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

From: Ivan Herman <ivan@w3.org>
Date: Thu, 2 Aug 2012 10:26:41 +0200
Cc: "Evain, Jean-Pierre" <evain@ebu.ch>, 'Dan Brickley' <danbri@danbri.org>, "public-vocabs@w3.org" <public-vocabs@w3.org>
Message-Id: <D0A86787-7967-4587-8E79-DDBE12B581DB@w3.org>
To: Martin Hepp <martin.hepp@ebusiness-unibw.org>
FWIW, both xsd:time and xsd:date are perfectly fine datatypes in RDF now:

http://www.w3.org/TR/2012/WD-rdf11-concepts-20120605/#xsd-datatypes

Ivan


On Aug 2, 2012, at 10:19 , Martin Hepp wrote:

> 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
>> Stiftung Az: 14-0563.1 Regierungspräsidium Karlsruhe
>> Vorstand: Dipl. Wi.-Ing. Michael Flor, Prof. Dr. Ralf Reussner,
>> Prof. Dr. Rudi Studer, Prof. Dr.-Ing. J. Marius Zöllner
>> Vorsitzender des Kuratoriums: Ministerialdirigent Günther Leßnerkraus
>> ..........................................................
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> 
> --------------------------------------------------------
> martin hepp
> e-business & web science research group
> universitaet der bundeswehr muenchen
> 
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----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
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FOAF: http://www.ivan-herman.net/foaf.rdf
Received on Thursday, 2 August 2012 08:27:07 GMT

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