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Re: [RDF+OWL] Way of connecting RDF and RIF lists

From: Jos de Bruijn <debruijn@inf.unibz.it>
Date: Fri, 01 May 2009 11:11:04 +0200
Message-ID: <49FABCA8.6020707@inf.unibz.it>
To: Dave Reynolds <der@hplb.hpl.hp.com>
CC: Sandro Hawke <sandro@w3.org>, RIF <public-rif-wg@w3.org>


Dave Reynolds wrote:
> Sandro Hawke wrote:
>>>> The argument against the one-to-one mapping is that it's harder to
>>>> implement.  I believe you cannot implement it in a rule system that
>>>> does
>>>> not support function symbols, unless you have specific machinery for
>>>> manipulating the RDF list structures while you manipulate the RIF
>>>> structures: the construction of an RIF list implies the existence of a
>>>> bunch of objects used for the structure of the RDF list.
>>> I think any system likely to implement this would have such specific
>>> machinery anyway.
>>>
>>> However, the "as extensions" option is still useful and a lower
>>> implementation barrier so perhaps that would be the better choice. It
>>> doesn't stop people doing future extensions which strengthen the link.
>>>
>>> I won't be there on Tuesday (though I may be able make the first 25m
>>> of the call if that would be useful). At the moment my preference
>>> would be "as extensions" as top choice (on grounds of lower
>>> implementation cost) but "1-to-1" also acceptable.
>>
>> How big is the implementation cost?  Jos said he thought Jena had this.
> 
> The implementation should be easy for Jena (if we do implement at all).
> Indeed it would harder to do it the other way since by default our lists
> would actually be RDF lists. I was commenting in general rather than for
> Jena but indeed I'm not sure the implementation cost is significant.
> 
> On reflection I've some slight unease about the 1-to-1 option and was
> using "implementation cost" as a cop out to avoid me having to figure
> out why I was uneasy. :-)
> 
> Thinking out loud here ... consider the rule set[*]:
> 
>    p(:a).
>    q(List(?x, ?x)) :- p(?x).
> 
> Under 1-1 semantics I believe this entails the RDF list (using Turtle
> notation):
> 
>    (:a :a)
> 
> i.e.
> 
>    [rdf:first :a; rdf:rest [rdf:first :a; rdf:nil]]
> 
> Right?
> 
> So creating a list, even just as an argument to a predicate, implies
> creating an RDF list structure (with a bNode for the list root) even
> though the rest of the predicate is invisible to the RDF view.
> 
> So for a PR engine if they want to implement RIF-RDF combinations with
> the 1-1 semantics then every time they cons up a list they will need to
> assert the corresponding frame formulae into their working memory. This
> seems like a high overhead.

Indeed. This is what makes the 1-1 semantics a lot harder.

> 
> Does that make sense? Is this a genuine problem or am I off base here?
> 
> I realize the term "RDF view" is not a defined notion in RIF, we only
> define the semantics of RIF-RDF combinations. However, in an RDF
> implementation I would expect to provide a means to extract an entailed
> RDF graph from the RIF-RDF combination, that's the value of such
> combinations for RDF processing. The fact that RIF predicates are not
> part of the entailed graph is rather useful. Having lists which aren't
> otherwise referenced (by non-list frames) leak out into that view is
> problematic.

Actually, RIF-RDF compatibility defines a notion of entailed RDF graphs.
One could call those the "RDF view".

> 
> Now, to argue against myself, in the rule set:
> 
>   p(:a).
>   :f[:p->List(?x ?x)] :- p(?x).
> 
> Then you would prefer the RDF view of the entailments to include:
> 
>   :f :p (:a :a) .
> 
> Since the RDF view of the RIF-RDF entailments is not defined then an
> implementer could do what they like. In the 1-1 case they could omit the
> orphaned lists from the view, in the extensions case they could add the
> non-orphaned lists into the view. So I can make something useful work
> either way.
> 
>> The main reason I want one-to-one is for people doing SPARQL queries on
>> the frames in a RIF rule base.  But I guess that SPARQL interface could
>> do this conversion without it really being a mandatory part of RIF (just
>> like it would do the conversion from rdf:text to RDF's plain literals).
> 
> Yes, this sounds like what I mean by an "RDF view" of the combination.
> 
> Dave
> 
> [*] By the way, is this rule set in RIF Core? During the f2f discussion
> on "grounded lists" in the core I found it hard to follow on the phone
> what people were meaning by grounded lists.

No. We were talking about ground lists, and ground=no variables.


Best, Jos

> 

-- 
+43 1 58801 18470        debruijn@inf.unibz.it

Jos de Bruijn,        http://www.debruijn.net/
----------------------------------------------
Many would be cowards if they had courage
enough.
  - Thomas Fuller


Received on Friday, 1 May 2009 09:11:51 GMT

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