- From: Antoine Zimmermann <antoine.zimmermann@emse.fr>
- Date: Tue, 18 Jun 2013 18:35:17 +0200
- To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>
- CC: public-rdf-wg@w3.org, Pat Hayes <phayes@ihmc.us>
Peter: Le 15/06/2013 20:30, Peter F. Patel-Schneider a écrit : > Another way of looking at this issue is as follows. > > Take an RDF graph G, and then divide the triples in it into two subgraphs, G1 and G2. The meaning of G is can be stronger than combining the separate meanings of G1 and G2. This was true in 2004 and remains true now. > > The simplest example of this (which is also in Pat's response) is: > > Let b be a blank node, > let G be the graph with two triples ex:John ex:child b and ex:Mary ex:child b > let G1 be the graph with one triple ex:John ex:child b > let G2 be the graph with one triple ex:Mary ex:child b What you are saying is that bnodes denote. When you reuse a bnode, it must denote the same thing wherever it appear. That's not the semantics of bnodes. In FOL, if I use the same variable in two formulas, and these formulas have an existential quantifier before the variable, then the set of two formulas is equivalent to a formula with two different variables. You could argue that you are losing information by turning a set of formulas into a single formula, since you have lost the fact that the two were using the same variable, but that fact has no semantic value. It's knowledge about syntax that truth preserving operations do not have to preserve. the triple: ex:John ex:child b is saying that John has a child. the triple: ex:Mary ex:child b is saying that Mary has a child. How can this two pieces of information lead to the conclusion that Mary and John have a child together? Especially since the information would be strictly the same if I had: ex:John ex:child [] and ex:Mary ex:child [] On the contrary, you pretend that bnodes have an identity, they are not exactly equivalent to any other bnode. In any case, it's not possible to have a compliant implementation of union. As soon as you realise union, you produce a serialisation that you can't guarantee to be more than isomorphic to the union. However, it is very much possible to implement merge, with a slight and minimal modification to the 2004 definition. Anyway, if you do not see what's wrong, I'll stick to my objection. I will make a concrete proposal indicating the kind of text I would like to see. AZ. > > In the separate meanings of G1 and G2 John and Mary need not have the same child, so combining these separate meanings doesn't get you back to the meaning of G. > > peter > > > On Jun 15, 2013, at 11:20 AM, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote: > >> Here is my response on unions. I have deliberately not included the previous discussion in this response. >> >> >> In 2004 it was assumed that having two RDF graphs sharing a blank node was a mistake. One way to go would have been to say that it was an error to combine RDF graphs that shared blank nodes. However, the notion of a merge was defined, I think mostly so that there was something to say about what to do in surface syntaxes. >> >> It is already the case that RDF graphs share blank nodes, and the new version of RDF allows for this fact of life. >> >> Now what to do when combining two RDF graphs that share blank nodes? Well, what should happen? It seems ludicrous to say that if you take part of an RDF graph and then combine it back with the graph itself that you get something different from the original graph, so merge doesn't seem to be a viable option. So we are left with simple union. >> >> So combining two RDF graphs that share blank nodes is no longer logically equivalent to conjunction. So what? >> >> So nothing! If the two RDF graphs don't have some inherent connection then they really can't share blank nodes, so combination is conjunction. If they do share blank nodes then they have some inherent connection and it should not be much of a surprise that their combination might not be conjunction. >> >> peter >> >> PS: It should be possible to come up with a more-complex semantics that captures some stronger intuition about blank nodes, but there is then the distinct possibility of ruling out some existing or potential use of blank nodes. Of course the way around this is to expand the expressive power of RDF (to, for example, include explicit existential quantification), but I'm pretty sure that no one wants to go there at this time. > > > -- Antoine Zimmermann ISCOD / LSTI - Institut Henri Fayol École Nationale Supérieure des Mines de Saint-Étienne 158 cours Fauriel 42023 Saint-Étienne Cedex 2 France Tél:+33(0)4 77 42 66 03 Fax:+33(0)4 77 42 66 66 http://zimmer.aprilfoolsreview.com/
Received on Tuesday, 18 June 2013 16:35:41 UTC