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From: Antoine Zimmermann <antoine.zimmermann@deri.org>
Date: Fri, 16 Jul 2010 16:17:31 +0100
Message-ID: <4C40780B.30302@deri.org>
To: Dave Reynolds <dave.e.reynolds@googlemail.com>
CC: foaf-dev@lists.foaf-project.org, Semantic Web <semantic-web@w3.org>, Linked Data community <public-lod@w3.org>
Beware, technical stuff follows.

Le 16/07/2010 13:07, Dave Reynolds a écrit :
> Looks interesting.
> In the description you say "(1) foaf:mbox_sha1sum, foaf:jabberID,
> foaf:aimChatID, foaf:icqChatID, foaf:yahooChatID and foaf:msnChatID are
> not owl:InverseFunctionalProperties anymore; instead, they are defined
> as owl:Keys for foaf:Agents, which is practically the same"
> I agree that making them owl:Keys is the only option for DL but the
> comment "practically the same" is maybe overstating it.
> My understanding was that the semantics of Keys [1] only applies to
> named individuals and so isn't effective on anonymous individuals (which
> is a common use case in FOAF). Is that correct?

Yes, you are correct.

Here, I made a simplifying short cut but saying "practically the same". 
In most cases, a blank node can be simply considered as a named 
individual, using the blank node ID (regardless of whether it is 
specified in the serialisation or internally represented) as a "name" 
for the individual.
In this case, the un-named individual are only those that exists because 
of inferences but which have no identifier at all (for instance, when 
using the OWL construct "owl:someValueFrom".

Such shortcuts are practically used in reasoners to deduce things about 
blank nodes in the same way they deduce things about URIs.

Using this approach on the following data:

foaf:yahooChatID a owl:DatatypeProperty ;
                  rdfs:domain foaf:Agent .
foaf:Agent owl:hasKey ( foaf:yahooChatID ).
_:bnode1 foaf:yahooChatID "xyz" .
_:bnode2 foaf:yahooChatID "xyz" .

one can infer:

_:bnode1 owl:sameAs _:bnode2 .

When the serialisation does not specify a name for a blank node, their 
is still an internal name somewhere.  For instance:

[ a :Person ] foaf:yahooChatID "xyz" ;
    foaf:firstName "John" .
[ a :Person ] foaf:yahooChatID "xyz" ;
    foaf:lastName "Doe" .

allows one to infer that there is one person with first name "John" and 
last name "Doe" (but its local identifier is only known to the reasoner).

An example where the inference would not hold is as follows:

:chatID a owl:DatatypeProperty ;
         rdfs:domain :Agent .
:Agent owl:hasKey ( :chatID ).
:hasFather a owl:ObjectProperty, owl:FunctionalProperty .
:john :chatId "xyz" .
:bob a [ owl:Restriction ;
          owl:onProperty :hasFather ;
          owl:allValuesFrom [ a owl:Restriction;
                              owl:onProperty :chatId;
                              owl:hasValue "xyz" ]
        ] .

from this, we cannot conclude that John is the father of Bob, although 
Bob has a father (un-named) whose :chatId is exactly the same as John.
If :chatId was inverse functional, we could conclude it. However, I said 
"practically the same" because this case is unlikely to occur in practice.

Hope it's clear enough. For the specs for keys, see [1,2,3].

[1] OWL 2 Web Ontology Language - Structural Specification and 
Functional-Style Syntax, Sect.9.5 Keys. 
[2] OWL 2 Web Ontology Language - New Features and Rationale, Sect.2.2.6 
F9: Keys. http://www.w3.org/TR/owl2-new-features/#F9:_Keys
[3] OWL 2 Web Ontology Language - Direct Semantics, Sect.2.3.5 Keys. 

> Dave
> [1] http://www.w3.org/TR/owl2-direct-semantics/#Keys
> On Fri, 2010-07-16 at 12:16 +0100, Antoine Zimmermann wrote:
>> Dear all,
>> I know that the compatibility of FOAF with OWL DL has been discussed a
>> lot in the past (and still sometimes surfaces again).  However, I'm
>> wondering, would it be reasonable to provide a DL version of FOAF in
>> complement of the official FOAF ontology?
>> More generally, wouldn't it be reasonable to provide alternative
>> versions of an ontology?  Think of XHTML: there are three different XML
>> Schemas for XHTML [1].  One could imagine alternative versions like FOAF
>> (Full), FOAF-DL, FOAF-lite...
>> Anyway, I did it: I've made a FOAF-DL ontology which modifies the FOAF
>> ontology such that (1) it is in OWL 2 DL and (2) it maximally preserves
>> inferences of the original FOAF ontology [2].
>> Interestingly, FOAF-DL is an OWL 2 RL ontology (in a nutshell, OWL 2 RL
>> is a subset of OWL 2 DL with low computational complexity and that is
>> compatible with rule-based inference engine).
>> You may notice that there are strange annotation properties for this
>> ontology:
>> <owl:Ontology rdf:about="http://purl.org/az/foaf#">
>>     ...
>>     <yoda:preferredVersion rdf:resource="http://xmlns.com/foaf/0.1/"/>
>>     ...
>> </owl:Ontology>
>> The Yoda vocabulary [3] is used to relate alternative versions of an
>> ontology. Here, it is said that there is a preferred version, which is
>> the official FOAF ontology.
>> Critiques to any of the previous comments are welcome.
>> [1] http://www.w3.org/TR/xhtml1-schema/#schemas
>> [2] The FOAF-DL ontology. http://purl.org/az/foaf
>> [3] Yoda: A Vocabulary for Linking Alternative Specifications of a
>> Vocabulary. http://purl.org/NET/yoda
>> Regards,

Antoine Zimmermann
Post-doctoral researcher at:
Digital Enterprise Research Institute
National University of Ireland, Galway
IDA Business Park
Lower Dangan
Galway, Ireland
Received on Friday, 16 July 2010 15:18:07 UTC

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