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Re: MuSim Ontology problems (was Re: Share, Like Ontology)

From: Yves Raimond <yves.raimond@gmail.com>
Date: Tue, 15 Jun 2010 19:23:38 +0100
Message-ID: <AANLkTimAKrqs2BxVKYUpeo0exZcV4zI3fxh8ISMB2SmC@mail.gmail.com>
To: Antoine Zimmermann <antoine.zimmermann@deri.org>
Cc: Bob Ferris <zazi@elbklang.net>, Kurt J <kurtjx@gmail.com>, Linked Data community <public-lod@w3.org>, Semantic Web <semantic-web@w3.org>, pedantic-web@googlegroups.com

>>> owl:Class is defined as a subclass of rdfs:Class *in the OWL
>>> specifications*.  The RDF/RDFS specification does not say anything about
>>> owl:Class.  So, from a pure RDFS perspective, owl:Class has as much
>>> meaning
>>> as, e.g., xyz:abc.  The fact that someone defines *somewhere* that
>>> xyz:abc
>>> is a subclass of rdfs:Class is irrelevant from a pure RDFS system point
>>> of
>>> view.  As I said in my example, a SPARQL query would not be able to
>>> retrieve
>>> the OWL classes or properties that are not directly asserted as RDFS
>>> classes
>>> or properties (unless the SPARQL engine implements part of the OWL spec,
>>> which is rarely the case).
>>> Now, that's a small issue but there is no disadvantage of putting the
>>> additional types, as far as I know.
>> Frankly, I don't think this is something we need to change. Yes, the
>> rdfs:subClassOf is in the OWL specification, so what? If you follow
>> your nose, you'll end up in the RDFS world, so that's OK.
> There is nothing wrong with not writing rdfs:Class (similarly to "there is
> nothing wrong in writing webpages in HTML 4.01 as opposed to XHTML").  I
> only point out that rdfs:Class (and rdf:Property) allows you to play nicely
> with RDF/RDFS tools, such as SPARQL (similarly to "XHTLM plays nicely with
> XML tools").

Can you explain what you mean by "such as SPARQL" here? It looks to me
like it's a completely orthogonal issue. You can plug some reasoning
behind a SPARQL end-point. It's a bit like saying "you can't use views
in relational databases if you want to use SQL" :-)

> You can obviously choose to ignore this, for various reasons. But here, the
> changes are so simple that I don't see any problem in adding it. May you can
> tell me where is the problem?
> As an example, consider the following SPARQL query:
> SELECT ?classOrProp WHERE {
>  {?classOrProp a rdfs:Class .} UNION {?classOrProp a rdf:Property .}
> most standard SPARQL implementations would return no result at all when
> evaluated against the Similarity ontology (or the Music Ontology). SPARQL
> engines have to implement simple graph matching but have no obligation to
> implement any entailment regime. In practice, there are extremely few SPARQL
> engines that go beyond simple RDF and I'm not aware of any implementation of
> SPARQL with built-in OWL entailment. Moreover, some Linked Data people are
> not very concerned about OWL entailment and advocate the use of simple
> RDF/RDFS modelling and tool usage. Yet, they want to take advantage of the
> RDFS underlying an OWL ontology. Let us give them what they want (at almost
> no extra cost).

So, if I follow your reasoning to the end, are you suggesting that
*every* inferrable triples should be added to the spec (not only
rdfs:Class, but also all others triples inferred by following
rdfs:subClassOf links, or rdfs:subPropertyOf, owl:inverseOf,
owl:equivalentClass etc)? Or to any Linked Data sources out there,
e.g. all triples inferred by following an owl:sameAs link? If so, even
if you limit yourself to RDFS entailment, you're on quite a big job!

I think it's reasonable to assume that RDF publishers don't want to
assume this kind of responsibility. I agree that the different
reasoning capabilities of different tools is an issue, but it's a much
wider issue than the one you're highlighting here.


> FYI, please notice that FOAF is defining all its classes and properties with
> both the OWL vocabulary *and* the RDFS vocabulary.
> I actually started to realise the interest of such practice because I was
> wondering why there was these two Class types.
> Cheers,
> AZ.
>>> [...skip...]
>>>>>> =====
>>>>>> 4) musim:distance and musim:weight
>>>>>> =====
>>>>>> I notice that you are defining two datatype properties with multiple
>>>>>> range
>>>>>> restriction:
>>>>>> :distance a owl:DatatypeProperty;
>>>>>> rdfs:range xsd:float;
>>>>>> rdfs:range xsd:int;
>>>>>> rdfs:range xsd:double .
>>>>>> and
>>>>>> :weight a owl:DatatypeProperty;
>>>>>> rdfs:range xsd:float;
>>>>>> rdfs:range xsd:int;
>>>>>> rdfs:range xsd:double .
>>>>>> I'm quite sure that it is not what you intend to mean and I imagine
>>>>>> that you
>>>>>> would like to say that the weight or the distance can be either a
>>>>>> float, a
>>>>>> double or an int. Here you actually specify that the distance and the
>>>>>> weight of something is necessarily a float, a int and a double at the
>>>>>> same
>>>>>> time.
>>>>>> Furthermore, the OWL spec [1] says that:
>>>>>> """As specified in XML Schema [XML Schema Datatypes], the value
>>>>>> spaces of
>>>>>> xsd:double, xsd:float, and xsd:decimal are pairwise disjoint."""
>>>>>> This implies that :distance and :weight are in fact empty relations
>>>>>> since it
>>>>>> is impossible to have a value which is both a float and a double.
>>>>>> Using
>>>>>> :distance or :weight in the predicate position of any triple would
>>>>>> make the
>>>>>> knowledge base inconsistent.
>>>>>> If you want to say that a distance or weight has to be in *one of*
>>>>>> the three
>>>>>> datatypes, you should rather say:
>>>>>> :weight a owl:DatatypeProperty, rdf:Property;
>>>>>> rdfs:range [ owl:unionOf ( xsd:float xsd:int xsd:double ) ] .
>> Yes, you're right - it should be an union, not an interesection.
>>>>>> However, I feel unsatisfied by this because it is slightly
>>>>>> overconstraining.
>>>>>> Why not allow xsd:decimal or even owl:real as well? Or untyped
>>>>>> literals
>>>>>> such as:
>>>>>> ex:a :distance "1879.42" .
>>>>>> I imagine that the value for such a distance will be computed
>>>>>> automatically
>>>>>> and the programme which does it will ensure that it is indeed a
>>>>>> number.
>>>>> another rookie mistake i'm afraid! i think leaving the rdfs: range
>>>>> unspecified perhaps makes the most sense - yes it is a common
>>>>> occurence to get a "NaN" distance in audio signal based similarity and
>>>>> other similarity calculations.
>>>> Here the issue is that the programme, which computes the number, knows
>>>> of course that it is a number, but the reason to define it at least as a
>>>> kind of number is for reusing this values.
>>>> I'm somehow satisfied with the restriction rdfs:range [ owl:unionOf (
>>>> xsd:float xsd:int xsd:double ) ], because it is a well-defined range,
>>>> which expresses that the values are number. I can't really imagine other
>>>> values that are might used here. The XSD namespace is a kind of best
>>>> practice for defining the Datatypes.
>>> Reusing the value would be straightforward. In practice, the value will
>>> be
>>> computed in such a way that it is a number (or maybe "NaN", if relevant)
>>> and
>>> will most likely be given a datatype. In the end, the data will contain
>>> something like:
>>> ex:sim :distance "389.009"^^xsd:float .
>>> There is no problem reusing this value, regardless of the range
>>> definition.
>>>  However, *if* the range constraint is maintained as you suggest, the
>>> following triples would be each individually inconsistent wrt the
>>> ontology:
>>> ex:sim :distance "389.009" .
>>> ex:sim :distance "NaN" .
>>> ex:sim :distance "389.009"^^xsd:decimal .
>>> ex:sim :distance "389.009"^^owl:real .
>>> ex:sim :distance "0fb7"^^xsd:hexBinary .
>>> ex:sim :distance "6z2b76aa"^^xsd:base64Binary .
>>> Yet, it's easy to make a programme that deals equally well with all these
>>> values, whereas it is difficult to ensure that everybody will use the
>>> three
>>> datatypes mentioned in the range assertion.
>>> In the absence of range assertion, such values as:
>>> ex:sim :distance "very similar" .
>>> ex:sim :distance "+++"^^xsd:string .
>>> would be consistent wrt the ontology but they can be simply ignored by
>>> any
>>> programme using these values. In the presence of the range assertion,
>>> these
>>> triples would be inconsistent wrt the ontology, but this does not prevent
>>> anybody from writing them, so they would have to be dealt with somehow
>>> too.
>> This discussion reminds me a bit of
>> http://www.w3.org/DesignIssues/InterpretationProperties.html.
>> I don't think there's anything wrong with either approach. It is
>> perfectly ok to put such a range constraint in the ontology. When
>> building an aggregator of MuSim data, it is much easier to know what
>> to expect (and what to eventually reject - in case it is inconsistent
>> wrt the ontology) rather than committing to support all possible
>> datatypes! On the other hand, it's fine to leave it open - you gain in
>> flexibility.
>> Kind regards,
>> y
> --
> Antoine Zimmermann
> Post-doctoral researcher at:
> Digital Enterprise Research Institute
> National University of Ireland, Galway
> IDA Business Park
> Lower Dangan
> Galway, Ireland
> antoine.zimmermann@deri.org
> http://vmgal34.deri.ie/~antzim/
Received on Tuesday, 15 June 2010 18:24:11 UTC

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