W3C home > Mailing lists > Public > public-rdf-dawg@w3.org > October to December 2010

Re: Another thing on property paths...

From: Axel Polleres <axel.polleres@deri.org>
Date: Fri, 17 Dec 2010 12:14:36 +0000
Cc: "SPARQL Working Group" <public-rdf-dawg@w3.org>
Message-Id: <98541990-5DAC-4894-ABCD-428CB1E75778@deri.org>
To: "Andy Seaborne" <andy.seaborne@epimorphics.com>
On 17 Dec 2010, at 10:54, Andy Seaborne wrote:
> What changes to
> 
> http://www.w3.org/2009/sparql/docs/query-1.1/rq25.xml#defn_evalZeroPath
> 
> are you proposing?

Do I understand correctly, that the current definition does exactly what I propose? :-)

If so - phew :-) Probably, I was just mislead by:

"
Definition: ZeroLengthPath

A zero length path matches all subjects and all objects in the graph, 
            and also any IRIs explictly given as endpoints of the path pattern.
"

in Section 18.1.6. Because this sounded to me like Cartesian product.

Can I suggest to reformulate this as:

"
Definition: ZeroLengthPath

A zero length path matches all subjects, objects, and also any IRIs explictly given as endpoints of the path pattern,
to themselves."

BTW, I am not clear why you include IRIs mentioned in SERVICE patterns, what about IRIs explicitly mentioned in GRAPH 
patterns then? Shouldn't there at least be aremark about why those aren't matched

Thanks for the quick clarification & best, 
Axel






> On 17/12/10 09:53, Axel Polleres wrote:
> > Independent from Jorge's remarks on the comments list, I have had another
> > question on PropertyPaths, particulary about ZeroLengthPath:
> >
> >
> > Admittedly, find the e.g. the zeroLengthPath operator quite unintiuitive at the moment,
> > linking *ALL* nodes with each other...
> > that means that e.g.
> >
> > ?X knows* ?Y
> 
> In the real work probably meant
> 
> ?X knows+ ?Y
> 
> > for not only the transitively linked pairs of resources via the knows property,
> > ALL pairs of nodes in the graph... Actually, I isn't a more standard way of treating 0-length paths just as
> > reflexive, i.e. only linking each node reflexively with itself?
> 
> It's a possibility.
> 
> Do you have pointers to the "more standard way"? (noting that there
> isn't an implied forward or backwards direction to arcs in an RDF
> graph).  ^:p is related to the intuition of the (not currently legal)
> :p{-1}.
> 
> > I would at least find this more intuitive and returning less noisy results, i.e. "what I can reach from one node
> > in 0 steps is just the node itself" sounds intuitive to me. Am I mistaken here? If yes, why?
> > At least, I don't understand why we *need* to return the pairs of all nodes here?
> 
> It does not *need* to - it's a choice point in the design provdiing it's
> consistent with the cases of:
> 
> ?X knows* <Y> ==> ?X=<Y>
> 
> and
> 
> <X> knows* ?Y ==> ?Y=<X>
> 
>         Andy
> ...
> 
> >
> > Can someony explain to me why the reflexive only version of ZeroLengthPath wouldn't work with the rest?
> >
> > Axel
> 
Received on Friday, 17 December 2010 12:15:06 GMT

This archive was generated by hypermail 2.3.1 : Tuesday, 26 March 2013 16:15:44 GMT