Re: XML schema and RDF datatypes [was: comments...]

From: Peter F. Patel-Schneider (pfps@research.bell-labs.com)
Date: 10/02/01


From: Dan Connolly <connolly@w3.org>
Subject: Re: XML schema and RDF datatypes [was: comments...]
Date: Tue, 02 Oct 2001 14:42:00 -0500

> "Peter F. Patel-Schneider" wrote:
> > 
> > > [Dan Connolly]
> > > >It's also possible to design a language where the type of
> > > >a literal may *depend* on a declaration from an XML schema:
> > > >
> > > >     <kr:KRLang xmlns:rdf="http://...new-kr-lang..."
> > > >             xmlns:ex="http://example/vocab">
> > > >     <ex:Person>
> > > >       <ex:name>John Doe</ex:name>
> > > >       <ex:shoeSize>10</ex:shoeSize>
> > > >     </ex:Person>
> > > >
> > > >so that the "10" above is not a logical constant at all;
> > > >not until you find a/the schema for http://example/vocab
> > > >do you know how to parse/interpret "10"... i.e. the
> > > >meaning of that chunk of XML is dependent on all the
> > > >trust issues around following links from one document
> > > >to another (not to mention a complete implementation
> > > >of XML Schema, an effort several orders of magnitude
> > > >larger than an RDF 1.0 parser).
> > > >
> > > >This sort of language is not a candidate for a future
> > > >version of RDF: it fails to meet
> > > >one of the basic requirements of RDF: that an RDF document
> > > >stands on its own as a logical formula.
> > 
> > I fail to see how this follows.
> 
> I'm not sure I can explain it any better. Maybe Pat H. can;
> he seems to have understood.

My point is that it is possible to create a semantics for RDF/RDFS/DAML+OIL
in which literals are untyped but still functions quite well as a logical
formalism in which each document (or any other collection of statements)
has a perfectly well-defined meaning.  For example the datatype extension
to DAML+OIL works this way.  To me, this forms a counterexample to the
conclusion of the rule.

> > I would also appreciate a pointer that provides some measure of support for
> > the premise of this inference.  (In fact, I would find it very instructive
> > to see a concise, authoritative enumeration of the ``basic requirements of
> > RDF''.)
> 
> Er... I'm not sure what sort of authority you're after.
> The RDF Model & Syntax WG didn't make requirements part
> of the 1.0 spec.

Yes, and I view this as a serious problem.

> This excerpt is from a NOTE that has no formal standing,
> but perhaps you'd find it convincing regardless of the source...
> 
> [[[
> Lack of ambiguity 
> 
> Some programming languages allow one to introduce identifiers
> from new name spaces in such a way that it is not possible to
> know which namespace a local identifier belongs to without
> accessing both the module interface specifications and checking
> which one has with the highest priority, or  most recently in the
> document, redefined a given local identifier. 
> 
> This may have some uses in a programming language such as
> Java[Java], but it has a serious flaw in that when one module
> changes (without the knowledge of the designers of the other
> module), it can unwittingly redefine a local identifier used by the
> second module, completely changing the meaning of a previously
> written document. Clearly, in the Web world in which modules
> evolve but documents must have clearly defined meanings, this is
> unacceptable. 
> ]]]
> 
> --        Web Architecture: Extensible languages
> http://www.w3.org/TR/NOTE-webarch-extlang#Ambiguity
> W3C Note 10 Feb 1998 
>
> Perhaps you would find a statement from one of the RDF 1.0 editors
> compelling:
> 
> 	"This is indeed an explicit a design decision ..."
> 	-- Ora to www-rdf-logic Mon, 01 Oct 2001 04:28:11 -0400

Yes, I agree that a comment from Ora should have great weight.  However,
Ora's comment was to the effect that RDF should be context free, I think,
and I also think that an approach similar to that of the one in DAML+OIL
would pass muster.

> As to concise, I condense the design principles of RDF
> as follows when I present it:
> 
> [[[
> Semantic Web
> Principles

> 
>      terms grounded in URI space 
> 
>      simple XML usage for use with XSLT etc. 
> 
>      explicit translation to statements 
>          natural language statements 
>          logical formulas 
> ]]]
>  -- http://www.w3.org/2001/Talks/0103daml-kt/slide7-0.html
> 

All fine, and none of which prevent untyped literals with typing provided
by RDF(S) mechanisms, at least in my opinion.

peter


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