W3C home > Mailing lists > Public > www-rdf-logic@w3.org > June 2002

Re: properties as nodes etc.

From: Seth Russell <seth@robustai.net>
Date: Sun, 30 Jun 2002 13:40:47 -0700
Message-ID: <004b01c22076$6b21c440$657ba8c0@c1457248a.sttls1.wa.home.com>
To: "Enrico Franconi" <franconi@cs.man.ac.uk>, <www-rdf-logic@w3.org>

From: "Enrico Franconi" <franconi@cs.man.ac.uk>

> Let's call R the n-ary relation, RC the class which reifies R, and ri
> the i-th property of RC in correspondence with the i-th argument of R.
> The following would be the additional constraint that RDF is unable to
> state, but that is necessary to properly encode the n-ary relation:
>
> \forall x1...xn . R(x1,...,xn) <-->
>     \exists-unique z . RC(z) \and r1(z,x1) \and ... \and rn(z,xn)

I believe that is already implied by the Model Theory.  Every RDF triple of
the form (<SUBJECT1> <PROPERTY1> <OBJECT1>) is a unique instance of
<PROPERTY1> ....that's what RDF already does for us.

> On June 30, Seth Russell writes:
> > if you want to expliticidly designate that each event is unique,
> > then give it a uri, instead of leaving the node anonymous.
> > Otherwise just continue to add restrictions until only one such
> > event can qualify.  I guess I don't see the problem.
>
> In this way you impose by hand the above uniqueness constraint, by
> explicitly writing the URI for the unique event. But does this URI
> really exist? Most of the time it is just a fictitious object of which
> you want just to state the existence. This would correspond to FORCE
> any user of a database to have an EXPLICIT key for each n-ary
> relationship: most of the time you don't have it -- it is just the
> combination of the keys of the participating entities.

Well like I said there are two ways to do it.  The best way, imho, is to put
sufficeint constraints on the Bnode such that only one event in the world
will be satisfied.   But using URIs works in RDF systems too.  And there are
URI schemes which can be automatically generated, so the URI string does not
need to 'really exist'.

My point is that your objection to RDF for n-array relationships is
unfounded.  Could you please supply us with an actual real world  test case
if you really think RDF is deficient here?

http://robustai.net/mentography/ConceptualDependency2.gif
http://robustai.net/mentography/arity.gif

Seth Russell
http://robustai.net/sailor/
Received on Sunday, 30 June 2002 16:47:10 GMT

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