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

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
>
>


-- 
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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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