W3C home > Mailing lists > Public > semantic-web@w3.org > July 2010

Re: Subjects as Literals

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 6 Jul 2010 15:51:18 -0500
Cc: Linked Data community <public-lod@w3.org>, Semantic Web <semantic-web@w3.org>
Message-Id: <FC2C642A-88B5-4229-B857-CF4D237C47FD@ihmc.us>
To: Toby Inkster <tai@g5n.co.uk>

On Jul 6, 2010, at 2:05 AM, Toby Inkster wrote:

> On Mon, 5 Jul 2010 17:43:17 -0500
> Pat Hayes <phayes@ihmc.us> wrote:
>> Well, nobody is suggesting allowing literals as predicates (although
>> in fact the RDF semantics would easily extend to this usage, if
>> required, and the analogous structures are allowed, and do have
>> genuine use cases, in ISO Common Logic.)
> Actually, I have suggested allowing them just to make things simpler -
> URIs, blank nodes and literals would all be allowed in any position.
> However, a statement with a literal in the predicate position would be
> officially defined to have no meaning.

Well (and now we really are in the ivory tower, by the way), the  
"right" thing to do here is to follow the semantics. The RDF semantics  
(http://www.w3.org/TR/rdf-mt/#interp) assumes that there is a mapping,  
called IEXT, from the universe to the set of property extensions. That  
universe, called IR in the mathematical jargon part of the spec, has a  
subset LV of literal values, which (well-formed) literals are required  
to denote. OK, so a literal denotes a literal value, to which - like  
anything else in the universe - the 'extension' mapping IEXT can be  
applied, making it meaningful to be used as a predicate in a triple.  
So in fact, the RDF semantics CAN handle triples with a literal in the  
property position, and they can be perfectly meaningful.

If you write something like {  "ab" "cd" "ef" . } and ask me what (the  
hell) it means, of course I have no idea; but then I also, and for the  
same reason, have no idea what { ex:ab ex:cd ex:ef . } means, either,  
until you show me some larger graph which uses these symbols in some  
organized way, ie an RDF "ontology" of some kind. But there is no need  
to declare that such literals-as-property triples MUST be meaningless.  
The semantics (and the entailment rules, etc.., suitably modified)  
work perfectly well on them, they can be allowed to have any meaning  
that anyone wants them to have, as long as they can somehow express  
that meaning in RDF and any surrounding semantic tools with enough  
clarity. And as I've said in other postings, when we allowed such  
constructions into CL, more as a matter of doctrine than with any  
actual applications in mind, to our great surprise they turned out to  
have several useful applications almost immediately. My experience is,  
in fact, that almost any syntactic structure that can be made to  
support a meaning will quickly find a use. Just to get your  
imagination working, one might for example say that a number used as a  
property means the property which raises its subject to that power, so  
it would be true to write

"2"^^xsd:number "3"^^xsd:number "8"^^xsd:number .

However, before I lose any more of my SW friends, let me say at once  
that I am NOT arguing for this change to RDF.

Pat Hayes

> -- 
> Toby A Inkster
> <mailto:mail@tobyinkster.co.uk>
> <http://tobyinkster.co.uk>

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Received on Tuesday, 6 July 2010 20:52:20 UTC

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