is wrong. If it were true then there could never be a URI Collision > http://www.w3.org/TR/webarch/#URI-collision > and there would be no point in the AWWW discussing it or admonishing > against it. > > An IRI can and often does denote different resources in different > *interpretations*. And this, in practice, means that an IRI often > denotes different resources in different *graphs*, because any graph has > a set of satisfying interpretations, and different graphs may have > different sets of satisfying interpretations. For example, suppose > graphs g1 and g2 have sets of satisfying interpretations s1 and s2, > respectively, and those sets may be disjoint. Then colloquially (and > technically) we can say that an IRI may map to one resource in g1 (i.e., > in some interpretation in s1) and a different resource in g2 (i.e., in > some interpretation in s2). > > This requires thinking about graphs in terms of sets of satisfying > interpretations -- an important and valid perspective -- rather than > assuming that one looks at them only through the lens of a single > interpretation. > > As a simple example of how a URI can denote different things in > different graphs, suppose Alice sends this graph G1 from her smart phone > to her home computer to turn *on* her porch light (assuming the usual > URI prefix definitions): > > G1: { @prefix db: <http://dbooth.org/> > ex:alicePorchLight rdf:value db:x . > db:x owl:sameAs ex:on . > ex:on owl:differentFrom ex:off . } > > and her light turns on. > > In contrast, Bob sends this graph G2 from his smart phone to his home > computer to turn *off* his oven: > > G2: { ex:bobOven rdf:value db:x . > db:x owl:sameAs ex:off . > ex:on owl:differentFrom ex:off . } > > and his oven turns off. > > It is perfectly reasonable and natural to ask "What resource does db:x > denote in G1?", and it is reasonable and natural to ask the same of G2. > The RDF Semantics (along with OWL) tells us that in G1 db:x denotes > whatever ex:on denotes, whereas in G2 db:x denotes whatever ex:off > denotes. That is useful! Furthermore, the semantics tells us that if > we merge those graphs then we have a contradiction -- there are no > satisfying interpretations for the merge -- and that is useful to know > also, because it means that Alice and Bob's graphs **cannot be used > together**. > > Furthermore, the RDF Semantics notion of an interpretation maps well to > real life applications: in effect, an application chooses a particular > interpretation when it processes RDF data. This is a very useful aspect > of the model theoretic style of the semantics. In this example, Alice's > home control app interpreted db:x to denote "on" and Bob's home control > app interpreted it to denote "off". And *both* were correct (in > isolation): they both did The Right Thing. > > In short, I think the above statement needs to be qualified somehow, > such as: > > "IRIs are *intended* to have global scope: Two different > appearances of an IRI are *intended* to denote the same resource." > (However, the RDF Semantics explains how an IRI may denote > different resources in different interpretations.) > > David >
Received on Wednesday, 2 October 2013 11:24:16 UTC