Re: weakness of embedded triples

Dear Peter,

you are right with both observations. The question is whether we want 
that behavior or not.

> In https://w3c.github.io/rdf-star/ there is a section on referential opacity.
> The main claim there is that triples are referentially opaque.
>
>
> But embedded triples are much weaker than just being referntially opaque.  To
> see this consider the following RDF* graph under the RDF* version of RDF
> entailment recognizing xsd:decimal and xsd:integer.
>
> :loisLane :believes << :clarkKent :height "6"^^xsd:decimal >> .
>
> In this semantics "6"^^xsd:decimal means the same as "6"^^xsd:integer so one
> would expect that
>
> :loisLane :believes << :clarkKent :height "6"^^xsd:integer >> .
>
> is RDF*-entailed.
>
> But it is not.  There are two reasons for this.
>
> First, there is no requirement that satisfying interpretations for the first
> graph map < :clarkKent :height "6"^^xsd:integer > to anything and if a
> satisfying interpretation does map the triple there is no requirement that its
> ITEXT mapping gives the triple its correct meaning.  (The value of ITEXT for
> the triple could have the real number pi as its third element.)
>
> Second, "6"^^xsd:integer is a different node from "6"^^xsd:decimal. So even if
> the intepretation treats the second embedded triple nicely, and thus gives it
> the same meaning as the first embedded triple, they are still two different
> triples and :loisLane can believe one but not the other.  So very little of
> the semantics of RDF gets into embedded triples.
>

We wanted different that different representations are treated 
differently if they have the same meaning. The reason for that is that 
we expected that RDF* would also be used to make statements about 
triples as they were stated, for example to be able to explain the 
reasoning performed on the triples but also for simple provenance. In 
these cases there should be a difference between

:loisLane :believes << :clarkKent :height "6"^^xsd:decimal >> .

and

:loisLane :believes << :clarkKent :height "6"^^xsd:integer >>

since we still talk about different representations.

>
> Each triple is, in effect, its own context.  So, in an RDFS version of RDF*,
> even if :loisLane believes several triples that should imply another, they
> generally don't.  For example:
>
> :loisLane :believes << :clarkKent rdf:type :man >> .
> :loisLane :believes << :man rdfs:subClassOf :human >> .
>
> Does not imply
>
> :loisLane :believes << :clarkKent rdf:type :human >> .
>
>
>
> So embedded triples are incredibly weak in RDF*.   Making them useful will
> likely require quite a bit of work.

Here, "useful" depends again on your intended use. We wanted to have a 
rather weak semantics which allows users with more complex use cases to 
add their semantics. It is easier to make the semantics more complex by 
adding extensions than to ignore certain parts. I for example remember 
that Jos De Roo announced some time ago that his EYE reasoner supports 
rules on RDF*. Of course that alone would not allow you to cover all 
cases, but it could be very helpful in practice.


>
>
>
> On the other hand, there are some unusual inferences that can be made in
> RDF*.  In an RDF* version of RDFS++ it is possible to state that two triples
> are the same.   The graph
>
> :loisLane :believes << :superman :can :fly >>.
> << :superman :can :fly >> owl:sameAs << :clarkKent :can :fly >> .
>
> is consistent here and implies
>
> :superman owl:sameAs :clarkKent .
> :loisLane :believes << :clarkKent :can :fly >>.

This last case is an interesting one. We indeed wanted the triple

:loisLane :believes << :clarkKent :can :fly >>.

to be a consequence of your statements. The question is whether

:superman owl:sameAs :clarkKent .

should follow (it does indeed follow, just as you describe). We made the 
semantics of embedded triples the way it is to be able to deal with 
blank notes. Here, I can't give a concrete answer whether (at least to 
my understanding) it should be that way. I will think about it (and read 
Pierre-Antoine's thoughts in the mean time, which just arrived as well) 
and come back to you.

Kind regards,
Doerthe



-- 
Dörthe Arndt
Researcher Semantic Web
imec - Ghent University - IDLab | Faculty of Engineering and Architecture | Department of Electronics and Information Systems
Technologiepark-Zwijnaarde 122, 9052 Ghent, Belgium
t: +32 9 331 49 59 | e: doerthe.arndt@ugent.be

Received on Wednesday, 14 October 2020 20:46:09 UTC