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RE: ISSUE-3: REPORTED: Lack of anonymous individuals

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Tue, 13 Nov 2007 10:13:17 +0900
To: "'Alan Ruttenberg'" <alanruttenberg@gmail.com>, "'Web Ontology Language \(OWL\) Working Group WG'" <public-owl-wg@w3.org>
Message-ID: <001701c82592$63b469c0$eb63dcd3@wolf>

Hello,

I actually like the suggestion of dropping blank nodes from OWL 1.1 DL, but leaving them in OWL 1.1 Full. No OWL 1.0 DL ontology I
know of actually uses blank nodes, so the loss of them is no big deal in practice. I don't believe that anyone in practice of OWL
1.1 DL would ever mourn the lack of blank nodes. Furthermore, I am not aware of any OWL 1.0 DL tool that implements blank nodes
exactly as it is said in the spec: most tools skolemize blank nodes when reading an ontology. This makes the tools sound and
complete for ontology satisfiability, but not for ontology entailment. No OWL 1.0 DL tool actually supports ontology entailment
directly: the inferences traditionally supported are instance checking, subsumption checking, classification, etc.

Thus, the simplest solution would be to leave the specification of OWL 1.1 DL as it is, and to allow blank nodes to appear only in
RDF data in OWL 1.1 Full. This would make OWL 1.1 DL technically not 100% backwards compatible, but I doubt that anyone in practice
would really care.



Now I don't completely understand what you meant with the following:

> We couldn't
> generate, within OWL-DL, bnodes. I'm not sure this is a big deal. It
> means we can't generate idiomatic foaf. It also means that OWL
> rendered in turtle gets potentially uglier.

Please note that we could only drop bnodes in instance data; however, bnodes would still be allowed in the translation from the
structural specification into the RDF syntax. Hence, I don't understand why OWL rendered in turtle gets potentially uglier. The RDF
rendering of OWL 1.1 DL would be unchanged by this: we would only just prohibit anonymous individuals in instance data.



> We could read bnodes within rdf and interpret them within OWL by way
> of the translation to the explicit existential (restriction on
> property blah blah), as long as there were only trees of bnodes, i.e,
> as Bijan notes

If we define OWL 1.1 DL without bnodes, then we should probably not handle a potential translation of bnodes while parsing in the
spec. Implementations are free to read bnodes in instance data and translate them into skolem constants; however, this is then
something that the implementation *may* choose to do. Strictly speaking, this should not affect the definition of the language in
any way; otherwise, we are risking incompatibility with OWL 1.1 Full.


To summarize, the solution would be the following:

1. Leave the structural spec and the mapping to RDF as is -- that is, disallow blank nodes in instance data.

2. Allow for bnodes in OWL 1.1 Full.

Regards,

	Boris

> -----Original Message-----
> From: public-owl-wg-request@w3.org [mailto:public-owl-wg-request@w3.org] On Behalf Of Alan Ruttenberg
> Sent: 10 November 2007 17:57
> To: Web Ontology Language (OWL) Working Group WG
> Subject: Re: ISSUE-3: REPORTED: Lack of anonymous individuals
> 
> 
> Related to the question of skolems versus existentials, I note that
> both the OWL 1.0 and the OWL 1.1 RDF mappings make liberal use of
> bnodes. Is there any reason that we couldn't use skolems (which are
> not bnodes)?
> 
> --
> 
> It doesn't seem like a good idea to me to *interpret* bnodes as
> skolems. Better to assume that we serialize skolems as urn:uuid: as
> Reto suggested in the email Bijan cites.
> 
> It seems to me that we could actually support both.
> 
> Individual(type(owl:Thing)) could be a skolem
> SomeIndividual(type(owl:Thing)) could be an existential.
> 
> Suppose we dropped bnodes(old style anonymous individuals) from OWL-
> DL, but not OWL-Full, what would be the consequences? We couldn't
> generate, within OWL-DL, bnodes. I'm not sure this is a big deal. It
> means we can't generate idiomatic foaf. It also means that OWL
> rendered in turtle gets potentially uglier.
> 
> We could read bnodes within rdf and interpret them within OWL by way
> of the translation to the explicit existential (restriction on
> property blah blah), as long as there were only trees of bnodes, i.e,
> as Bijan notes
> 
> Can anyone say why doing things this way would be problematic?
> 
> -Alan
> 
Received on Tuesday, 13 November 2007 01:14:12 GMT

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