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Re: {Disarmed} Re: blank nodes (once again)

From: Enrico Franconi <franconi@inf.unibz.it>
Date: Thu, 24 Mar 2011 22:30:58 +0100
Cc: Pat Hayes <phayes@ihmc.us>, Hugh Glaser <hg@ecs.soton.ac.uk>, Mark Wallace <mwallace@modusoperandi.com>, Alan Ruttenberg <alanruttenberg@gmail.com>, Reto Bachmann-Gmuer <reto.bachmann@trialox.org>, Ivan Shmakov <oneingray@gmail.com>, Ivan Shmakov <ivan@main.uusia.org>, Dieter Fensel <dieter.fensel@sti2.at>
Message-Id: <651BFA3F-A63D-406E-9BA0-1CFAF1800B92@inf.unibz.it>
To: "<semantic-web@w3.org> Web" <semantic-web@w3.org>
I believe that there is a major misunderstanding by the non-logicians about bnodes, existential variables, and skolem constants.

At the end of this message I compiled a sequence of statements from this thread which show this misunderstanding.

As we all know it is correct that a bnode in RDF can be equivalently represented as a skolem constant (or as an existential variable, but here I'm not concerned about that).
The whole point is that a bnode once skolemised does NOT behave like any other constant, since it should be UNIQUE all around the world and at any time; indeed, it is a skolem constant. To avoid this impossible requirement in practice, 'scoping' within a graph is introduced, with all the consequences it bears. The scoping annoyances have nothing to do with logicians pretending them :-)
So, the idea of dropping bnodes by transforming them to plain constants would be incompatible with the original bnode semantics, since you can not guarantee the unicity wherever whenever. Under this simplifying idea, bnodes would be just as any other constant and therefore there would be no reason to even consider them.

For this reason, I agree (again!) with Pat: 

On 24 Mar 2011, at 15:45, Pat Hayes wrote:
> Which is why it would probably not be useful or practical to *change* the interpretation of blank nodes in RDF. On the other hand, it might be useful to define a simplified version of RDF which simply does not have blank nodes in it.

If you don't like bnodes, the only option is to drop them completely. This RDFminus data model would become at this point just a special case of the relational data model:

On 24 Mar 2011, at 19:31, Pat Hayes wrote:
>> Why not just create a new triple based rdf-subset compatible specification, call it "web data" or something.
> Or 'ground RDF' . Yes. I confess however that part of my own private agenda at this point is just to see how much easier the entire RDF experience (including SPARQL and OWL and RIF) would be if we simply wrote blank nodes out of RDF altogether. 



On 24 Mar 2011, at 00:44, Dieter Fensel wrote:
> Maybe a straight-forward way is to think about [bnodes] as unique constants, i.e., use the idea of skolemization. I think this is also in line with a proposal of Pat, a down-sized version of the Jos & Enrico paper

On 24 Mar 2011, at 10:08, Graham Klyne wrote:
> bNodes don't fundamentally add expressive power when making assertions about the world.  I.e. that Skolemization achieves the same effect.  I think it was mainly the convenience (maybe not for logicians!) argument that carried the day.

On 24 Mar 2011, at 16:13, Sandro Hawke wrote:
> In particular, I think the system which first exposes the RDF content on the Web should be the one which Skolemizes it, since it knows what URL prefix to use. (If there isn't one such system, then Skolemizing is a problem.) This system has the interesting challenge of minimizing changes if/when it re-reads modified content destined for the same URL. That's the most interesting problem in this space, to me....
> (...)
> In practice, imagine I have a hand authored page of turtle with maybe 150 triples, much of it lists. I click "publish" and it gets Skolemized and published at URL U. Then I change my mind about something, make a tiny edit, click "re-publish" and it gets Skolemized again, and the new version gets published at U. If someone is watching U, I want them to see that only a little change was made. A naive (uuid) Skolemization would make the change look huge, as every blank node got an entirely new label. 

On 24 Mar 2011, at 20:10, Dieter Fensel wrote:
>> There are lots of places where no name is preferable to a name that may mislead or confuse, or the clutter of lots of UUIDs.
> Yes, but for this you do not need existential quantified variables.
> You need a syntactical mechanism to provide you do-note-care-names.

On 24 Mar 2011, at 22:05, Dieter Fensel wrote:
> At 19:31 24.03.2011, Pat Hayes wrote:
>> Or 'ground RDF' . Yes. I confess however that part of my own private agenda at this point is just to see how much easier the entire RDF experience (including SPARQL and OWL and RIF) would be if we simply wrote blank nodes out of RDF altogether.
> Maybe you want to keep them as syntactical sugar.
Received on Thursday, 24 March 2011 21:31:34 UTC

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