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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: Tue, 8 May 2012 08:49:03 +0200
Cc: 'Michael Schneider' <schneid@fzi.de>, Bijan Parsia <bparsia@cs.man.ac.uk>, Ian Horrocks <ian.horrocks@cs.ox.ac.uk>, "public-owl-wg@w3.org" <public-owl-wg@w3.org>, "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>, "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, Sandro Hawke <sandro@w3.org>
Message-Id: <0FF875AE-A839-41F2-9376-95582EC9E2DD@w3.org>
To: "Evain, Jean-Pierre" <evain@ebu.ch>

On May 8, 2012, at 08:26 , Evain, Jean-Pierre wrote:

> Michael,
> 
> Thanks for the documented explanations.
> 
> However, that looks like very convoluted. Anytime, those who want to solve the issues that you mention can use dateTime.
> 
> But, I am trying to bring RDF and OWL in audiovisual production and I need to express things as simple as:
> 
> - segment video "y" start at time "t" for a duration "d"
> 
> I do not see why I should use any date information in this, it is not relevant. Also because dateTime is a frequent source of errors.
> 
> But maybe it is a bad idea altogether to bring RDF and OWL in audiovisual production?

Jean-Pierre,

without taking side on whether duration should or should not be part of OWL 2 for reasoning (I do not have enough technical baggage to comment on that): if you use duration today, it is valid RDF (even if RDF2004 still refers to duration as a SHOULD NOT be used, but that might change now with XSD1.1). Not being part of OWL 2 means that you do not get OWL 2 datatype reasoning on those literal values; I presume an OWL 2 reasoner would treat those literal values pretty much as strings. So the question is what you expect from your tooling v.a.v. that particular data. 

Ivan

> 
> Jean-Pierre
> 
> -----Original Message-----
> From: Michael Schneider [mailto:schneid@fzi.de] 
> Sent: mardi, 8. mai 2012 01:42
> To: Bijan Parsia
> Cc: Ian Horrocks; Ivan Herman; public-owl-wg@w3.org; Public-Rif-Wg (E-mail); Peter F. Patel-Schneider; Sandro Hawke; Evain, Jean-Pierre
> Subject: Re: status of xsd:duration in OWL (and RIF and SPARQL) - ACTION-164: RDF WG
> 
> Am 07.05.2012 12:42, schrieb Bijan Parsia:
>> On 7 May 2012, at 11:29, Michael Schneider wrote:
>> 
>>> Am 07.05.2012 00:19, schrieb Ian Horrocks:
>>>> Hi Bijan (et al),
>>>> 
>>>> According to my understanding, we agreed to keep the WG alive so that we could fix any OWL 2 problems caused by changes to XSD 1.1 and update the OWL 2 Rec to reference the XSD 1.1 Rec. It was also foreseen that we could take advantage of this update to fix any editorial errata in the OWL 2 Rec.
>>>> 
>>>> While I agree that the dividing line between editorial errata and substantive changes is not 100% clear, it does seem pretty obvious to me that adding support for a new datatype goes beyond the spirit of this agreement.
>>> 
>>> I agree!
>> 
>> But, you know, who the heck cares about the spirit of some agreement?
> 
> Oops, sorry for having been so short on words - that's not what people 
> generally expect from me and, certainly, I have more to say than simply 
> agreeing to Ian. :-)
> 
> Fine, not talking about procedural stuff here, and also not mentioning 
> (ok, I do) that I believe that it's not so easy to bring a working group 
> happily back to work after 2 1/2 years, there is still the question 
> whether these three datatypes are technically appropriate for inclusion 
> in OWL at all (whether in OWL 2, or OWL 2.1, or whatever). So here is 
> the situation as I recall it:
> 
> First to say, it's not that these datatypes were simply forgotten to be 
> considered, as some people seem to believe. Rather, at least two of them 
> were discussed and then excluded deliberately. Below is my understanding 
> of why and when this happened.
> 
> NOTE: In the remainder, whenever I refer to the XSD datatype spec, I 
> refer to the Candidate Recommendation as of 30 April 2009, which was the 
> one to which the OWL 2 and RIF specs currently refer to.
> 
> 1) xsd:time
> 
> IIRC, the OWL WG had adopted a certain design principle for the 
> time-related datatypes of OWL 2, which was the "timeOnTimeline" 
> property, as defined in the XSD datatype spec [1]: each data value in 
> the value space of a datatype must be exactly one point on the 
> everlasting time line.
> 
> This is certainly fulfilled by xsd:dateTimeStamp, which is included in 
> OWL 2. In xsd:time, however, each data value specifies infinitely many 
> times on the time line: "12:00:00 o'clock" happens every day! In fact, 
> The XSD spec at that time said [2]:
> 
>     time represents instants of time that recur
>     at the same point in each calendar day, or
>     that occur in some arbitrary calendar day.
> 
> For OWL 2 reasoners (well, actually for the OWL 2 semantics), such a 
> definition is a problem, because they have to decide for two given times 
> whether the two times are equal or not and, if not, whether one time 
> value is smaller or larger than the other one. Comparisons of this sort 
> become particularly troublesome, if one tries to compare, say, a 
> xsd:time value with a xsd:dateTimeStamp value: the dateTimeStamp value 
> is a fixed point on the time line, but any xsd:time value will refer to 
> time points both before and after that fixed time point.
> 
> The corresponding OWL WG issue was ISSUE-126 [3]. The description of the 
> issue swiftly mentions the problems of the time-related datatypes. There 
> was a long discussion on this issue, as you can see from the list of 
> references below the issue description; I guess, these mails contain 
> more discussion on the xsd:time datatype problem, but I haven't read 
> them all. The eventually accepted proposal (which only mentions 
> xsd:dateTime but not any of the other time-related datatypes anymore) to 
> resolve the issue was [4], with resolution at F2F3 [5] and 
> implementation by ACTION-177 [6].
> 
> 2) xsd:date
> 
> Here, the situation was different compared to xsd:time, as data values 
> in the value space of xsd:date do not represent time points, but time 
> /intervals/, as stated in the old version of the spec [7]:
> 
>     date represents top-open intervals of exactly
>     one day in length on the timelines of dateTime,
>     beginning on the beginning moment of each day,
>     up to but not including the beginning moment
>     of the next day.
> 
> This, of course, is also not compatible with the 
> single-point-on-timeline design criteria. Consequently, xsd:date was 
> excluded by the resolution of ISSUE-126 as well.
> 
> 3) xsd:duration
> 
> Unfortunately, I wasn't able to find any discussion on xsd:duration in 
> the OWL WG mailing list. The reason might be that this datatype was 
> already excluded in the XSD datatype map of RDF [8]:
> 
>     The other built-in XML Schema datatypes are
>     unsuitable for various reasons, and SHOULD NOT
>     be used: xsd:duration does not have a well-defined
>     value space
> 
> In retrospect, this first came to a surprise to me, so let's see how the 
> value space of xsd:duration was defined in the old CR of XSD 1.1 [9] 
> (long after the RDF rec, which still referred to XSD 1.0!):
> 
>     Duration values can be modelled as two-property
>     tuples. Each value consists of an integer number
>     of months and a decimal number of seconds.
> 
> Hrmpf, that appears to me a strange definition: why months? The length 
> of a month is not well-defined, it may be anything between 28 and 31 
> days. So it would be easy to give two xsd:duration literals, basically 
> consisting of month-second tuples, for which it becomes non-well-defined 
> whether they are equal or, if not, which of them is smaller or larger. 
> In fact, the text continues by:
> 
>     duration is partially ordered.
> 
> Sorry, but... I'm not clear where this idea of using months as part of 
> the value space definition came from. Anyhow, for OWL 2 reasoners, 
> adopting xsd:duration would make things troublesome, as some OWL 2 
> ontologies would not have a well-defined semantics then.
> 
>    * * *
> 
> Now, so far for the situation at the time of OWL 2 recommendation. All 
> three datatypes, according to their definitions in the XSD candidate 
> recommendation at the time of OWL 2 and RIF spec, provided problems for 
> OWL 2, be it with respect to the one-point-on-timeline design criteria 
> (xsd:time and xsd:date), or with regard to well-definedness of the (two) 
> OWL 2 semantics (xsd:time and xsd:duration). I leave it to the 
> proponents of these datatypes to explain what has changed in the XSD 
> spec since the recommendation of OWL 2 to allow for these datatypes to 
> be included now, such that (a) the design decisions made for OWL 2 are 
> retained and (b) without rendering the semantics of OWL 2 non-well-defined.
> 
> Best,
> Michael
> 
> References:
> 
> [1] 
> <http://www.w3.org/TR/2009/CR-xmlschema11-2-20090430/#vp-dt-timeOnTimeline>
> [2] <http://www.w3.org/TR/2009/CR-xmlschema11-2-20090430/#time>
> [3] <http://lists.w3.org/Archives/Public/public-owl-wg/2008Jun/0138.html>
> [4] <http://lists.w3.org/Archives/Public/public-owl-wg/2008Jul/0433.html>
> [5] <http://www.w3.org/2007/OWL/meeting/2008-07-29#resolution_1>
> [6] <http://www.w3.org/2007/OWL/tracker/actions/177>
> [7] <http://www.w3.org/TR/2009/CR-xmlschema11-2-20090430/#date>
> [8] <http://www.w3.org/TR/2004/REC-rdf-mt-20040210/#XSDtable>
> [9] <http://www.w3.org/TR/2009/CR-xmlschema11-2-20090430/#duration>
> 
> -- 
> .........................................................
> 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
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> .........................................................
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----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
FOAF: http://www.ivan-herman.net/foaf.rdf
Received on Tuesday, 8 May 2012 06:46:20 UTC

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