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Re: status of xsd:duration in OWL (and RIF and SPARQL) - ACTION-164: RDF WG

From: Martin Hepp <martin.hepp@ebusiness-unibw.org>
Date: Thu, 2 Aug 2012 10:19:08 +0200
Cc: 'Dan Brickley' <danbri@danbri.org>, "public-vocabs@w3.org" <public-vocabs@w3.org>
Message-Id: <D3C76A70-3A72-4B21-BE80-35EA70FA0F4C@ebusiness-unibw.org>
To: "Evain, Jean-Pierre" <evain@ebu.ch>
As a side note, the flow of argument in the forwarded reply shows that the core Semantic Web community is designing formal languages but not formal languages for Web ontologies.

The limitations of reasoning with xsd:time and xsd:data are microscopic in comparison to the challenges of any reasoning over data published at Web scale - by millions of site owners, from millions of existing databases etc. 

Best

M. Hepp


On Aug 1, 2012, at 7:45 PM, Evain, Jean-Pierre wrote:

> Hi Dan,
> 
> Here is one part of the discussion as a response to my request to consider date and time separately in addition to duration...
> 
> There is more... still digging
> 
> JP
> 
> -----Original Message-----
> From: Evain, Jean-Pierre 
> Sent: mardi, 8. mai 2012 11:18
> To: 'Michael Schneider'
> Cc: 'Ivan Herman'; Bijan Parsia; Ian Horrocks; public-owl-wg@w3.org; Public-Rif-Wg (E-mail); Peter F. Patel-Schneider; Sandro Hawke
> Subject: RE: status of xsd:duration in OWL (and RIF and SPARQL) - ACTION-164: RDF WG
> 
> Dear Michael,
> 
> I appreciate your time and effort in trying to bring more background around the current situation.
> 
> I must say that I am growingly puzzled. This is definitely making me question my resolution to move for these technologies. If it cannot provide simple answers to simple questions, then maybe I am wasting my time. If I show your answer to some of my colleagues in my expert community, I may get some buying from those who have an academic background, but not from implementers (who are those who count to me in my daily business).
> 
> I believe that the semantics of time, date and duration are clear and I am surprised that they may be considered as being not mathematically univocally representable. For me there is nothing more semantically defined than a datatype bound to a particular format (and you'll always find cases where representation of date and time is ambiguous whether you use date, time or dateTime). A class instantiating such a datatype is also semantically defined in the context of a given ontology. Etc.
> 
> I believe working on the duration example would seem to answer part of the question but it is taking the easy way and in this particular case:
> - you have taken an arbitrary time reference that is a second (what about tenth or thousandth of a second)
> - you are facing the problem of defining the type of month according to its duration and resolving this as suggested looks interesting :--(
> - then once you have calculated the value, how do you say on which basis it was calculated (e.g. how do you signal the unit unless it has to be seconds:--(, etc.)?
> 
> Why not simply reuse the xsd datatypes? That would solve all the above problems with a simple expression in a well defined format. What do I miss?
> 
> But you didn't really answer my question about expressing a start time in a video.  This is semantically perfectly clear and defined. I'd like to see an example of how this would be done and could be recognised as good practice by implementers.
> 
> I am not saying that you are wrong. You seem to have been thinking about it.. But I believe we do not live in the same world.:--) I am personally trying to be very practical and I realise that this technology may never fulfil my requirements to serve uniquely some theoretical purpose on improbable queries.
> 
> I am really wondering if this makes sense. Please convince me.
> 
> Jean-Pierre
> 
> 
> 
> 
> 
> 
> -----Original Message-----
> From: Michael Schneider [mailto:schneid@fzi.de] 
> Sent: mardi, 8. mai 2012 10:39
> To: Evain, Jean-Pierre
> Cc: 'Ivan Herman'; Bijan Parsia; Ian Horrocks; public-owl-wg@w3.org; Public-Rif-Wg (E-mail); Peter F. Patel-Schneider; Sandro Hawke
> Subject: Re: status of xsd:duration in OWL (and RIF and SPARQL) - ACTION-164: RDF WG
> 
> Hi Jean-Pierre!
> 
> Am 08.05.2012 09:25, schrieb Evain, Jean-Pierre:
> 
>> I understand the point which is being made being what is useful for reasoning or not.
> 
> Replace "useful for reasoning" by "required for the well-definedness of 
> the semantics" of OWL 2 or RIF!
> 
> It is a basic technical requirement for the specifications of these 
> languages that for every syntactically well-formed expression (aka an 
> OWL 2 ontology or a RIF rule set), the semantic meaning can be 
> determined by mathematical means. At a minimum, for OWL 2 and RIF, this 
> means that it can always be determined whether an input ontology is 
> satisfiable or not, or whether one given ontology entails another given 
> one or not. Only in the cases of OWL 2 DL and its profiles, it is an 
> additional requirement (by design) that there are reasoning procedures 
> that are able to do these determinations in an automated way for all 
> input, because these languages are required to be computationally 
> decidable. But having a well-defined semantics is always needed. 
> Clearly, if there are ontologies for which it cannot uniquely be deduced 
> (mathematically) whether they are satisfiable or not, a reasoner cannot 
> give the "right" reasoning result for them, because it cannot then be 
> determined whether it's answer is right or not, or just one correct 
> answer out of many.
> 
> To illustrate this problem, take the case of xsd:duration in its 
> definition as of the time of finalizing OWL 2, where each literal of 
> xsd:duration would essentially denote a pair (m, s) consisting of a 
> certain number m of month plus a certain number s of seconds. Let there 
> be two such durations:
> 
>     d1 := (2, 0)
>     d2 := (1, 30*24*60*60)
> 
> Now, depending on what is meant by "a month", these two durations can 
> represent either (i) the same value (if a month has 30 days), or (2) d1 
> can be greater than d2 (if a month has 31 days), or (3) d1 is smaller 
> than d2 (if a month has, say, 28 days = 4 weeks). I may well have missed 
> a precise definition of "a month" in the (newest version of the) XSD 
> spec, in which case the above example may be void. But if not, then it 
> is clear that any OWL 2 (+xsd:duration) ontology for which the question 
> of satisfiability depends on whether the above two durations are the 
> same or not, or which of them is greater, does not have a uniquely 
> defined semantic meaning.
> 
> An example for the need of being able to determine whether equality 
> between two duration values holds or not would be an ontology with data 
> enumerations consisting of duration values (denoted by "d1" and "d2", as 
> defined above, but in a real ontology one would use their correct 
> literal form, of course):
> 
>     :D a rdfs:Datatype ;
>        owl:oneOf ( d1 ) .
>     :dp a owl:DatatypeProperty ;
>         rdfs:range :D .
>     :s :dp d2 .
> 
> This set of axioms should be satisfiable if and only if d2 = d1, because 
> only in this case, the object d2 of the property assertion (last 
> statement) would denote an instance of the singleton datatype :D = {d1}. 
> But if it cannot be determined whether d2 equals d1 or not, then it 
> cannot be determined whether the axiom set is satisfiable or not.
> 
> An example for the need of comparison of two durations (greater or 
> lesser than) could be constructed from the use of OWL 2 datatype 
> restrictions.
> 
>> But does that mean that all other information is garbage?
> 
> Everyone can say everything about everything. But an OWL 2 or RIF or 
> whatever language specification with a formal semantics at its core 
> would, if not well-defined, IMO count as just that: garbage. :-)
> 
> Best,
> Michael
> 
> -- 
> ..........................................................
> Dipl.-Inform. Michael Schneider
> Research Scientist, IPE / WIM
> 
> FZI Forschungszentrum Informatik
> Haid-und-Neu-Str. 10-14
> 76131 Karlsruhe, Germany
> Tel.: +49 721 9654-726
> Fax: +49 721 9654-727
> 
> michael.schneider@fzi.de
> www.fzi.de
> 
> ..........................................................
> Forschungszentrum Informatik (FZI) an der Universität Karlsruhe
> Stiftung des bürgerlichen Rechts
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> ..........................................................
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--------------------------------------------------------
martin hepp
e-business & web science research group
universitaet der bundeswehr muenchen

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Received on Thursday, 2 August 2012 08:19:39 GMT

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