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Re: question about new-style RDF lists

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Mon, 23 Sep 2002 10:06:51 -0400 (EDT)
Message-Id: <20020923.100651.65456888.pfps@research.bell-labs.com>
To: phayes@ai.uwf.edu
Cc: www-rdf-comments@w3.org

From: pat hayes <phayes@ai.uwf.edu>
Subject: Re: question about new-style RDF lists
Date: Fri, 20 Sep 2002 21:02:30 -0500

> >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.

A weak theory of lists is just like the weak theory of containers in RDF.
No lists are assumed to exist at all, except the empty list.  This seems to
me to be consistent with the RDF/RDFS philosophy, and appears to me to be a
viable theory of lists.

> We have to assume that containers exist, in order to provide 
> interpretations of the container constructions in the language. 

Huh?  Which containers have to exist in RDF?  I don't see any requirement
that any container exists in the RDF MT.

> 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. 

The container properties do have to exist in RDF.  However, containers
themselves do not.

> 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. 

Why?  If you say
	_:x rdf:first ex:foo .
	_:x rdf:rest rdf:nil .
you have just constructed an RDF list, just as
	_:y rdf:type rdf:Bag .
	_:y rdf:_1 foo .
constructs an RDF bag.

> 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.

But there is no RDF need for any lists to exist beforehand.

> 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.

Sure, it is not a strong requirement, but it is a requirement, and can have
noticable consequences.

> Pat Hayes

Peter F. Patel-Schneider
Bell Labs Research
Received on Monday, 23 September 2002 10:07:01 GMT

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