W3C home > Mailing lists > Public > www-rdf-logic@w3.org > January 2007

Re: weird example in RDF-MT document...

From: Denny Vrandecic <dvr@aifb.uni-karlsruhe.de>
Date: Thu, 25 Jan 2007 22:48:19 +0100
Message-ID: <45B925A3.90005@aifb.uni-karlsruhe.de>
To: axel@polleres.net
CC: www-rdf-logic@w3.org

Hi Axel,

ah, now I see, it's that darned existential semantics of blank nodes 
again. I thought the inferred triple was meant to be a part of the 
original graph.

Now I agree, if those are two separate graphs than this looks like a bug 
indeed. The problem here is not the reification, but the blank node. The 
doc should have used URIs here instead of blank nodes, this would have 
made a much stronger point, and be less confusing.

Cheers,
denny



Axel Polleres wrote:
> 
> Hi denny,
> 
> Denny Vrandecic wrote:
>> this makes sense to me. Consider the following addition of the graph:
>>
>> _:xxx dc:creator ex:Axel.
>> _:xxx dc:date "Jan-24-2007".
>> _:yyy dc:creator ex:Denny.
>> _:yyy dc:date "Jan-25-2007".
>>
>> basically saying that the statement was done by you yesterday, and by 
>> me today, you would not want to entail that it was said by you today 
>> as well.
> 
> sure, but this is not the point of the example in the spec.
> 
> 1) Given the additional information you mention and lets call this graph 
> S' :
> 
>  _:xxx rdf:type rdf:Statement .
>  _:xxx rdf:subject <ex:subject> .
>  _:xxx rdf:predicate <ex:predicate> .
>  _:xxx rdf:object <ex:object> .
>  _:yyy rdf:type rdf:Statement .
>  _:yyy rdf:subject <ex:subject> .
>  _:yyy rdf:predicate <ex:predicate> .
>  _:yyy rdf:object <ex:object> .
>  _:xxx <ex:property> <ex:foo> .
>  _:xxx dc:creator ex:Axel.
>  _:xxx dc:date "Jan-24-2007".
>  _:yyy dc:creator ex:Denny.
>  _:yyy dc:date "Jan-25-2007".
> 
>  Now this makes a difference of course, but even then, the graph S' 
> still entails the graph E consisting of the single triple,
> 
>   _:yyy <ex:property> <ex:foo> .
> 
> (the blank node id in E isn't supposed to have any meaning in commong 
> with _:yyy in the original graph).
> 
> it would though indeed not entail the graph E'
> 
>   _:yyy dc:creator ex:Denny.
>   _:yyy dc:date "Jan-25-2007".
>   _:yyy <ex:property> <ex:foo> .
> 
> Do you see my point now?
> (looks like we jointly elaborated an example which makes more sense to 
> me at least, but I still have problems with the original one in the 
> spec...)
> 
> 
> 2) Note that the original graph S as in the example (without any 
> additional triple):
> 
>  _:xxx rdf:type rdf:Statement .
>  _:xxx rdf:subject <ex:subject> .
>  _:xxx rdf:predicate <ex:predicate> .
>  _:xxx rdf:object <ex:object> .
>  _:yyy rdf:type rdf:Statement .
>  _:yyy rdf:subject <ex:subject> .
>  _:yyy rdf:predicate <ex:predicate> .
>  _:yyy rdf:object <ex:object> .
>  _:xxx <ex:property> <ex:foo> .
> 
> is not lean since
> 
>  _:xxx rdf:type rdf:Statement .
>  _:xxx rdf:subject <ex:subject> .
>  _:xxx rdf:predicate <ex:predicate> .
>  _:xxx rdf:object <ex:object> .
>  _:xxx <ex:property> <ex:foo> .
> 
> entails the whole graph. That is basically my problem with the example 
> where I got stuck.
> 
> best,
> axel
> 
> 
> 
> 
>> One could discuss if the identity of a statement is constituted by its 
>> three components, subject, predicate, object, but the standard clearly 
>> says it does not, and so the identity of a statement seems constituted 
>> by some other kind of magic (well, I'd go for an URI).
> 
> 
>> This also has the advantage that a statement with an URI as its 
>> identifier can actually be dynamic, like ex:Denny ex:listens_to 
>> ex:Jewel, which changes from time to time -- but nevertheless it could 
>> always have the same URI.
>>
>> Cheers,
>> denny
>>
>> Axel Polleres wrote:
>>
>>>
>>> Hi all,
>>>
>>> I was studying the RDF semantics document once again in some detail 
>>> and it looked to me I found a bug in an example in the end of section 
>>> 3.3.1.
>>> Since I doubt that, I was asking myself whether somebody here can 
>>> help me to get the knot out of my head....
>>>
>>>
>>>    http://www.w3.org/TR/rdf-mt/#Reif
>>>
>>>
>>>
>>> In the end of that section, it is stated that
>>>
>>> "For example,
>>>
>>> _:xxx rdf:type rdf:Statement .
>>> _:xxx rdf:subject <ex:subject> .
>>> _:xxx rdf:predicate <ex:predicate> .
>>> _:xxx rdf:object <ex:object> .
>>> _:yyy rdf:type rdf:Statement .
>>> _:yyy rdf:subject <ex:subject> .
>>> _:yyy rdf:predicate <ex:predicate> .
>>> _:yyy rdf:object <ex:object> .
>>> _:xxx <ex:property> <ex:foo> .
>>>
>>> does not entail
>>>
>>> _:yyy <ex:property> <ex:foo> ."
>>>
>>>
>>>  This is at the very least strange for me...  and I think simply wrong.
>>>
>>> I mean, I understand what is *meant* to be said here, but of course
>>> the single triple graph
>>>
>>> _:yyy <ex:property> <ex:foo> .
>>>
>>> IS entailed by the former.
>>>
>>> Can anybody shed light on me? I must admit that I have some 
>>> difficulties to understand the non-normative reificaiton proposed in 
>>> that section, but I would assume that simple entailments still hold.
>>>
>>> I would be grateful if anybody can shed light on me or tell me 
>>> whether this was already answered elsewhere?
>>>
>>> Thanks,
>>> axel
>>>
>>>
>>>
>>>
>>>
>>
>>
> 
> 
Received on Thursday, 25 January 2007 21:49:49 UTC

This archive was generated by hypermail 2.3.1 : Wednesday, 2 March 2016 11:10:47 UTC