W3C home > Mailing lists > Public > public-rdf-wg@w3.org > February 2013

RE: Problem with auto-generated fragment IDs for graph names

From: Markus Lanthaler <markus.lanthaler@gmx.net>
Date: Mon, 18 Feb 2013 19:36:43 +0100
To: "'Eric Prud'hommeaux'" <eric@w3.org>
Cc: "'Andy Seaborne'" <andy.seaborne@epimorphics.com>, <public-linked-json@w3.org>, "'RDF-WG'" <public-rdf-wg@w3.org>
Message-ID: <015001ce0e06$e825ff40$b871fdc0$@lanthaler@gmx.net>
On Monday, February 18, 2013 7:08 PM, Eric Prud'hommeaux wrote:

> > Yes. Would that also be the case if bNodes would *not* denote the
> graph they
> > label? As I understand it, if bNodes wouldn't denote the graph, you
> couldn't
> > look up a graph labeled with a bNode ID in a dataset because you
> wouldn't
> > know if that bNode ID denotes that graph or not. Is that correct?
> Aha! Would "does not formally denote the graph" mean there's no usable
> mapping from label to graph? I believe we can factor out whether
> bnodes are permitted as graph labels as this question is arises in
> either case.

I think it does. I think that's exactly the problem that Pat outlined in one
of his previous emails:


> > If you have the following dataset:
> >
> > {
> >   _:b1 x:signature "... signature ..." .
> > }
> > _:b1 {
> >   ... some triples ...
> > }
> >
> > Do the two _:b1 above refer to the same, i.e., the named graph? Does
> this
> > mean that "... signature ..." is the signature of the graph labeled
> with
> > _:b1? Or could it be that the signature is about something completely
> > different?
> Yeah, it'd really be useless if the system were permitted to have _:b1
> (or even <http://a.example/graphs/b1>, for that matter) refer to
> something other than the graph which was paired with that signature.

Unfortunately, I think that's the case for http://a.example/graphs/b1.

> I don't know how to utter that in the semantics doc 'cause I don't
> know what "denotes" means. The semantics that we're trying to avoid
> implying is that dataset1's graph <foo> is the same as dataset2's
> graph <foo>. (Some systems may make such a promise, but it's not
> generally required of e.g. deployed linked data or SPARQL systems.)

Why's that? Why does "foo" not identify (to use a different term) the graph.
The two datasets may contain the same triples of the graph "foo", or maybe
just a subset of them. Why is this different from an IRI in the subject

> All the semantics has to capture is that for a given dataset, there is
> a map from graph label to graph. I suspect we don't want to go a
> step further and say that the mapping is 1:1 because of:
>     {
>       <b1> dc:author "Bob" .
>       <b2> dc:author "Bob" .
>       <b1> owl:sameAs <b2> .
>     }
>     <b1> { ... some triples ... }
>     <b2> { ... some triples ... }

What would be the problem if we would do that?

> I suspect that saying
>     Within a dataset, a graph node label denotes a graph.
>    Graph node labels may appear as subjects or objects in graphs.
> would do the trick, but again, I don't understand what drove us from
> "denotes" to "is paired with".

Neither do I.. I'm trying to find it out because I find it very confusing
and inconsistent. Thanks for your patience. I know this all has already been
debated to death but I wasn't able to find any arguments baking the decision
up (apart from "the SPARQL WG is silent about it").

Markus Lanthaler
Received on Monday, 18 February 2013 18:37:16 UTC

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