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Re: question about new-style RDF lists

From: pat hayes <phayes@ai.uwf.edu>
Date: Fri, 20 Sep 2002 21:02:30 -0500
Message-Id: <p05111b1fb9b17fe32ee7@[65.217.30.172]>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: www-rdf-comments@w3.org

>Pat Hayes has recently
>(http://www.w3.org/Archives/Public/w3c-rdfcore-wg/2002Sep/0185.html)
>proposed a semantics for the new-style RDF lists. 
>
>This semantics is a divergence from the general RDF and RDFS philosophy
>that minimal solutions are to be preferred.

I disagree: see below.

>  (This is evident in the
>semantics for rdfs:domain and rdfs:range, in particular.)  Why would a
>strong semantics for new-style lists, where all lists exist in all
>interpretations, be chosen over a weak semantics for new-style lists,
>particularly as RDF containers exhibit a very weak semantics?

It all depends on what you mean by 'weak' and 'strong'. Seems to me 
that the style in the draft is in fact the weaker of the 
alternatives, since it doesn't go beyond first-order assumptions in 
the models.  Assuming that lists have to be finite takes us into 
recursion theory.

We have to assume that containers exist, in order to provide 
interpretations of the container constructions in the language. The 
non-list (old) RDF container vocabulary does not provide any 
general-purpose recursive accessing mechanism; each 'place' in a 
container has its unique property for accessing it.  Thus, most 
(all?) of the 'structure' of the containers is hidden in the domain 
of properties. RDF domains are required to contain an infinite set of 
container properties, and nobody seems to find this particularly 
difficult to swallow. Lists are different, however. Allowing 
arbitrary S-expression constructions in the syntax (which is what the 
rdf:first/rest/nil/List effectively does) requires that we have 
things in the domain which can serve to be denotations of all such 
expressions; if we did not, then the list 'constructors' might have 
nothing to construct. The suggested MT only requires an 
interpretation to contain *some* set of lists over the domain: in 
effect, it reproduces the recursive idea implicit in the Sexpression 
syntax but phrases it as a recursion over the universe.

It might be worth emphasizing that simply requiring the semantic 
domain to *contain* some large, even infinite, set is not a very 
strong semantic requirement in itself. Datatyping for example 
routinely requires semantic domains to contain infinite sets of 
integers, strings and so on.

Pat Hayes


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Received on Friday, 20 September 2002 22:02:28 GMT

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