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Re: Why contexts? (was: Klyne Contexts: 5.3-5.5 resources, languages and frames)

From: Jonas Liljegren <jonas@rit.se>
Date: 28 Nov 2000 15:56:24 +0100
To: Graham Klyne <GK@Dial.pipex.com>
Cc: RDF interest group <www-rdf-interest@w3.org>
Message-ID: <87g0kcrvqf.fsf@jonas.rit.se>
Graham Klyne <GK@Dial.pipex.com> writes:

> I believe that resources are too low-level a concept for describing
> complex systems.  My intuition, which I am planning to demonstrate
> through examples built over the coming months, is that construction
> of complex models requires more flexible, higher-level constructs
> than resources and statements.

I would like to see those examples.

> I must be clear, I have no reason to believe that resources and
> statements are not _logically_ sufficient to describe complex systems
> -- I am not seeking to build a logical framework beyond that
> supportable by RDF;  rather it is the _expressivity_ that concerns me.
> I am seeking to create structures that do for information modelling
> what functions and subroutines do for computer programming:  allow us
> to create higher-level constructs from lower level pieces.
> There is one particular issue I have identified when trying to
> construct complex system descriptions:  it is difficult to construct a
> description from just nodes and arcs without having an prior knowledge
> of the complete ontological framework.  I contend that when describing
> complex systems we do not start out with complete knowledge of the
> ontological structure that will describe that system.  Development of
> the statements and the ontology move forward together.

Yes. It's important that RDF data can handle schema evolution.  My
hope and vision is that RDF will solve the compability problem.  We
should not have to upgrade the data in orde to use new schemas.

I have taken TBL's concept of conversions [1] as the basis
for handling schema evolution.  You can inventy your own predicates
for what you want to say. Someone else can later state that those
predicates has some relation to other predicates.

As an example:  Say that we have a couple of statements about peoples

 p1 --type--> Person
 p1 --name--> "Jonas liljegren"

 p2 --type--> Person
 p2 --first_name--> "Graham"
 p2 --last_name--> "Klyne"

What happens if I make a query to retrieve the names of all persons
from the DB?  We can have conversion rules what will infere the
implicit properties.

A simpler form of this is the stating that one predicate is an alias
of another predicate in another schema/model.

Some conversions rules can be encoded in RDF.  Other conversion rules
can be programmed (hard coded) into applications.

One conversion rule used in Wraf, is the one from RDFS that inferes
implicit rdf:type properties from the rdfs:subClassOf of the explicit
types.  (Please ask for clarification if this is of intrest.)

[1] http://www.w3.org/DesignIssues/Semantic.html

> One of my reasons for using contexts is that it gives me a framework
> for making statements that do not depend on detailed ontological
> structure.  Thus, in the 1st example you cite, I know that my car has
> an engine and a body without necessarily knowing how they are
> ontologically related to the car as a whole, and I can make meaningful
> statements about them.

You could use my:has as a predicate.  That would suffice.  Your
example doesn't make any connection between MyCar and TheBody.

> >One example says, in part:
> >
> >       [MyCar] --isa--> [FordEscort]
> >       [     ] --rdfc:asserts-->
> >         {
> >         [TheBody] ----color-----> "red"
> >         [TheEngine] --capacity--> "1600cc"
> >         }
> >
> >
> >The normal way to do this would be:
> >
> >  S1: [my:Car] --type--> [FordEscort]
> >  S2: [my:Car] --body--> [my:Body]
> >  S3: [my:Body] --color--> [red]
> >  S4: [my:Car] --engine--> [my:Engine]
> >  S5: [my:Engine] --capacity--> [my:Capacity]
> >  S6: [my:Capacity] --unit--> [cc]
> >  S7: [my:Capacity] --value--> "1600"
> By comparison, this form of description requires that the ontological
> relationship between the components is known.

Well. You will have to know something about their relation.  You could
choose between the very general predicates my:isRelatedTo and
my:isNotRelatedTo.  More precise relations can later be added or

> >Maby should continue this discussion in rdf-logic?
> >   Haven't compared this to DAML yet...
> My focus here isn't primarily on inferencing, and I'm not trying to
> define an alternative DAML or similar framework.  As far as possible,
> I'm trying to avoid issues of pure logic, other than how they affect
> the construction of descriptions.

I intend to (try to) implement logic in the form of conversion rules,
starting by conversion rules used in connection to handling of
statements about collections of statements.  I hope that this system
will handle which statements should be considered fact, based on
trust, inference and property distribution.

> My concern is how to use RDF in the process of constructing models of
> complex systems.  We have tried to construct descriptions of systems
> of only moderate complexity, and found that strict ontological
> structures tend to lock us into rigid and inflexible forms of
> expression, making the task of constructing a description very
> difficult, and definitely not practical for our purposes.

I would like to see examples of this.

/ Jonas Liljegren

The Wraf project http://www.uxn.nu/wraf/
Sponsored by http://www.rit.se/
Received on Tuesday, 28 November 2000 09:53:20 UTC

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