W3C home > Mailing lists > Public > w3c-rdfcore-wg@w3.org > October 2001

Re: Literals as subjects, labels for nodes

From: Jan Grant <Jan.Grant@bristol.ac.uk>
Date: Tue, 16 Oct 2001 16:38:49 +0100 (BST)
To: Pat Hayes <phayes@ai.uwf.edu>
cc: Dave Beckett <dave.beckett@bristol.ac.uk>, w3c-rdfcore-wg <w3c-rdfcore-wg@w3.org>
Message-ID: <Pine.GSO.4.31.0110161632270.11492-100000@mail.ilrt.bris.ac.uk>
On Tue, 16 Oct 2001, Pat Hayes wrote:

> >Rather than get into detailed N-Triples syntax issues, can we discuss
> >what this change to the model means; an outline of why it is required
> >and its implications.
> >
> >Consequent syntax changes can be discussed in another thread.
>
> OK, fair enough (though it really is mostly a syntax issue, in fact).
> Let me try to summarize.
>
> First, there are two separate issues/proposals which can be discussed
> separately, though they do fall naturally together:
>
> 1. Provide a way to allow two different occurrences of the same
> literal to be distinguished in the syntax. (must-do)
>
> 2. Allow literals as subjects. (optional)
>
> Let me focus on the first as it is the most important.
>
> In the current draft of the MT document, RDF graphs are required to
> be 'tidy', which is defined to mean that labels uniquely identify
> their node, ie no two nodes have the same label. "Label" here refers
> to both urirefs and literals; that means that, with this definition,
> any literal can occur in a graph only in one place, labelling one
> single node.
>
> This is not acceptable if we are ever going to allow nontrivial
> datatyping, since (if we do) that would mean that we will want one
> occurrence of a literal to mean one thing, and a different occurrence
> to mean something else, so we cannot require that they be the same
> occurrence in the graph. (Exactly how the different occurrences of,
> say, "010701" are inferred to be of type integer, or string, or date,
> or whatever, is another issue that we can discuss separately; the
> present point is only that different occurrences of the same literal
> might be somehow treated differently, so cannot be forced to be
> syntactically identical.) So, if we have (or even contemplate the
> possibility of some extension of RDF ever having) nontrivial literal
> datatyping, then we have to relax the strict tidiness condition on
> literals, and allow the same literal label to appear at several
> places in the graph.

No; because we can consider the typed datum to be the literal, not its
(unicode) representation. These are all distinct wherever we need them
to be, and the "same" for the purposes of tidyness where we need them to
be. If you do this the problem you describe goes away (I think).

> OK, that's the pressing matter. The other matter is less pressing. If
> we were to allow literals as subjects, then this proposed extension
> to the Ntriples syntax could also be used to specify graphs in which
> datatyping information was specified directly, by simply asserting
> that a (particular occurrence of a) literal has a datatype as its
> rdf:type. (see
> http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Oct/0277.html
> )

This is an interesting point. What I'm suggesting is that there is an
"inherent type" associated with a literal. This is distinct from any
assertions made in a graph. We can go one step beyond a model theory by
talking about categorising which interpretations we consider to be
valid; for instance, we might require that

	integer(23) <rdf:type> <xsd:Integer> .

should be ok in all valid interpretations, but

	integer(23) <rdf:type> <foo:NegativeInteger> .

should not be. This becomes less of a problem if you adopt the notion of
multiple "literal instances" as was recently suggested.

> This would provide a simple, obvious mechanism for declaring
> datatypes which would integrate smoothly into both the (extended) RDF
> syntax, and also the proposed model-theory treatment of datatypes.
> (sketched in
> http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Oct/0164.html )
> And I can see no rational reason to prohibit it, other than it
> possibly being beyond our charter. So this seems to me to be some
> extra support for the proposal to allow literals as subjects.
> (However, even if this proposal is rejected, the previous point
> stands on its own. )
>
> I hope this covers the issues adequately.
>
> Pat
>
>

-- 
jan grant, ILRT, University of Bristol. http://www.ilrt.bris.ac.uk/
Tel +44(0)117 9287088 Fax +44 (0)117 9287112 RFC822 jan.grant@bris.ac.uk
"My army boots contain everything not in them." - Russell's pair o' Docs.
Received on Tuesday, 16 October 2001 11:41:30 EDT

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