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Re: RDF and OWL compatibility

From: Dave Reynolds <der@hplb.hpl.hp.com>
Date: Mon, 09 Jan 2006 17:50:12 +0000
Message-ID: <43C2A254.1010005@hplb.hpl.hp.com>
To: Enrico Franconi <franconi@inf.unibz.it>
CC: public-rif-wg@w3.org

Enrico Franconi wrote:

> On 5 Jan 2006, at 15:51, Dave Reynolds wrote:
> 
>> On the question of bNodes in the head, I hear the argument that it  is 
>> not sufficient to just treat these as new Skolem constants but  my 
>> intuitive understanding of the issue is too weak. It would be  really 
>> helpful if someone could construct a test case which  demonstrates the 
>> difference in results that arise between correct  treatment of bNodes 
>> in the head versus treatment as Skolem  constants. In the concrete 
>> cases I've seen where bNodes are used in  the head of rules they seem 
>> to be intended as a form of anonymous  gensym - so the Skolem constant 
>> semantics may be the more  practically useful interpretation.
> 
> 
> A a naive gensym would fail the use case  <http://www.w3.org/2005/ 
> rules/wg/wiki/Managing_incomplete_information>, where two examples  (in 
> section "9.4. (Rules involving generation of unknown)") show how  you 
> can make things wrong with a naive use of skolem constants to  implement 
> the existential variables in the head.

Sorry to be slow on the uptake but I didn't follow why the examples break 
with naive gensym.

The example in 9.4 on that page shows the rules:

[[[
       rdf:type(X,db:employee) :- kb:works-with(X,Y).
       kb:works-with(Z,X) :- rdf:type(X,db:employee).
]]]

A naive implementation of bNodes-by-gensym would rewrite the unsafe second 
rule to something like:

       kb:works-with(Z,X) :- rdf:type(X,db:employee), gensym(Z).

where "gensym/1" returns some freshly minted bNode on each call;

or, better,

       kb:works-with(Z,X) :- rdf:type(X,db:employee), gensym(X, Z).

where "gensym/2" binds Z to some bNode which is a (skolem-)function of X.

As far as I can see either of these would return the same answer as the 
classical semantics for both of your test cases 9.4.1 and 9.4.2.

Dave
Received on Monday, 9 January 2006 18:54:07 GMT

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