directSubClassOf, directType, directSubPropertyOf

[VladimirAlexiev]

(This is an addition to the RDFS vocab not a SPARQL feature, but I still think this group is an appropriate forum to post such addition.)

Why?

rdfs:subClassOf, rdfs:subPropertyOf are reflexive and transitive, and rdf:type is transitive over subClassOf.

But reflexivity is most often useless, and having the direct (step, non-transitive) variants of such properties is useful in many situations, eg:

• to display a tree of classes
• for other detailed subclass work (eg see https://github.com/edmcouncil/fibo/issues/1187)
• to implement transitive reasoning more efficiently (eg see https://graphdb.ontotext.com/documentation/enterprise/rules-optimisations.html#transitiveover)

Previous work

• rdf4j has props sesame:directSubClassOf, directType and directSubPropertyOf:
  • https://rdf4j.org/javadoc/3.1.0/org/eclipse/rdf4j/model/vocabulary/SESAME.html
  • http://archive.rdf4j.org/javadoc/sesame-4.1.0/org/openrdf/sail/inferencer/fc/DirectTypeHierarchyInferencer.html
  • https://graphdb.ontotext.com/documentation/standard/query-behaviour.html#how-to-use-rdf4j-direct-hierarchy-vocabulary
  • Note: Update DirectTypeHierarchyInferencer vocabulary to rdf4j namespace eclipse-rdf4j/rdf4j#2460 is a task by @jeenbroekstra to move them to an rdf4j namespace.
• Jena has ReasonerVocabulary, which includes directSubClassOf "The property that represents the direct/minimal version of the subClassOf relationship", directSubPropertyOf and directRDFType.
  • These are Java parameters not RDF props
  • @afs are there RDF props to access these? Or it depends on reasoner? What RDF props does the Jena reasoner make available?

Workarounds

• One could hack it with complex and inefficient queries like this:

select * {
  ?x rdfs:subClassOf ?y
  filter (?x != ?y && 
    !exists {?x rdfs:subClassOf ?z. ?z rdfs:subClassOf ?y. 
      filter(?x != ?z && ?z != ?y)})}

• an approximation is also possible by using Entailment Regimes or vendor specific mechanisms like onto:explicit, eg

# Q1
select * from onto:explicit {
  ?x rdfs:subClassOf ?y
}
# Q2
select * from onto:explicit {
  ?x rdf:type ?y
}

But this doesn't work in some situations:

• if a transitive relation is inserted explicitly (I'm not sure whether directSubClassOf eliminates such relation instance, either)
• in Q2, if rdf:type itself is inferred (eg from rdfs:domain)

Proposed solution

It would increase interoperability if this group can pick a w3c namespace for such properties and standardize the names, and rdf4j and jena agree to expose their props under these standard names

Considerations for backward compatibility

It would be best if rdf4j and jena continue to support these props under both their current names, and the unified names. I hope that such "aliasing" of magic props is easy to achieve.

[afs]

@afs are there RDF props to access these?

There is a general mechanism to get direct forms:

urn:x-hp-direct-predicate:http_//www.w3.org/2000/01/rdf-schema#subClassOf

It can be used via a prefix.

Often it is done by having the inference and base data graphs available in the same dataset so it is a matter of using GRAPH.

[VladimirAlexiev]

Often it is done by having the inference and base data graphs available

I described this as a Workaround above (Q2). Let me clarify: given these triples

:x :p :y.
:p rdfs:domain :a.
:a rdfs:subClassOf :b.

An RDFS inferencer will infer:

:x rdf:type :a. # domain
:x rdf:type :b. # subclass
:a rdfs:subClassOf :a. # reflexivity
:b rdfs:subClassOf :b. # reflexivity

But I'd like only these as "direct":

:x sesame:directType :a. # domain
:a sesame:directSubClassOf :b. # explicit