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Re: RDF Extensibility

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 6 Jul 2010 16:05:03 -0500
Cc: Toby Inkster <tai@g5n.co.uk>, Michael Schneider <schneid@fzi.de>, Linked Data community <public-lod@w3.org>, Semantic Web <semantic-web@w3.org>
Message-Id: <EB0E593A-83B9-4D87-956D-6DDDA7FB496F@ihmc.us>
To: Jiří Procházka <ojirio@gmail.com>

On Jul 6, 2010, at 9:34 AM, Jiří Procházka wrote:

> On 07/06/2010 03:35 PM, Toby Inkster wrote:
>> On Tue, 6 Jul 2010 14:03:19 +0200
>> "Michael Schneider" <schneid@fzi.de> wrote:
>>
>>> So, if
>>>
>>>    :s "lit" :o .
>>>
>>> must not have a semantic meaning, what about
>>>
>>>    "lit" rdf:type rdf:Property .
>>>
>>> ? As, according to what you say above, you are willing to allow for
>>> literals in subject position, this triple is fine for you
>>> syntactically. But what about its meaning? Would this also be
>>> officially defined to have no meaning?
>>
>> It would have a meaning. It would just be a false statement. The
>> same as the following is a false statement:
>>
>> 	foaf:Person a rdf:Property .
>
> Why do you think so?
> I believe it is valid RDF and even valid under RDFS semantic  
> extension.
> Maybe OWL says something about disjointness of RDF properties and  
> classes
> URI can be many things.
>
> I think there are issues about RDF extensibility which haven't been
> solved and they concern:
> a) semantics
> b) serializations
>
> In case of a) I don't have cleared up my thoughts yet, but generally I
> would like to know:
> How are semantic extensions to work together in automated system?

Well, the semantics always defines some notion of entailment, and your  
system is supposed to respect that notion: not draw invalid  
conclusions, draw as many valid conclusions as you feel are useful,  
don't say things are inconsistent when they aren't, etc.. Otherwise,  
you have free rein. So, if you have several semantic extensions, they  
are each provide a set of such entailments and they should add up to  
one single set of legal entailments.

> How to let agent know that the data is described using new RDF
> extension, which the client doesn't know and the data could be (or
> definitely are) false if it is interpreted using vanilla RDF  
> semantics?

NOt false, if its a semantic extension (they can't contradict the RDF  
semantics., only extend it.) BUt same point more generally: how do we  
know, given some RDF, what semantic extensions are appropriately to be  
used when interpreting it? That is a VERY good question. This is  
something that RDF2 could most usefully tackle, if only in a first- 
step (ham-fisted?) kind of a way. We were aware that this was an issue  
in the first WG, but it was just too far outside out charter, and our  
energy level, to tackle properly. One obvious (?) thing to say is that  
using a construction from a namespace which is associated with the  
definition of any RDF semantic extension is deemed to bring along the  
necessary interpretation conditions from the extension, so that for  
example if I use owl:sameAs in some RDF, then I mean it to be  
understood using the OWL semantic conditions. We all do this without  
remarking upon it, but loosely, and to make this precise and normative  
would be a very interesting (and useful) exercise. (An issue already  
here is, which version of the OWL semantics is intended? Does the use  
in RDF also "import" the OWL-DL syntactic restrictions on its use, for  
example?)

Pat


>
> b) How should my system know that the data which is just being  
> processed
> is new revision of RDF/XML and not malformed RDF/XML when forward
> compatibility was out of sight, out of mind when RDF/XML was designed?
>
> Best,
> Jiri Prochazka
>

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Received on Tuesday, 6 July 2010 21:06:16 UTC

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