Re: RDF* and grouping relation properties

Hi Thomas,

Thanks for your thoughts on this issue. I've come to the conclusion that it
may be best practice in both RDF or RDF* to bundle a set of property-value
pairs as a blank node. However, I'm not sure of your view of the RDF
semantics for a reified triple.

Section 3.3.1https://www.w3.org/TR/rdf-mt/#Reif <http://3.3.1> of the W3C
RDF Semantics recommendation has this example, which seems to contradict
the view that two rdf:Statement instances with the same subject, predicate
and object properties are owl:sameAs

Since the relation between triples and reifications of triples in any RDF
graph or graphs need not be one-to-one, asserting a property about some
entity described by a reification need not entail that the same property
holds of another such entity, even if it has the same components. For
example,

_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject <ex:subject> .
_:xxx rdf:predicate <ex:predicate> .
_:xxx rdf:object <ex:object> .
_:yyy rdf:type rdf:Statement .
_:yyy rdf:subject <ex:subject> .
_:yyy rdf:predicate <ex:predicate> .
_:yyy rdf:object <ex:object> .
_:xxx <ex:property> <ex:foo> .

does not entail

_:yyy <ex:property> <ex:foo> .


The RDF 1.1 Semantics recommendation section on Reification
<https://www.w3.org/TR/rdf11-mt/#reification> also includes this same
example.

When I was first wondering about this I checked the Stardog implementation
and found this was the case for my example.  I've just now checked the
Hermit and Pellet reasoners via Protege and found that they also do not
infer that the two Statement instances are owl:sameAs. Are you familiar
with any developed reasoners that do infer this?

Tim


On Sun, Jun 21, 2020 at 6:14 AM thomas lörtsch <tl@rat.io> wrote:

>
>
> > On 18. Jun 2020, at 17:44, Tim Finin <finin@umbc.edu> wrote:
> >
> > While experimenting with RDF* I realized one issue: for some relations,
> we may have several properties that should be treated as a group.  For
> example, the provenance of a relation extracted from the text of a web page
> might include a link to the page and the date retrieved.
> >
> > Using the following two RDF* expressions merges the four properties so
> that we can no longer determine which :source and :retrieved values go
> together.
> >
> > << :man :hasSpouse :woman >>
> >     :source <http://foo.com/>;
> >     :retrieved "2020-06-17"^^xsd:date .
> > << :man :hasSpouse :woman >>
> >     :source <http://bar.com/>;
> >     :retrieved "2020-01-01"^^xsd:date .
> >
> > Using a traditional RDF reification approach maintains the pairing.
> >
> > :man2 :hasSpouse :woman2 .
> > [ ]  a rdf:Statement ;
> >      rdf:subject :man2 ;
> >      rdf:predicate :hasSpouse ;
> >      rdf:object :woman2 ;
> >     :source <http://foo.com/> ;
> >     :retrieved "2020-06-17"^^xsd:date .
> > [ ] a rdf:Statement ;
> >     rdf:subject :man2 ;
> >     rdf:predicate :hasSpouse ;
> >     rdf:object :woman2 ;
> >    :source <http://bar.com/>;
> >    :retrieved "2020-01-01"^^xsd:date .
>
> In my understanding of the RDF Standard Reification semantics your two
> blank nodes are owl:sameAs as the reification quad refers to the abstract
> triple type, not any concrete token. That contradicts the intuition of the
> provenance statements (:source and :retrieved) but that’s just how things
> are right now in RDF. There is no sound meta modelling in RDF.
>
> It would of course be possible to define subclasses of rdf:Statement, like
> rdf:TripleType and rdf:TripleToken, define a context (e.g. a Named Graph)
> in which a triple becomes an actual token, and then reify such tokens. Only
> then the above provenance statements would have sound semantics.
>
> To the best of my knowledge RDF* binds its reification semantics back to
> RDF Standard Reification. The RDF* <<…>> construct is syntactic sugar for
> the RDF Standard Reification quadlet - nothing more, nothing less [0]. So
> merging those provenance statements is indeed the right thing to do. One
> might even argue that RDF* has a slight advantage over RDF Standard
> Reification as it represents the semantics more faithfully. However not
> what anybody would expect from the solution to reification in RDF.
>
> Thomas
>
>
> [0] Well, not exactly, as there are those modes too - SA and PG - which
> may implicitly add the cited triple to the graph or not. That’s a proposal
> and I’m not sure if implementations support mode switching or which mode(s)
> they support.
>
>
>
> > A possible solution when using RDF* is to encapsulate associated
> properties as a blank node entity, as in the following
> >
> > :man3 :hasSpouse :woman3 .
> > << :man3 :hasSpouse :woman3 >>
> >     :provenance [ :source <http://foo.com/>;
> >                            :retrieved "2020-06-17"^^xsd:date ] .
> > << :man3 :hasSpouse :woman3 >>
> >     :provenance [ :source <http://bar.com/>;
> >                            :retrieved "2020-01-01"^^xsd:date ] .
> >
> > However, this approach seems to violate the normal key/value pattern of
> property graph properties, which could be a compatibility issue.
>
> >
> >
> > --
> > Tim Finin,  Willard and Lillian Hackerman Chair in Engineering,
> Computer Science and
> > Electrical Engineering, U. Maryland, Baltimore County, 1000 Hilltop
> Circle, Baltimore MD
> > 21250. http://umbc.edu/~finin, finin@umbc.edu, tfinin@gmail.com,
> mobile:410-499-3522
>
>