union (Re: Review RDF 1.1 Semantics (ED 3rd June 2013))

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.

Received on Saturday, 15 June 2013 18:20:35 UTC