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

Re: weakness of embedded triples

From: Holger Knublauch <holger@topquadrant.com>
Date: Mon, 19 Oct 2020 18:28:50 +1000
To: public-rdf-star@w3.org
Message-ID: <e1dd8845-7ef5-338f-a3e0-4dc3f714b698@topquadrant.com>
Similar situation here at TopQuadrant, see

     http://datashapes.org/reification.html#uriReification

Holger


On 10/19/2020 6:24 PM, Pavel Klinov wrote:
> This is roughly how Stardog supports RDF* and so far we find it 
> sufficient in the enterprise context. It's pretty easily understood by 
> users familiar with edge properties in the property graph data model, 
> which is one of the most important factors for us.
>
> Cheers,
> Pavel
>
> On Sat, Oct 17, 2020 at 9:54 PM Martynas Jusevičius 
> <martynas@atomgraph.com <mailto:martynas@atomgraph.com>> wrote:
>
>     Does RDF* need new semantics at all? Couldn't it be a syntax-level
>     convention for unique triple IDs?
>
>     E.g. <<s>, <p>, <o>> being syntactic sugar for
>     uri(concat("urn:rdf:id:", hash(str(<s>)), hash(str(<p>)),
>     hash(str(<p>)))).
>
>     For example, the triple
>
>     <<https://www.w3.org/People/Berners-Lee/card>
>     <http://xmlns.com/foaf/0.1/primaryTopic>
>     <https://www.w3.org/People/Berners-Lee/card#i>>
>
>     gives
>
>     URI(CONCAT("urn:rdf:id:",
>     SHA1(STR(<https://www.w3.org/People/Berners-Lee/card>)),
>     SHA1(STR(<http://xmlns.com/foaf/0.1/primaryTopic>)),
>     SHA1(STR(<https://www.w3.org/People/Berners-Lee/card#i>))))
>
>     gives
>
>     <urn:rdf:id:63874e34ff5f326e67e888f6818f72d5033ecb343cadd8c2120281d72cefce4481485c937b6a95a656beaa67c13db29f3d7be801328b7c9125976c5f>
>
>     which essentially would become the "5th element", in addition to
>     quads.
>
>     On Thu, Oct 15, 2020 at 1:38 PM Pierre-Antoine Champin
>     <pierre-antoine.champin@ercim.eu
>     <mailto:pierre-antoine.champin@ercim.eu>> wrote:
>     >
>     >
>     > 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 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 Monday, 19 October 2020 08:29:10 UTC

This archive was generated by hypermail 2.4.0 : Monday, 19 October 2020 08:29:12 UTC