RDF star "working baseline"

RDF-star baseline (WORKING DRAFT 2024.08.06)

ABSTRACT SYNTAX

graph                 ::= triple*
triple                ::= tripleStructure
tripleStructure       ::= subject predicate object
subject               ::= NoLiteralTerm
predicate             ::= iri 
object                ::= term
NoLiteralAtomicTerm   ::= iri | BlankNode
atomicTerm            ::= NoLiteralAtomicTerm | literal
NoLiteralTerm         ::= NoLiteralAtomicTerm | tripleTerm
term                  ::= NoLiteralTerm | literal
tripleTerm            ::= tripleStructure

Notes:

• RDF 1.1 syntax is the above without the tripleTerm category and its dependents.
• A term is denoted by r, a triple by t, and a graph by g.
• Given a triple structure s, we denote the subject, predicate, object of s as s.s, s.p, s.o, respectively.

SIMPLE SEMANTICS

An RDF simple interpretation I is a structure <IR, IP, IS, IL, IEXT, RE> consisting of:

1. A non-empty set IR of resources, called the domain or universe of I.
2. A set IP, called the set of properties of I.
3. A mapping IS from IRIs into IR ⋃ IP, called the interpretation of IRIs.
4. A partial mapping IL from literal into IR, called the interpretation of literals.
5. A mapping IEXT from IP into 2IR x IR, called the extension of properties.
6. ⏩ An injective function RE from IR x IP x IR into IR, called the denotation of triple terms. ⏪

A is a mapping from BlankNode to IR.

Given I and A, the function [I+A](.) is defined over terms, triples, and graphs, and the function [[I+A]](.) is defined over terms, as follows.

• [I+A](r) = IS(r)   if r is a iri
• [I+A](r) = IL(r)   if r is a literal
• ⏩ [I+A](r) = RE([I+A](r.s), [I+A](r.p), [I+A](r.o))   if r is a tripleTerm ⏪️
• [I+A](r) = A(r)   if r is a BlankNode

• [I+A](t) = TRUE   if and only if   <[I+A](t.s), [I+A](t.o)> ∈ IEXT([I+A](t.p))

• [I+A](g) = TRUE   if and only if   ∀ t ∈ g . [I+A](t) = TRUE

A simple interpretation I is a model of a graph g   if and only if   ∃ A . [I+A](g) = TRUE.

The set of all models of a graph g is called models(g).

Simple entailment: g ⊨ g'   if and only if   models(g) ⊆ models(g').

Notes:

• RDF 1.1 simple semantics is the above without the parts within ⏩...⏪ marks.

ABSTRACT SYNTAX OF WELL FORMED RDF

graph                 ::= triple*
triple                ::= tripleStructure
tripleStructure       ::= ( subject predicate object ) | 
                          ( reifier rdf:reifies tripleTerm )
subject               ::= noLiteralAtomicTerm
predicate             ::= iri
object                ::= atomicTerm
noLiteralAtomicTerm   ::= iri | BlankNode
atomicTerm            ::= noLiteralAtomicTerm | literal
noLiteralTerm         ::= noLiteralAtomicTerm | tripleTerm
term                  ::= noLiteralTerm | literal
reifier               ::= subject  
tripleTerm            ::= tripleStructure

Notes:

• Observe that RDF 1.1 is always well formed.

RDF SEMANTICS

RDF semantics is defined over the well formed fragment of RDF.
RDF semantics restricts interpretations as follows:

A RDF interpretation I is a RDF model of a graph g   if and only if

• ∃ A . [I+A](g) = TRUE   and
• all the RDF 1.1 metamodelling stuff   and
• all the RDF 1.1 axiomatic triples stuff   and
• ⏩ metamodelling characterising reification ⏪️   and
• ⏩ axiomatic triples characterising reification ⏪️

The set of all RDF models of a graph g is called rdf-models(g).

RDF entailment: g ⊨ g'   if and only if   rdf-models(g) ⊆ rdf-models(g').

Notes:

• RDF 1.1 RDF semantics is the above without the parts within ⏩...⏪ marks.

TURTLE SYNTAX

A triple term is represented in Turtle as follows:

<<( s p o )>>

The following shortcut for a term (a triple term reifier) is added to Turtle, which can be used in arbitrary subject or object position:

<< e | s p o >>

which translates in Turtle as follows:

<< e | s p o >> p1 o1 .   ==>   e rdf:reifies <<( s p o )>> .  e p1 o1 .
p1 o1 << e | s p o >> .   ==>   e rdf:reifies <<( s p o )>> .  p1 o1 e .

Notes:

• A Turtle graph with triple term reifiers and without triple terms is always well-formed, and a well-formed graph can always be expressed as a Turtle graph without triple terms but possibly with triple term reifiers.