Details
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Improvement
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Status: Closed
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Major
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Resolution: Fixed
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None
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None
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Description
From https://github.com/commons-rdf/commons-rdf/issues/56
afs:
RDF 1.1 says "IRIs, literals and blank nodes are distinct and distinguishable." [my emphasis]
http://www.w3.org/TR/rdf11-concepts/#section-rdf-graph
This is a consequence of RDF being an abstract syntax - there is no logic/entailment at this level - it was true in RDF 1.0 but now it is explciitly stated in RDF Concepts.
Distinguishable blank nodes mean that unique characteristics need to align to the Java identity contract.
At least, the same (= RDFTerm.equals) blank node, even when different java objects, must have the same internal string. (.equals)
It's a one-way implicition: same internal string does not imply equality so this works across independent implementations.
An extreme implementation is to always return the same internal string (may not be helpful but should be legal).
afs:
This also related to the proposed BlankNode.ntriplesString().
The choice of output string is dependent on the writing process. It only needs to be unique across the file being written. A choice for output is short forms like ":b0", ":b1" etc.
The ntriples output form is not a unique property of the blank node. I think we should not include ntriplesString in the core common API.
stain:
Not sure what this is proposing, but :-1: to remove BlankNode.ntriplesString - and :+1: to improve the contract text for BlankNode.
I found ntriplesString very useful as it becomes an interoperability point and have (largely) predictable outputs.
The commons RDF API stays very close to the rdf11-concepts http://www.w3.org/TR/rdf11-concepts/ , which I like. The ntriplesString are however trivial to implement - and almost all implementations are probably going to have something like that anyway. I never liked much that the name doesn't include get - but I guess that is because it is a derived value and might need further calculations.
The only contentious part is in BlankNode - so perhaps add a specialization of ntriplesString that clarifies the pitfalls here (as we did with equals). The long paragraphs of BlankNode on the top does not currently help to clarify this.
See the simple implementation of BlankNode for one simple way to deal with those "non-ntriples-valid internal identifiers".
Always keeping an internal UUID field or similar is another - implementations can decide on what is most natural to their implementations - they probably have already dealt with this already, although possibly not within their equivalent of the BlankNode class. The BlankNode is also free to keep an internal reference to the Graph or "local scope" and use that to generate identifiers.
There is no requirement anywhere for Blank Node identifiers to always be re-generated in serialization - this is simply a liberty that is available. A serializer based on Commons RDF can still do that - he can simply ignore BlankNode.ntriplesString and create a temporary Map from internalIdentifier to b1, b2, etc. I do however not see why we need to REQUIRE a serializer do such an operation - that is taking this API beyond its scope and into "best practice" (in which case we would also deal with prefixes, preserving prefix names, canonicalizing URIs, etc).
As an example of the current strength, I was able to write an N-triples serializer in simple by just concatenating the ntriplesString of the components from TripleImpl.toString and then just joining with \n:
This is powerful - for nothing else it's great for debugging. I am not proposing to add ntriplesString() for Triple, as it might need to be much closer to the Graph - but at least RDFNode.toString() could have a default method that calls ntriplesString() (which is 200 times more useful than LiteralImpl 2bd85b1f529302f9 from Object.toString
)
afs:
Some display string is useful but reading the contract for ntriplesString, it is not (just) for display purposes. c.f. Java toString. There is a different in escaping. I see that TripleImpl.toString does not do syntax escaping.
Providing a readable RDFNode.toString() would separate the development dsplay concerns (e.g. no escapes maybe) from serialization concerns.
Some RDF systems implement blank nodes from a sequence (e.g. Mulgara). Actually that policy can be quite convenient for debugging development.
We could include N-Triples in commons-rdf but to me v1 should targetted as "use RDF data". Parsing and serialization is the concern of the implementation. The simple impl is one such example, not a new RDF system (is it?
ansell:
I commented on the pull request to remove some of the tests that test or rely on the BlankNode internal identifier structure, particularly that it be a valid n-triples identifier. However, those tests made it into the merged version because it was otherwise basically okay and we are continually evolving anyway so there is no need to have perfect pull requests at this stage. I will review and merge #55 and then work on any further cases that we may not be testing for yet.
I am all for defining/clarifying the contract for .toString in the API, even if it says that there is no specific escaping or formatting done on the output of .toString.
Supporting N-Triples in the base API seems to be natural for two reasons to me. Firstly, it is the simplest syntax, so it doesn't require any particular optimisations and Triples can be streamed out without relying on a particular framework or serialiser. Secondly, for a long time it has been the sole established test case format for RDF, although it is defined on its own for RDF-1.1, so it is a natural single serialisation to support.
As long as the output of ntriplesString is defined to be implementation and local scope specific for BlankNodes (no confusion with IRI or Literal), I am fine with having it. Given the number of times the BlankNode API references "local scope" right now, we are unlikely to have more users commenting that it is unusual than we already have had for the last 10 years with RDF-1.0.
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- is related to
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COMMONSRDF-8 simple .GraphImpl.add() must clone BlankNode
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- Closed
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- Closed
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