Re: Slides for Berlin Data Workshop

On 2/27/19 5:28 AM, Antoine Zimmermann wrote:
> On 26/02/2019 18:01, Gregg Kellogg wrote:
>>> On Feb 26, 2019, at 1:49 AM, Antoine Zimmermann
>>> <antoine.zimmermann@emse.fr> wrote:
>>>
>>> Hello,
>>
>> Hi Antoine,
>>
>
> [skip]
>
>>
>> My understanding of the rationale behind the reasoning that the names
>> of named graphs do not denote those graphs is for the URI case. It’s
>> true, what you say, that there are numerous deployed systems (for
>> better or worse) that have their own interpretation. However, my
>> position is that blank nodes allow for a different interpretation, as
>> they can have no meaning outside of any given serialization. This
>> allows us to allow for the interpretation that blank node names of
>> named graphs _could_ actually denote those graphs. This is evident in
>> the usage of anonymous named graphs in specs such as Verifiable
>> Credentials [1].
>
> Yes, in absence of standards that tell how named graphs should be
> interpreted, IRI-named-graphs _could_ be interpreted differently from
> bnode-named-graphs. But I expect that those who have their idea about
> how to interpret IRI-named-graphs would not be happy that
> bnode-named-graphs should be interpret in a different way.
> That's why a strong assertion like "the only reasonable
> interpretation" is daring.
>
>
>> In any case, these are my own opinions, and not those of either the
>> JSON-LD WG or CG. The purpose of the workshop is to lay out areas for
>> future development, and I think such an interpretation would help
>> bring RDF into line with Notation3 formula and relate to actual
>> real-world use.
>
> Seeing that you are connecting your proposal to N3, I think I
> understand your line of thoughts. It makes sense, as far as I
> understand the N3 specification. But N3 is not a standard, N3 spec has
> some ambiguities, and not all RDF-minded people have N3 in mind when
> they think of named graphs.
>
>
>>> Similarly, slide 5 is not about "Reasoning in JSON-LD": it is
>>> explaining how to map N3 formulas to JSON-LD. Then people can
>>> decide to interpret JSON-LD documents as N3, following slide 5
>>> representation, and do *N3 reasoning*, not "JSON-LD reasoning".
>>> They could also just map this representation to a normal RDF
>>> dataset and apply other kinds of reasoning.
>>
>> Yes, I should be more explicit, it is about a way to extend the RDF
>> model to allow for a syntax to describe things that can later be
>> interpreted for reasoning. I’d like to see some of the N3 reasoning
>> concepts be made more broadly applicable to an extended RDF dataset;
>> one which allows universally quantified variables as resources.
>
> I've seen that N3 is appealing to some semweb enthusiasts and it would
> be good to be able to embed N3Logic inside RDF serialisations like
> JSON-LD. As far as I can see for now, your proposal is nice at the
> syntactic level.
>
> But I would consider that this is analogous to the RDF encoding of OWL
> ontologies. OWL ontologies are defined independently of RDF, according
> to the structural specification. For instance:
>
> Ontology( <o>
>  Declaration( Class(:A) )
>  Declaration( Class(:B) )
>  Declaration( Class(:C) )
>  SubClassOf( :C ObjectUnionOf( :A :B ) )
> )
>
> According to the OWL semantics, "ObjectUnionOf( :A :B )" *denotes* the
> union of what :A and :B denote.
>
> Additionally, OWL defines a mapping from the structural syntax to RDF
> graphs, such that this graph:
>
> <o> a owl/ontology .
> :A a owl:Class .
> :B a owl:Class .
> :C a owl:Class;
>   rdfs:subClassOf _:x .
> _:x owl:unionOf (:A :B) .
>
> represents the same OWL ontology as before. In this graph, the blank
> node _:x is there to "represent", in some sense, the class
> ObjectUnionOf( :A :B ). Yet, under no circumstances can anyone say
> that this bnode *denotes* a set of things. Blank nodes *do not*
> denote, in RDF.
>
> Still, it is possible to interpret this graph to the effect that the
> whole structure "_:x owl:unionOf (:A :B)" is interpreted as the union
> of the instances of :A and :B (using the reverse mapping from RDF to
> structural spec).
>
> Similarly, in an N3 formula like:
>
> <a> <b> {<s> <p> o>} .
>
> the expression {<s> <p> <o>} denotes the graph serialised as "<s> <p>
> <o>", according to what apparently the N3 specification says. There
> could be an N3 mapping to RDF dataset translating this to:
>
> <a> <b> _:x .
> _:x { <s> <p> <o> }
>
> according to which we could interpret the *whole* structure "_:x { <s>
> <p> <o> }" directly as a graph. But from an RDF point of view, the
> bnode _:x should be treated as an existential that, maybe, if there
> was a standard interpretation of named graphs, would map the bnode to
> the RDF graph "<s> <p> <o>", or something else.
>
> But in spite of what OWL says, the RDF graph "_:x owl:unionOf (:A :B)"
> does not have to be interpreted as a set of things. There is the RDFS
> semantics where it's not the case. Even in the OWL 2 RDF-based
> semantics, this is not the case. In OWL 2 RDF-based semantics, _:x is
> interpreted as an existential (as it should, following RDF semantics)
> that declares the existence of some resource that in turn is related
> to a set of resources according to the ICEXT function.
>
> This duality of interpretations in OWL (Direct Semantics vs. RDF-based
> semantics) is peculiar and a source of misunderstanding, but there is
> worse: RDF graphs can be interpreted in extremely different ways, even
> in standards! An R2RML mapping is represented as an RDF graph that has
> to be interpreted completely differently from the RDF semantics. A
> SHACL shapes graph is an RDF graph that's also interpreted in a very
> special way.
>
>
> I would like to see different standards specifying different ways of
> interpreting RDF datasets. There could be the "RDF dataset for graph
> metadata" semantics (wink wink Sandro). There could the "RDF dataset
> for contextual reasoning" semantics. There could be the "RDF dataset
> for N3logic" semantics (win wink Gregg).
>
> All those ways of interpreting RDF datasets can coexist without having
> JSON-LD enforce one or another.
>
>
> Best,
> --AZ


+1

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