Re: bNodes as local constants

> 
> Jos writes:
> > > In RDF we have
> > > 
> > > john rdf:type Person   entails   _:x rdf:type Person
> > > 
> > > where _:x is a bNode. The entailment relationship holds because there is
> > > an assignment for _:x which is the same as the   interpretation of the
> > > constant john.
> > > 
> > > 
> > > If we would translate _:x to a local constant, say, x, then this
> > > entailment relation no longer holds:
> > > 
> > > john rdf:type Person   not-entails   x rdf:type Person
> > > 
> > > Although john and x may denote the same individual in some
> > > interpretation, this is by no means the case in every interpretation
> > > which is a model of    john rdf:type Person.
> 
> Michael Kifer replies:
> > Actually this is not a good example. A better way to approximate bnodes is
> > to translate them into skolem in the head and into existentials in the
> > body. This is how it is done in FLORA-2 and (I think I saw somewhere) cwm.
> > 
> > In your example _:x rdf:type Person to the right of "entails"
> > is a query, i.e., it is a body literal. So, it is translated into an
> > existential and everything is hunky-dori. (Existentials in rule bodies are
> > fine.)
> > 
> > This is not to say that this simulation is completely adequate, but this
> > particular case is not a problem.
> 
> I understood Jos to only be talking about facts (not rules) in this
> example.   Is that also your understanding?    When you talk about
> "entails" as if it were a rule, are you making an analogy?

I am not making any analogies. Formally, if P is a set of rules and 
Exists X Q is a query then

P entails Exists X Q

is what we are checking to find answers to the query in a rule-based language.
An existential conjunctive query is even a Horn clause.
This was exactly the case in Jos' example.


	--michael  


> If so, then I think I agree.  Jos, your example manifests the difference
> only when RIF Core is used to express what is entailed, and we never
> need to to do that.  Use FOL to talk about what is entailed, and we
> don't see a difference, do we?
> 
>    rif("john rdf:type f:Person")   |= exists x triple(x, rdf:type, f:Person)
>    rdf("_:john rdf:type f:Person") |= exists x triple(x, rdf:type, f:Person)
> 
> (Neither "john" nor "_:john" can be named in the entailments, since they
> are both in a sense local, and so not in scope for use elsewhere.)
> 
> You mentioned that SPARQL result sets include b-Node identifiers, but
> those are only in-scope for a particular result set (like the x in my
> entailments).   They are not actually revealing the local names which
> might have been used in the data-being-queried:
> 
>          There need not be any relation between a label _:a in the
>          result set and a blank node in the data graph with the same
>          label. 
> 
>          An application writer should not expect blank node labels in a
>          query to refer to a particular blank node in the data. 
> 
>                 -- Current SPARQL Draft [1]
> 
> So I don't see a problem there.  (I'm also kind of baffled on how the
> semantics of local names in RIF are supposed to be different from the
> semantics of file-scope existentials.  Example anyone?  I think it's the
> same question.)
> 
>    -- Sandro
> 
> 
> [1] http://www.w3.org/TR/2007/WD-rdf-sparql-query-20070326/#BlankNodesInResults
>

Received on Wednesday, 25 April 2007 06:19:55 UTC