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Re: [Fwd: Some questions about Named Graphs]

From: Yuzhong Qu <yzqu@seu.edu.cn>
Date: Tue, 29 Jun 2004 12:12:09 +0800
Message-ID: <00c201c45d8f$4161ee20$fd0b77ca@xobjects>
To: "Jeremy Carroll" <jjc@hplb.hpl.hp.com>
Cc: "www-archive" <www-archive@w3.org>, "pat hayes" <phayes@ihmc.us>
> > 
> > Some questions arised when I read the section 2 "Abstract Syntax and
> > Semantics" of this paper.
> > 
> > 1. 5-tuple <N,V,U,B,L> where: U is a "set" of URIrefs; L is a "set" of
> > literals (both plain and typed); B is a "set" of blank nodes; ......;N
> > is a "set" of pairs forming a partial function from U to V  U  V .
> > 
> >    Are they finite sets? I guess they should be.
> 
> I haven't thought. With RDF abstract syntax we tried to avoid specifying
> finiteness - it seems limiting in some ways.
> I guess I would make a finiteness assumption only when I needed it. The
> named graphs paper is currently so high-level that finiteness doesn't
> seem necessary.

All right.

> >   As to "N is a set of pairs forming a partial function from U to V  U
> >  V", why not a partial function from U to P(V  U  V)? (The power set
> > of ...) because you use "partial function" instead of "partial mapping".
> > 
> 
> thank you, that was a bug. I will work that into the next version.
> 
> > 2. For ng, ng' belonging N with ng != ng'then the blank nodes used in
> > triples from rdfgraph(ng) are all distinct from those used in triples
> > from rdfgraph(ng'), i.e. blank nodes cannot be shared between different
> > graphs named in N.
> > 
> > What's the criteria to test ng = ng'? I guess it depends on the equality
> > of the first elements of the two pairs, i.e. whether or not name(ng) =
> > name(ng'), then it depends on the equality of URIrefs?
> >
> 
> This looks like an editing problem. In earlier versions (maybe
> retrievable from www-archive), we allowed multiple names for a single
> graph. In the current version I am not sure whether we do or not.
> 
> I think I need to look back over the history.
> 
> It seems to me that we could allow multiple names for one-and-the-same
> graph by changing the test to
>   rdfgraph(ng) != rdfgraph(ng') where != is the identity inequality not
> the isomorphism one.

If I get your idear, then it means that 

 rdfgraph(ng) != rdfgraph(ng') doesn't exclude the possiblity of 

 ng rdfg:equivalentGraph ng'.

And whether or not rdfgraph(ng) = rdfgraph(ng') is a syntax issue (may at an abstract syntax level).
 
> > In addition, why you must require "the blank nodes used in triples from
> > rdfgraph(ng) are all distinct from those used in triples from
> > rdfgraph(ng')"?  I mean that the above requirement (or assumption) seems
> > redundant. Even worse, in some cases, two blank nodes within two
> > different graphs may be equal due to the fact that they have the same
> > value for an inverse-functional dataProperty (in OWL), or the two blank
> > nodes are equal to each other within some context.
> 
> No, that's your misunderstanding I think.
> So, two blank nodes in two graphs may denote the same thing, that's OK.
> What we were trying to ensure is that there were not problems at the
> semantic level. We took a conservative line of not extending the RDF
> semantics except where unavoidable. So the blank node condition is
> simply meant to reflect that two graphs in the abstract syntax for named
> graphs look and behave like two separate graphs in the RDF abstract
> syntax. The RDF semantics does not really cover the case when you have
> two RDF graphs sharing a blank node; it really isn't clear what that is
> meant to mean, so we avoid it by this restriction.

OK. The meaning of "distinct" is at syntax level.

> > 3. Can a blank node denote a named graph (or graph)?
> > 
> 
> Yes.
> 
> >    If it can, how about the corresponding syntax?
> >
> 
> No syntax is provided for saying which graph, but for example
> 
> eg:a {
> _:b rdf:type rdfg:Graph
> _:b rdfg:subGraphOf eg:a
> }
> 
> makes _:b a graph and it must be one of the four subgraphs of eg:a.

I see. But what four subgraphs?

Based on my understanding, an rdfgraph without a name doesn't make sense in this framework,does it?

So, a blank node can denote a named graph, but not any unnamed rdfgraph.

> This restriction is not in the earlier TriX paper, but when we came to
> look at the semantics we felt the more conservative line introduced
> fewer questions.

> > 4. As to the Namespace and RDF Schema:
> > xmlns:rdfg="http://www.w3.org/2004/03/trix/rdfg-1/"
> > 
> >    The class "rdfg:Graph" is to represent the concept of named graph,
> > graph, or other thing?
> 
> named graph
> 
> > 
> >    I guess it's about named graph, then why not use the word "NamedGraph"?
> > 
> 
> I prefer graph, for political reasons. It emphasises the continuity from
> RDF. I don't now about anyone else.
> 
> > 
> > Thanks for your concern!
> > 
> > 
> > Yuzhong Qu
> > 
> Jeremy
> 

Yuzhong

Received on Tuesday, 29 June 2004 00:09:55 UTC

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