W3C home > Mailing lists > Public > public-rdf-star@w3.org > October 2020

Re: weakness of embedded triples

From: Pierre-Antoine Champin <pierre-antoine.champin@ercim.eu>
Date: Thu, 15 Oct 2020 13:37:46 +0200
To: public-rdf-star@w3.org
Message-ID: <a4d34f98-2d0d-c3c4-8d9a-451f6f34834c@ercim.eu>

On 14/10/2020 23:13, Peter F. Patel-Schneider wrote:
> Let's make the height example even more stark.
>
>
> :loisLane :believes << :clarkKent :height "6.0"^^xsd:decimal >> .
>
>
> does not imply
>
>
> :loisLane :believes << :clarkKent :height "6.00"^^xsd:decimal >> .
>
>
> I would hope that any Tom, Dick, and Lois can realize that these two literals
> are the same.

I see your point, but this is really a matter of deciding where you put
the boundary...

So I would still prefer to be radical here and consider any lexical
difference as potentially significant.

>
>
> If you want to stick to literals that have to be supported in RDF
>
>
> :loisLane :believes << :clarkKent :name "Clark"@en-US >> .
>
>
> does not imply
>
>
> :loisLane :believes << :clarkKent :name "Clark"@en-us >> .

Are "Clark"@en-US and "Clark"@en-us really different literals, for the
abstract syntax??

I would have thought they are the same (and so the implication above
would hold).

Reading the spec again, I realize that things are not so clear: "Lexical
representations of language tags /MAY/ be converted to lower case", and
then Literal term equality
<https://www.w3.org/TR/rdf11-concepts/#dfn-literal-term-equality>
requires that language tags "compare equal, character by character". So
these 2 literals MAY be considered equal, and the implication MAY
hold... :-/ Add to this that BCP47 explicitly state that language tags
are case insensitive... I'd say that we are in gray area here.

>
>
>
> peter
>
>
>
> On 10/14/20 4:45 PM, Doerthe Arndt wrote:
>> 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
>>
>>
>>

Received on Thursday, 15 October 2020 11:37:56 UTC

This archive was generated by hypermail 2.4.0 : Thursday, 15 October 2020 11:37:57 UTC