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Re: DAML: restricting number of elements in a list

From: Ken Baclawski <kenb@ccs.neu.edu>
Date: Mon, 11 Feb 2002 22:13:11 -0500 (EST)
To: Ian Horrocks <horrocks@cs.man.ac.uk>
cc: Steven Gollery <sgollery@cadrc.calpoly.edu>, www-rdf-logic@w3.org
Message-ID: <Pine.LNX.3.96.1020211210541.7500A-100000@steam>
On Mon, 11 Feb 2002, Ian Horrocks wrote:

> On February 11, Ken Baclawski writes:
> > On Sat, 9 Feb 2002, Ian Horrocks wrote:
> > 
> > > On February 7, Ken Baclawski writes:
> > > > The daml:item property relates a list to each of its elements.  One can
> > > > impose a daml:minCardinality restriction on the daml:item property to
> > > > ensure that the number of vertices in a polygon is at least 3. 
> > > > 
> > > > Ken Baclawski
> > > > Ken@Baclawski.com
> > > > UBOT Project
> > > 
> > > I am afraid that this is a common misconception. You can write such a
> > > thing, but if you read the language specification you will find that
> > > daml+oil does not provide any semantics for it, so it does not have
> > > the effect that you desire (in fact, from the point of view of
> > > daml+oil, it has no effect whatsoever). 
> > 
> > You are correct that the intention of daml:collection is to be unordered,
> > but the axiomatic semantics specifies that DAML lists do have an order,
> > via the daml:first and daml:rest properties.  Furthermore, the axiomatic
> > semantics specifies that daml:item relates a list to each item in the list
> > (item-axiom-2).  Which one specifies the semantics of DAML+OIL:  the
> > axiomatic semantics (written in KIF) or the language specification
> > (written in English prose)? 
> 
> It isn't a question of whether lists are ordered or not (although as
> you point out lists are treated as sets DAML+OIL), it is the fact the
> the list construction forms part of the syntax specification of the
> language. If you check he model theoretic semantics (the definitive
> specification of the meaning of DAML+OIL), you will see how this
> works. 

Where does it say that the model-theoretic semantics is the definitive
specification of the meaning of DAML+OIL?  In the Reference Description it
states: "Two references that give a precise definition of the meaning of
the language constructs are the model-theoretic semantics and the KIF
axiomatization."  I could not find a statement in the Reference
Description that the model-theoretic semantics takes precedence over the
KIF axiomatization, nor a statement that either one is the "definitive
specification".

It seems that there are at least 4 specifications of DAML+OIL, and none of
them is fully consistent with any other:

1. Reference Description
2. Revised Language Specification
3. Model-Theoretic Semantics
4. Axiomatic Semantics

For example, the Axiomatic Semantics formally specifies lists, while the
Model-Theoretic Semantics handles lists informally.  In the Axiomatic
Semantics and the Revised Language Specification, the daml:List class
participates in the class hierarchy (e.g., List-axiom-1), while in the
Model-Theoretic Semantics daml:List, daml:first, daml:rest and daml:item
are not even mentioned.  The Reference Description states that lists are
unordered, but this is not reflected in any of the other specifications.

It isn't clear to me how this situation contributes to a "clean and well
defined semantics" as claimed by the Reference Description.

> The confusion is caused by the fact that RDF does not separate
> modelling layers, so the "meta-model" (i.e., the specification of the
> syntax of the language) is mixed up with the model (an ontology
> defined using the language).

IMHO, it doesn't help matters for one of the specifications to be
independently "correcting" this problem while the other specifications do
not.  Whatever solution is chosen for this problem should be consistently
reflected in all of the specifications.

Ken Baclawski
Ken@Baclawski.com
UBOT Project
Received on Monday, 11 February 2002 22:13:19 GMT

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