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Re: why I don't like named graph IRIs in the DATASET proposal

From: Pat Hayes <phayes@ihmc.us>
Date: Fri, 7 Oct 2011 22:30:11 -0500
Cc: <richard@cyganiak.de>, <pierre-antoine.champin@liris.cnrs.fr>, <public-rdf-wg@w3.org>
Message-Id: <B9424B09-B81E-4F09-9372-6D86F28F415D@ihmc.us>
To: Peter Frederick Patel-Schneider <pfps@research.bell-labs.com>

On Oct 6, 2011, at 11:21 PM, Peter Frederick Patel-Schneider wrote:

> From: Pat Hayes <phayes@ihmc.us>
> Subject: Re: why I don't like named graph IRIs in the DATASET proposal
> Date: Thu, 6 Oct 2011 11:06:07 -0500
> 
>> 
>> On Oct 6, 2011, at 5:54 AM, Peter Frederick Patel-Schneider wrote:
>> 
>>> From: Pat Hayes <phayes@ihmc.us>
>>> Subject: Re: why I don't like named graph IRIs in the DATASET proposal
>>> Date: Wed, 5 Oct 2011 18:05:04 -0500
>>> 
>>>> 
>>>> On Oct 4, 2011, at 10:08 AM, Richard Cyganiak wrote:
>>>> 
>>>>> On 2 Oct 2011, at 18:06, Pierre-Antoine Champin wrote:
>>>>>> As you stress it, RDF does not dictate which IRI should denote which
>>>>>> resource (including graphs). I don't think I ever suggested to change that.
>>>>>> 
>>>>>> However, RDF dictates that each time I use the same IRI (as a node), it
>>>>>> denotes the same resource.
>>>>> 
>>>>> No, it doesn't.
>>>> 
>>>> Yes, it does. 
>>> 
>>> Are you sure, Pat, especially formally?
>>> 
>>> peter
>> 
>> Well, formally, it requires that any RDF graph has an
>> interpretation. Now, suppose a URI U occurs in a graph G where it is
>> (for reasons that need not detain us here) required to denote some
>> thing, and also occurs in some other graph H where it is similarly
>> required to denote some other thing, different from the first
>> thing. Now consider the graph (G union H). If both requirements are in
>> force, this graph cannot be given an RDF interpretation. Which
>> violates the RDF semantics spec. So by modus tollens, those two
>> requirements, taken together, violate the spec.  
> 
> Sure, in a particular interpretation of a graph there is a single
> denotation for each IRI (in the graph).   However, I don't see anything
> above carrying either of these conditions.

If we say that an IRI is constrained to denote some particular thing (by some constraining conditions external to the model theory itself, but widely agreed, eg by treating http-range-14 as a normative rule, or deciding that http://dbpedia.org/resource/Ayn_Rand  denotes Ayn Rand) what we are in effect saying, in model-theoretic terms, is that we are only allowing interpretations which map this IRI to this thing. Any other interpretation is being ruled out as illegal. The phrase "all interpretations" in the semantics then has to be understood as meaning, **all interpretations which satisfy the globally agreed constraints.** In my example, I was assuming two such constaints which took a single IRI to two different things. There are no interpretations which satisfy both of those constraints. 

> 
> Different interpretations can have different denotations for the same
> IRI in a particular graph.

Of course, but if we have agreed that an IRI denotes some particular thing - which is exactly what a naming convention does -  then that agreement applies to all interpretations. 

> 
> I suppose that one could consider a single interpretation as it relates
> to two different graphs, where, of course, there would be a single
> denotation of each IRI, but the RDF Semantics doesn't consider this.  

Of course it does. It applies to ANY graph, and the union of two graphs is itself a graph. A single interpretation of two graphs is simply an interpretation of their union. 

> In
> any case the set of interpretations for one graph may not be the same as
> the set of interpretations of another.

Well, at present an interpretation is defined over a vocabulary, not a graph. So any interpretation of a vocabulary which includes that used in the graphs is a single interpretation of them both.

Pat


> 
> [...]
> 
>> Pat
> 
> peter
> 
> 

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Received on Saturday, 8 October 2011 03:31:20 GMT

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