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

From: Bijan Parsia <bparsia@isr.umd.edu>
Date: Mon, 12 Apr 2004 16:13:44 -0400
Message-Id: <E61253A3-8CBD-11D8-804E-0003939E0B44@isr.umd.edu>
Cc: public-sw-meaning@w3.org
To: Pat Hayes <phayes@ihmc.us>

On Apr 12, 2004, at 12:30 PM, Pat Hayes wrote:

>> On Apr 11, 2004, at 11:04 PM, John Black wrote:
>>  [snip]
>>  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.
>
>  Indeed.

Upon reflection, using whitespace for quoting in emaijl isn't the best 
idea ;)

>>  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.

Oy, yes.

But interpretations of distinct graphs need not assign the same domain 
entity to the same URI. So, unless you merge the graphs, the URIs can 
have different meanings. Right?

>  Second, if they did behave, then they ought to behave the same 
> independently of any merging, since merging graphs does not alter 
> URIref meanings.

Really? Huh.

There seems *some* sense in which it does, as mergine graphs can alter 
the set of axioms in effect (talking about OWL, now).

> 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.

I don't believe that.

> 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.

Sure there is: Behavior under merging.

>> 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.

What, no values?

> 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 .

And others!

> 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.

Ah, right. For some reason I got stuck in my head that literals are 
self denoting.

Cheers,
Bijan Parsia.
Received on Monday, 12 April 2004 16:14:17 GMT

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