Re: relational data as a bona fide member of the SM

Describing correctly lists requires a non-first order language (you need fix-points, for that matter). RDF has a classical first-order semantics, so there is no way to correctly capture them.
--e.

On 6 Nov 2011, at 17:03, Alexandre Riazanov wrote:

> 
> 
> On Thu, Nov 3, 2011 at 8:16 PM, Jeen Broekstra <jeen.broekstra@gmail.com> wrote:
> On 04/11/11 11:22, Alexandre Riazanov wrote:
> 
> I don't have a problem with the OWA in general. The problem is the
> OWA is there even when you don't want it, specifically when you want
>  to be able to specify a piece of data completely and unambiguously.
> 
> IMHO, this has little to do with the OWA. By all means, go ahead and specify your data completely and unambiguously. Me adding additional (or even conflicting) data about your data somewhere on the Web does not suddenly invalidate _your_ data, or make it any less complete or unambiguous. This is a provenance/trust-issue (whose data do you take into account, and whose do you ignore?), not an OWA issue.
> 
> The open world assumption is about allowing anyone to say anything about everything, but it is not forcing you to take what everyone else says at face value.
> 
> 
>  With OWA, you cannot compute the length of a list because somebody
> else can redefine the list somewhere.
> 
> 1) this is not true if you use the rdf:List construct, which specifically models a closed list (indeed, it was introduced into RDF for this very reason).
> 
> Assuming that the list is identified with a URI, someone can add different rdf:first and rdf:rest to it. Indeed, considering provenance may alleviate this problem sometimes. What if you want to trust both sources of information that define the list? 
> 
>  
> 
> 2) even if it were true, what's stopping you from just treating your dataset as closed and computing the length anyway?
> 
> 
> If you have two definitions/descriptions of the list, which length will you report to the user? Both?
> Moreover, you can have two definitions in the same graph from the same source. 
> 
> Another situation is when your graph is growing incrementally and at no point in time you can assume it is complete. Then, if you have bags or sets described with some sort of membership 
> predicates (like rdf:_1), to indicate when the set description is complete you have to use ad hoc tricks, like assigning the cardinality explicitly. It would be much easier with some general syntactic mechanism allowing to say "these are all the elements of the set". Sets, bags, lists
> are data structures. Why treat them as individuals?
> 
> Cheers,
> 
> Jeen
> 
> 
> 
> 
> -- 
> ======================================
> Alexandre Riazanov (Alexander Ryazanov), PhD
> Saint John, New Brunswick, Canada
> Skype: alexandre.riazanov
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> http://www.linkedin.com/in/riazanov
> http://www.unbsj.ca/sase/csas/faculty.php
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