W3C home > Mailing lists > Public > www-rdf-logic@w3.org > September 2001

Re: model theory for RDF/S

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Fri, 28 Sep 2001 10:57:10 -0500
Message-Id: <p05101027b7da48c3442f@[]>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: www-rdf-logic@w3.org


>  > But why bother with all this? The concept of a labelled graph is
>>  standard and uncontroversial, so this is one place where we can say
>>  some mathematics in a reasonably intuitive way without sacrificing
>>  either precision or readability by non-mathematicians.
>I thought that a labeled graph only allows at most one edge between a pair
>of nodes, so labeled graphs would not solve this problem.  Certainly I've
>seen lots of treatments where labeled graphs are defined in this fashion.

Hmm, after a quick web check I see that you are right. OK, this point 
requires another amplification sentence. Thanks for catching this 
possible misunderstanding.


>  >
>>  If we are dealing with untidy graphs, the graph syntax has no
>>  structural advantages over a lexical syntax, and it would then be
>>  preferable to simply attach the model theory directly to the
>>  N-triples notation (which in an earlier version of the model theory
>>  is what we in fact did, but that had problems of its own.).
>There may be advantages to untidy graphs when looking at complex strategies
>for literals.  In particular, in the model theory for DAML+OIL, merging
>nodes whose labels are literals may change the meaning of the graph.  (Of
>course the model theory for DAML+OIL doesn't use graphs, but if it did,
>such merging would not necessarily be meaning-preserving.)

Ah, I had not thought of that in the DAML+OIL context.  Can you give 
a simple example?

>>  However, Peter, a question for a professional Description Logician:
>>  this would allow literals to be assigned non-literal property values
>>  by an RDF assertion. Wouldn't that break DAML+OIL?
>DAML+OIL depends somewhat on the separation between resources and
>literals.  Some Description Logics may break severely if their separation
>between abstract (resources) and concrete (literals) domains is breached.

Right, that is what worries me. I recall this being a sticking point 
in the DAML discussions for some people, so I presume it is fairly 
critical there also, no?

>>  >Taking care of rdf:type:
>>  >
>>  >A core RDF interpretation, i.e., RDF without reification or containers, is
>>  >an interpretation over a vocabularly that includes rdf:type with the
>>  >following extra conditions
>>  >
>>  >      1. IS(rdf:type) is in IP
>>  >      2. IEXT(IS(rdf:type)) <= IR x IR
>>  Is there any real need for condition 2 here in RDF?
>I don't know.  The condition is directly stated in M&S.  It says that
>literals cannot have instances, which is probably a good thing.  I'm not sure
>what the instance of "2" could be.
>>  I hope it can be
>>  avoided, since it would mean that a triple
>>  aaa rdf:type LLL .
>>  where LLL is a literal, comes close to being a contradiction. Right
>>  now, all it implies is that IR and LV overlap, but if anyone were to
>>  ever claim that they didn't overlap, then it would be. I don't like
>>  having land-mines concealed in the model theory.
>I'm not sure how you get this implication,

from your condition 2 on rdf:type, which would be
rdf:type rdfs:range rdfs:Resource .
in RDFS.

>  nor am I sure why literals need
>to have instances.

I tend to agree, but there would be nothing in the syntax to prevent 
someone writing a triple that says they do, so we would have to give 
it a meaning.


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Received on Friday, 28 September 2001 11:57:17 UTC

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