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Re: rdf:value and RDF Schema (was: typed containers in RDF Schema)

From: Garret Wilson <garret@globalmentor.com>
Date: Tue, 11 Jun 2002 12:32:48 -0700
Message-ID: <02cf01c2117e$c55a5be0$7245a640@odysseus2001>
To: <fmanola@mitre.org>
Cc: <www-rdf-comments@w3.org>, "Brian McBride" <bwm@hplb.hpl.hp.com>

Frank,

Oh, this is getting curiouser! More below...

----- Original Message -----
From: "Frank Manola" <fmanola@mitre.org>
To: "Garret Wilson" <garret@globalmentor.com>
Cc: <www-rdf-comments@w3.org>; "Brian McBride" <bwm@hplb.hpl.hp.com>
Sent: Tuesday, June 11, 2002 11:31 AM
Subject: Re: rdf:value and RDF Schema (was: typed containers in RDF Schema)


> You're
> interpreting the RDF Schema as necessarily defining *redundant*
> information that you can check for consistency against the instances
> (e.g. that if dc:creator has a range of Person, the resource that's the
> value of a dc:creator must have a rdf:type property with value Person),
> but technically the Schema can be equally interpreted as being
> additional descriptive information about instances (so if you find a
> resource that's the value of a dc:creator, you can assume it's of type
> Person because the schema says it must be).

Oh, my. You're saying that RDF Schema is descriptive rather than
prescriptive! Heavens (quoting a good friend of mine)!

So my RDF Schema processor gets a canonical RDF Schema for "foo:" (it just
knows which one to use---we all can accept that step, I think), and the RDF
Schema says that the rdf:range of dc:creator for foo:Publication is
foo:Person. I then feed the following RDF instance into the RDF procesor:

<foo:Publication>
  <dc:creator>
    <rdf:Bag>
      <rdf:li rdf:resource="urn:x-people:jane-doe/>
      <rdf:li rdf:resource="urn:x-people:john-smith/>
    </rdf:Bag>
  </dc:creator>
</foo:Publication>

If you'll read my previous "typed containers in RDF Schema" posts (which
assumed RDF Schema to be prescriptive), I had thought that the problem would
be that the RDF instance above would be invalid---it couldn't see that the
*members* of the rdf:Bag object of dc:creator contains Persons.

Now, with a *descriptive* RDF Schema, it gets worse---the RDF Schema says
that, whatever you might think about this RDF instance, the rdf:Bag is
really of rdf:type Person.

With each correspondance, I think I'm understanding RDF Schema more and
more---and seeing its utility less and less...

> Depends on the Schema processor, because once again, you're assuming a
> particular interpretation of these additional schema declarations.

Darn interpretations! Hey, I have an idea: why doesn't someone invent a
description language that would unambiguously encode semantics that would
only be open to a single interpretation? ;)

> For
> example, if we'd declared an Age property with a domain of Person, that
> wouldn't necessarily mean that every Person resource has to have an Age
> property value (it would in an OO type system, but not in RDF).  That
> would actually be a separate constraint (not currently expressible, I
> think, in RDF), saying that if there is a Person resource, there must be
> a triple with that Person instance as subject, and Age as predicate (and
> some value as object); (and enforcing that constraint would make some
> assumptions about our being able to find that triple.

Well, that's certainly clarifying things, but again it begs the question of
RDF Schema utility.

I want something I can:

1. Give to those metadata-makers so they can create RDF metadata that that
expresses unambiguous meaning but is constrained to the vocabulary's rules.
2. Give to an RDF tool so that it will allow a GUI creation of an RDF
metadata instance constrained to the vocabulary's rules.
3. Use to determine whether the metadata given to my by others matches the
constrains of the vocabulary's rules.

RDF Schema no longer seems appropriate for any of these.

Garret
Received on Tuesday, 11 June 2002 15:34:28 GMT

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