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Re: RDF-ISSUE-79 (undefined-datatype): What is the value of a literal whose datatype IRI is not a datatype? [RDF Concepts]

From: Pat Hayes <phayes@ihmc.us>
Date: Fri, 18 Nov 2011 10:55:02 -0600
Message-Id: <AC595FC0-7C1D-4DB2-9A27-D593DA2AC7A4@ihmc.us>
To: RDF Working Group WG <public-rdf-wg@w3.org>, RDF Working Group Issue Tracker <sysbot+tracker@w3.org>

On Nov 9, 2011, at 2:53 PM, RDF Working Group Issue Tracker wrote:

> RDF-ISSUE-79 (undefined-datatype): What is the value of a literal whose datatype IRI is not a datatype? [RDF Concepts]
> http://www.w3.org/2011/rdf-wg/track/issues/79
> Raised by: Richard Cyganiak
> On product: RDF Concepts
> The RDF Concepts spec (in both 2004 and 1.1 versions) does not answer the question what's the value of a literal where the datatype IRI doesn't actually denote a datatype, like <"foo",http://example.com/not-a-datatype>. This is surprising, as there is a section that normatively defines the value of *all other* literals.

I dont find it surprising, and I think you have slightly mischaracterized it. A typed literal only has a fixed meaning relative to an actual datatype. So, to fix the meaning, you have to invoke a datatype denoted by the datatype URI. If this is not available, then the literal's value is not determined, and it becomes in effect something like an unknown URI. But why is this surprising? The typed-literal syntax implies that the datatype IRI is going to denote something that will determine its (the literal's) value. If that is not available, it has no determined value. Seems kind of obvious (and natural) to me. 

> There are many possibilities:
> (i) the spec leaves it undefined
> (ii) that's not a valid RDF graph
> (iii) it's a valid RDF graph, but the value, if any, is unknown
> (iv) it's a valid RDF graph, and the literal is ill-typed
> This should be made explicit.
> The status quo is (i). I believe that the model theory says it's (iii).

Yes, it is (iii) at the moment, if by "valid" you mean syntactically correct. (See below.) However, the semantics does (rather vaguely) talk about the possibility of having datatypes "declared" in a graph (see end of section 5.1, http://www.w3.org/TR/rdf-mt/#DTYPEINTERP ):

"If every recognized URI reference in a graph is the name of a known datatype, then there is a natural datatype map DG which pairs each recognized URI reference to that known datatype (and 'rdf:XMLLiteral' to rdf:XMLLiteral). Any rdfs-interpretation I of that graph then has a corresponding 'natural' DG-interpretation which is like I except that I(aaa) is the appropriate datatype and the class extension of rdfs:Datatype is modified appropriately. ApplicationsMAY require that RDF graphs be interpreted by D-interpretations where D contains a natural datatype map of the graph. This amounts to treating datatyping triples as 'declarations' of datatypes by the graph, and making the fourth semantic condition into an 'iff' condition. Note however that a datatyping triple does not in itself provide the information necessary to check that a graph satisfies the other datatype semantic conditions, and it does not formally rule out other interpretations, so that adopting this requirement as a formal entailment principle would violate the general monotonicity lemma described in section 6, below."

You can never know that literal is ill-typed unless you have the datatype to check that it is, so (iv) can't ever be right. 

(BTW, this phrase "valid RDF graph" seems to be blurring its meaning. In the semantics document, validity refers to truth in interpretations: being invalid means that a graph is false in every interpretation, ie it cannot be satisfied. It does not mean syntactically illegal. Validity in this sense requires an inference engine to check, not a parser. I know that "valid" has many meanings, but just wanted to make sure we don't start talking past one another, or at least be aware of it when we do, cf. this thread.)



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Received on Friday, 18 November 2011 16:55:43 GMT

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