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Re: RIF and QL

From: Enrico Franconi <franconi@inf.unibz.it>
Date: Thu, 26 Jan 2006 00:02:41 +0100
Message-Id: <13E4C60D-F3A7-4EDD-82F1-F31261EE1AAD@inf.unibz.it>
To: W3C RIF WG <public-rif-wg@w3.org>

On 25 Jan 2006, at 21:11, Ed Barkmeyer wrote:
> I need to understand what François has in mind by "expression in a  
> query language".  I can understand how something like:
>  (SPARQL 'RDFbase '<query text> <rule-expression> ... <rule- 
> expression>)
> could work as an "external query", where:
> - SPARQL designates a particular query service (or a class of query  
> service for which the rule engine is to find a server in  
> conjunction with the "RDFbase" parameter)
> - RDFbase designates the KB on which the service is to operate for  
> this query
> - <query text> is a query stated in the language of the designated  
> service that may contain references to "external parameters" using  
> the "external parameter syntax" *for that query language*
> - <rule-expression> is an operand expression in the rule language  
> (RIF) syntax.  The expression is evaluated by the rule engine  
> before invocation of the external query service, and the result of  
> the evaluation is passed in the position in which the expression  
> occurs, i.e. an "actual parameter".

I am sure (or hope) that what are you talking about is only *one*  
option out of the *many* that RIF has to characterise and define in  
order to interoperate with ontology languages (such as RDF, in this  
example): let's call this option the 'trivial'semantics.

Why am I saying 'ontology languages'?

What you call an "expression in a query language" is in the very  
general case an open formula in some ontology/knowledge- 
representation language, whose bindings make the formula 'somehow'  
formally connected with some knowledge base (the RDFbase in your  
example) -- in your case the connection is the bare logical  
implication. I guess that this is a fair (informal) formalisation of  
this 'trivial semantics'.

However, there may be several kinds of these 'connections': most of  
them are based on a model-theoretic characterisation rather than on  
entailment (see [1] - and I can really think of at least three  
additional important classes: FOL semantics (à la SWRL), LP-weak-safe  
semantics (à la Rosati), and autoepistemic semantics). So, we are  
really talking about at least 4 different semantic options to  
characterise the interoperability between a knowledge base and rules  
by means of 'query expressions' (as defined above) appearing in the  
body of some rules.

To be more concrete, let me take a very special case, and see how it  

Let us restrict attention to the RDF and OWL ontology/knowledge- 
representation languages (we have at least to consider those two, as  
per our charter). In order to super-simplify our life, let us in  
addition restrict our attention to the case when those queries are  
atomic: atomic binary predicates (a triple for RDF, a role for OWL)  
and atomic unary predicates (a class in OWL).

By adopting the 'trivial' semantics above, it is impossible to  
correctly capture correctly, for example, the function-free horn  
clause fragment of SWRL (which is, if you think a little about it, a  
special case of the above but with FOL semantics); basically none  
(but one) of the approaches surveyed in [1] would be captured  
correctly; and 100% of the use cases in <http://www.w3.org/2005/rules/ 
wg/wiki/Managing_incomplete_information>, while perfectly expressible  
in the trivial setting,  would miserably fail.

So, while I believe that there should be room for the 'trivial'  
semantics above, RIF should try to characterise also the several  
alternative approaches as known in the literature.


Enrico Franconi                  - franconi@inf.unibz.it
Free University of Bozen-Bolzano - http://www.inf.unibz.it/~franconi/
Faculty of Computer Science      - Phone: (+39) 0471-016-120
I-39100 Bozen-Bolzano BZ, Italy  - Fax:   (+39) 0471-016-129

[1] Enrico Franconi and Sergio Tessaris (2004). Rules and Queries  
with Ontologies: a Unified Logical Framework. Workshop on Principles  
and Practice of Semantic Web Reasoning (PPSWR'04).
Received on Wednesday, 25 January 2006 23:02:51 UTC

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