Re: To embed or combine

>>>> I'm afraid I don't really see the difference between "accessing RDF
>>>> data" and "entailment regimes", so I don't really understand why they
>>>> should be treated differently.
>>> I'm not suggesting they be treated differently.
>>>
>>> To write rules that work with the data, or to implement a translator for
>>> those rules, the data mapping does have to normatively defined. To me
>>> this means that tr and the treatment of bNodes in data (not necessarily
>>> queries), literals etc have to all be normative. In the current document
>>> almost all of that is covered - literals are in the normative part, tr
>>> is implicit in the model theory but the handling of bNodes is not
>>> spelled out. Spelling tr out and defining the bNode handling in the
>>> normative part would resolve this, and would further increase the
>>> clarity of the document, at the trivial cost of moving a very small
>>> number of lines of text around.
>>
>> I'm not sure what you mean with "spelling out tr". If you mean that we
>> need to show how using RDF data with RIF rules can be done, then it is
>> indeed something we still need to do, in the "Guide to using RIF with
>> the semantic web" document you mentioned.
> 
> We want that too but simply defining tr in the normative part is also
> simple and useful.

I'm not sure what it would buy us to define two semantics for the same
thing.

> 
>> The handling of bNodes is indeed not spelled out, because it is implicit
>> in the semantics.  They are symbols local to an RDF graph, so a
>> combination does not need to, and should not, touch these symbols.  We
>> do, however, need more explanatory text about this point.
> 
> No, we need to define the mapping so that builtins like isBlankLiteral
> are implementable. It makes a difference whether they are represented by
> undistinguisable URIs (your proposal) or by distinguisable URIs or by a
> datatype (my proposal).

In my proposed combined model-theoretic semantics, blank nodes are not
represented by anything in RIF, because they are local to RDF graphs,
and can thus not be accessed outside of the graph it is contained in.
In the proposed embedding which can be used to reduce reasoning with
combinations to reasoning with RIF, blank nodes are Skolemized.
So, the indistinguishable URI is not introduced for representation
purposes, but only for reasoning purposes.

If blank nodes were to be represented by distinguished URIs, then they
would no longer be blank nodes in the sense defined by the RDF semantics
specification.

Thinking about it a bit more, I actually do not think a built-in like
isBlanknode could be defined in a meaningful way while observing the
semantics of blank nodes.

Some implementations may not observe the RDF semantics, and provide an
implementation for the isBlanknode built-in based on an incorrect
treatment of blank nodes, but I think we should observe the semantics
defined in the W3C standard.

> 
>> So, with respect to the handling of blank nodes I think we are fine in
>> the normative part; we simply need more explanation in the informative
>> part.
> 
> 
>>>> Wouldn't one simply use a combination with the simple entailment regime
>>>> in this case?
>>> Yes, I pointed this out in the "at first I was concerned" paragraph
>>> (since subset semantics includes the trivial subset).
>>
>> I did not understand what is meant with "subset semantics".  Also, could
>> you send a pointer for rho-df?
> 
> It's the ESWC-07 best paper that you yourself referenced. They called
> their reduced RDFS {\rho}df did they not?

Ah, I did not recognize it without the latex markup :-)
I'm still wondering what you mean with "subset semantics".


Best, Jos

> 
> Dave

-- 
Jos de Bruijn            debruijn@inf.unibz.it
+390471016224         http://www.debruijn.net/
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