W3C home > Mailing lists > Public > www-rdf-logic@w3.org > June 2001

Re: Reification as nesting

From: David Allsopp <dallsopp@signal.dera.gov.uk>
Date: Wed, 06 Jun 2001 09:25:48 +0100
Message-ID: <3B1DE90C.B6F1CDE6@signal.dera.gov.uk>
CC: www-rdf-logic@w3.org

pat hayes wrote:

> There are two substantive things that nesting needs to be able to do.
> First, it must provide a way to distinguish triples from assertions.
> Second, nesting needs to be recursive, so that one can
> describe subexpressions. That is, a nest might have other nests
> inside it.
> No, three things. 

I didn't expect the Spanish Inquisition... 8-)

> Third, it must be possible to somehow label a nesting.

> Seems to me that all this can be done in one fairly simple way, by
> allowing the subject and object of RDF triples to themselves be RDF
> triples (not reifications of triples, but actual triples.) These
> 'inner' triples are not asserted, and the 'verb' of the triple that
> points to them provides the needed labelling. 

[Examples snipped]
> The only snag I can see with this idea is that it would require some
> work to reconstitute which triples in an arbitrary set of triples
> were being asserted and which were not. That is, if some of this
> 'nested' RDF is simply rendered down into a set of isolated triples,
> then the question of whether one of these rendered-down pieces is a
> top-level (asserted) triple or not will depend on what other triples,
> if any, are pointing to it. 

I guess the options are a) don't do that (i.e. always store the triples
in their two distinct sets)
or b) process the triples on receipt to partition them. Either way
should work. The serialisation syntax would determine whether a) is
enforced for serialised graphs, and the implementations will have to
distinguish the two types of triple anyway.

> If this kind of sensitivity of
> triple-assertions to their context is seen as a problem, there are
> several ways to overcome it, but all the ones I can think of seem to
> require either some extension of the triples model to something
> slightly larger (eg a triple with a marker bit), or some global
> extension of the entire triple-based model to something more complex
> (eg some global notion of context), or some kind of datastructure
> hacking, or else some global labelling convention whereby unasserted
> triples can be recognised. 

I would imagine that people will implement it in all sorts of ways
depending on the situation (and whether or not they are hacking RDF 1.0
code to make it compatible 8-).

> However, I would urge that this degree of
> contextual sensitivity should just be accepted as part of the burden
> of information trafficking, much as checksums are needed to make sure
> that all the packets have arrived. It seems a small price to pay for
> a great gain in expressiveness and flexibility.


David Allsopp.

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Received on Wednesday, 6 June 2001 04:30:17 GMT

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