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Re: Blank node identifiers in FILTER clauses

From: Bijan Parsia <bparsia@cs.man.ac.uk>
Date: Mon, 17 Jul 2006 18:46:39 +0100
Message-Id: <EB234424-A81B-4CBF-A965-996C6A7A2E4B@cs.man.ac.uk>
Cc: Enrico Franconi <franconi@inf.unibz.it>, RDF Data Access Working Group <public-rdf-dawg@w3.org>
To: Eric Prud'hommeaux <eric@w3.org>

Eric, you seem to have a number of confusions that have gotten all  
rather severely tangled up. Some are based on simple ignorance (e.g.,  
of the distinction between distinguished and non-distinguished  
variables, which explains the behavior of Pellet), but then others  
compound on those errors. I'm just going to try to hit the highlights  
tersely, because I don't see that trying to untangle everything  
inline will actually help. Plus it's too much work :)

I shall also prune rather ruthelessly.

On Jul 16, 2006, at 6:13 PM, Eric Prud'hommeaux wrote:

>
> On Fri, Jul 14, 2006 at 05:37:16PM +0200, Eric Prud'hommeaux wrote:
>>
>> On Fri, Jul 14, 2006 at 02:48:10AM +0100, Enrico Franconi wrote:
>>>
>>> On 13 Jul 2006, at 17:23, Eric Prud'hommeaux wrote:
[snip]
>>>> SELECT ?Meal ?WineColor
>>>> WHERE {
>>>>   ?Meal food:hasDrink _:Wine .
>>>>   _:Wine wine:hasColor ?WineColor }

_:Wine is treated as a nondistinguished variables. That is, it does  
not need to have a named entity as a binding and it cannot return  
bindings.

>>>> Pellet <http://www.mindswap.org/2003/pellet/demo> gives these
>>>> results:
>>>>
>>>> |  Meal 	| WineColor |
>>>> +---------------+-----------+
>>>> | test:MyLunch 	| :White    |
>>>> | test:MyDinner	| :Red	    |

Which explains why it returns these values for this. There is no  
named Wine in the KB that can be bound to this variable, but it is  
provable that there must be *some* such wine in all models.

>>>> When querying Pellet for:,
>>>>   ?Meal food:hasDrink ?Wine .
>>>>   ?Wine wine:hasColor ?WineColor
>>>>
>>>> it gives no results because it has no bindings for the Wine.

?Wine is treated as a distinguished variable. In this sense it  
doesn't not quite conform to the SPARQL semantics, but since there  
are no SPARQL semantics for OWL DL, and quasi-distinguished variables  
are a new phenomenon, I think this is acceptible. Distinguished  
variables bind *only* to named entities, thus their bindings may not  
be BNodes, which are *not* names, but existential variables.

>>>> But why
>>>> not?

Because of the semantics of the variables.

>>>> Certainly, it as deduced that there is something there, but it's
>>>> opaque to RDF. Why doesn't it infer the triples:?
>>>>  food:MyLunch hasDrink [ hasColor :White ] .
>>>>  food:MyDinner hasDrink [ hasColor :Red ] .

Well, it does infer something similar, but all this is besides the  
point.

If the scoped set for the OWL DL entailment regime contains BNodes  
(making ?vars quasi-distinguished), then we should expect the above  
two queries to return with bindings. If not, then the current  
behavior is correct. Since it's not yet defined, I think we're ok :)

>>> You can't do that since you can not represent all the possible
>>> answers. You couldn't represent the case when there is a cyclic
>>> coreference between bnodes.
>>> E.g., _:a R _:b.
>>>       _:b S _:a.
>>
>> Perhaps I misinterpret here, but I thought that would be an argument
>> against materializing a closure and then querying it, as opposed to
>> proving the existence of certain triples.

The above KB is not a legal OWL DL KB, so it cannot be entailed. (See  
the transformation to triples:
	<http://www.w3.org/TR/owl-semantics/mapping.html#4.1>
"""Bnode identifiers here must be taken as local to each  
transformation, i.e., different identifiers should be used for each  
invocation of a transformation rule. Ontologies without a name are  
given a bnode as their main node; ontologies with a name use that  
name as their main node; in both cases this node is referred to as O  
below.""")

>> What query pattern could one match with bNodes and SPARQL's current
>> entailment rules for matching that one couldn't match with variables
>> against the inferred graph? (Note, showing me why Pellet should not
>> infer
>>   food:MyLunch hasDrink [ hasColor :White ] .
>>   food:MyDinner hasDrink [ hasColor :Red ] .
>> would be one way to answer this question.)
>
> In order to give you more to argue with, I've worked out some triples
> that I think are implied by your little house example.
>
> We were trying match:
>   Paul :hasFriend _:Y .
>   _:Y rdf:type Employee .
>   _:Y hasFriend _:Z .
>   _:Z rdf:type Manager .

This isn't cyclic.
[snip]
> I think you should be able to infer it, and then query it:
> SELECT ?X
> WHERE { ?X rdf:type Worker .
>         ?X :hasFriend ?Y .
>         ?Y rdf:type Employee .
>         ?Y :hasFriend ?Z .
>         ?Z rdf:type Manager } # note ?vars

This is irrelevant if you interpret the variables as distinguished.

> I'm really surprised this is not how people currently handle OWL
> semantics.

I suspect it's because you don't understand OWL semantics, and have  
not studied how to implement a sound and complete reasoner. Or  
rather, you don't recognize the distinction between an anonymous or  
generated individual in a model, and a BNode.

> bNodes are perfect for this.

Nope.

> If anyone wrote a backward-
> chaining OWL engine, it seems they'd have to do this.

Well, it really doesn't make sense to talk of "backward chaining"  
since you just don't chain at all, really. Pellet does do some data  
driven inference (e.g., classification and realization) and some on  
demand inference (e.g., arbitrary entailments, and queries at the  
moment), but it's not really a helpful way to think about it.

> (Come to think
> of it, I think cwm is basically backward-chaining.)

Nope.
<http://www.w3.org/2000/10/swap/doc/cwm.html>

"""Cwm (pronounced coom) is a general-purpose data processor for the  
semantic web, somewhat like sed, awk, etc. for text files or XSLT for  
XML. It is a forward chaining reasoner..."""

> Or maybe I just don't get something...
[snip]

Yep. Looks like the cyclicity.

Cheers,
Bijan.
Received on Monday, 17 July 2006 17:47:12 GMT

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