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RDF* to RDF mapping (Re: First version of a test suite for RDF*)

From: Pierre-Antoine Champin <pierre-antoine.champin@ercim.eu>
Date: Tue, 24 Nov 2020 09:40:44 +0100
To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, public-rdf-star@w3.org
Message-ID: <6bb325c4-f02f-4e1a-8c25-f1efa39d3cc6@ercim.eu>
Peter,

thanks for this proposal; this is in fact quite similar to what I had in 
mind in my proposal in issue 37 [1].

Two remarks:

1) Is it really necessary that N transforms literals? I find this quite 
intrusive that a simple graph such as:

     :xavier :name "Xavier".

would be encoded as another graph... (of course, you can use a less 
probable marker than "X", but still...).

Since the only rdf*:subject*/rdf*:predicate/rdf*:object triples in the 
encoded graph are those generated by the mapping, I don't think values 
need to be "protected" by N.

2) In my idea, RDF semantics was extended to enforce some dependency 
between subject and subject*, between predicate and predicate*, and 
between object and object*, and to ensure that subject, predicate and 
object were functional (a statement describes only one "fact"). In other 
words, the following things would have been *inconsistent* (under RDF* 
entailment recognizing xsd:integer):

# G1

_:stmt rdf*:object* M("a"^^xsd:integer); rdf*:object 42.

# G2

_:stmt rdf*:object* M("42"^^xsd:integer); rdf*:object 43.

Disclaimer: those semantic constraints make the malformed-literal-bnode 
test [2] problematic, I don't have a solution for that, and I am not 
even sure that this test should be kept (but I put it here for 
discussion). The dedicated semantics in my pull-request [3], however, 
passes this test, and the others (I think).


[1] https://github.com/w3c/rdf-star/issues/37#issue-746823745
[2] 
https://w3c.github.io/rdf-star/tests/semantics/manifest.html#malformed-literal-bnode
[3] https://github.com/w3c/rdf-star/pull/19


On 24/11/2020 00:59, Peter F. Patel-Schneider wrote:
> Here is a mapping from generalized RDF* to generalized RDF that I believe
> satisfies all the test cases:
>
>
> Short and a bit sloppy version:
>
> Define M on RDF terms as follows:
>    M(I) is the string of I for I an IRI
>    M(L) is the long lexical form of L for L an RDF literal
>
> Let L be an injective mapping from RDF triples to "fresh" blank nodes.
>
> Define the mapping -* from a generalized RDF* graph G* to a generalized RDF
> graph G as follows:
>    Pick some embedded triple < s p o > such that none of s, p, and o are
> embedded triples, replace all occurrences of it by L(< s p o >), and add the
> triples
>      < L(< s p o >) rdf:type rdf:Statement >
>      < L(< s p o >) rdf:subject s > if s is not a malformed literal
>      < L(< s p o >) rdf:subject* M(s) > if s is not a blank node
>      < L(< s p o >) rdf:predicate p > if p is not a malformed literal
>      < L(< s p o >) rdf:predicate* M(p) > if p is not a blank node
>      < L(< s p o >) rdf:object o > if o is not a malformed literal
>      < L(< s p o >) rdf:object* M(o) >  if o is not a blank node
>    Finish when there are no embedded triples left.
>
>
>
>
>
> Longer and more careful version:
>
> Let rdf* be an IRI namespace.
>
> Let M be an injective mapping from IRIs and RDF literals to strings starting
> with "X"
>
> Let L be an injective mapping from RDF triples to blank nodes whose range
> excludes an infinite number of blank nodes.
>
> Let N be an injective mapping from IRIs to IRIs, strings to strings, and blank
> nodes to blank nodes that does not include any IRIs in the rdf* namespace or
> strings that start with "X" or blank nodes in the range of L in its range.
> (This is just used to ensure that the encoding of IRIs, strings, and blank
> nodes do not clash with RDF terms in the graphs.)
>
> Define the mapping -* from a generalized RDF* graph G* to a generalized RDF
> graph G as follows:
>    First replace each RDF term by its mapping under N.
>    Pick some embedded triple < s p o > such that none of s, p, and o are
> embedded triples, replace all occurrences of it by L(< s p o >), and add the
> triples
>      < L(< s p o >) rdf*:type rdf*:Statement >
>      < L(< s p o >) rdf*:subject s > if s is not a malformed literal
>      < L(< s p o >) rdf*:subject* M(s) > if s is not a blank node
>      < L(< s p o >) rdf*:predicate p > if p is not a malformed literal
>      < L(< s p o >) rdf*:predicate* M(p) > if p is not a blank node
>      < L(< s p o >) rdf*:object o > if o is not a malformed literal
>      < L(< s p o >) rdf*:object* M(o) >  if o is not a blank node
>    Finish when there are no embedded triples left.
>
>
>
>
>
> peter
>
>
>
>
>
> On 11/23/20 6:30 AM, Pierre-Antoine Champin wrote:
>> Hi all,
>>
>> as per my action assigned on our last call [1], I pushed a first version of
>> a test-suite. The first goal is to provide a set of concrete examples of how
>> RDF* implementations are expected to behave. An HTML rendering of that
>> test-suite is available here:
>>
>> https://w3c.github.io/rdf-star/tests/semantics/manifest.html
>>
>> so that everyone can review, reference, and comment each of the test cases.
>>
>> If "entail" sounds esoteric to you, think of the 2nd graph in each test case
>> as a SPARQL* ASK query, which is expected to return TRUE (or FALSE, in the
>> case of "not entail").
>>
>> Coming next: a similar test suite for SPARQL*.
>>
>>    best
>>
>> [1] https://github.com/w3c/rdf-star/issues/40
>> <https://github.com/w3c/rdf-star/issues/40>
>>

Received on Tuesday, 24 November 2020 08:40:55 UTC

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