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Re: How does RDF/OWL formalism relate to meanings?

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 12 Apr 2004 11:30:10 -0500
Message-Id: <p06001f4ebca06edfc1c5@[]>
To: Bijan Parsia <bparsia@isr.umd.edu>
Cc: "John Black" <JohnBlack@deltek.com>, public-sw-meaning@w3.org
>On Apr 11, 2004, at 11:04 PM, John Black wrote:
>>In the RDF Semantics Recomendation it states:
>>"1.2 URI references, Resources and Literals.
>>This document does not take any position on the way that URI references
>>may be composed from other expressions, e.g. from relative URIs or
>>QNames; the semantics simply assumes that such lexical issues have been
>>resolved in some way that is globally coherent, so that a single URI
>>reference can be taken to have the same meaning wherever it occurs."
>>- http://www.w3.org/TR/rdf-mt/#urisandlit
>>What is the effect of the language, "...so that a single URI
>>reference can be taken to have the same meaning wherever it occurs."?
>>How important is this assumption to RDF semantics?
>Upon reflection, that isn't the best wording.


>Roughly: In the *graph* there are only absolute URIs. There also are 
>no contexts, so every node labeled with the same uri is equivalent.
>*Between* graphs, however, URIs can behave quite differently (until 
>you merge them).

Oh, no, wait a minute. That's very misleading. First, URI's don't 
behave. Second, if they did behave, then they ought to behave the 
same independently of any merging, since merging graphs does not 
alter URIref meanings. It is important that a single URI is the same 
no matter what graph it occurs in. URIreferences scope across the 
entire Web, so a given URIref in your graph and in my graph really 
are the same URIref. If they weren't, there really would be no 
difference at all between URIrefs and blank nodes, and RDF graphs 
would be essentially all-blank graphs.

>I'd say it's pretty important :)
>Note that URIs in literals (e.g., in literals of datatype 
>xsd:anyURI) are exempt from this merging. So the above text isn't 
>quite right if you try to read it in full generality.

No, it is right and they aren't exempt.  A typed literal in RDF 
consists of a URIref denoting the datatype  and a STRING which is the 
lexical form of the datatype. So in your example, the only actual 
URIref in the literal is http://www.w3.org/2001/XMLSchema#anyURI 
which is a genuine first-class URIref and has the same meaning there 
as it does everywhere else, and indeed might also occur in 
non-literal triples, eg

<http://www.w3.org/2001/XMLSchema#anyURI>  rdf:type rdfs:Datatype .

The other part of the literal is a character string which indicates 
the lexical form of (in this case) a URI, but it is not itself an 
actual URI. The URI you are talking about is what this typed literal 
DENOTES, ie the datatype value of the lexical form under the 
lexical-to-value mapping (in this case, from strings to URIs) ; but 
it doesn't, strictly speaking, occur anywhere in the RDF syntax.


>Bijan Parsia.

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Received on Monday, 12 April 2004 12:30:13 UTC

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