[sc34wg3] Association items

[Murray Altheim]

Lars Marius Garshol wrote:
> * Patrick Durusau
[...]
> | Although it is represented in syntax as a "unary" association,
> | shouldn't it be modeled as an association with two roles, etc.?
> 
> Why would we do that? What would be the type of the other role, what
> would be the player? IMHO, unary associations are very useful for
> expressing binary conditions like
> 
>   is-consistent(lmg : consistent)
> 
> and I can't really think of an implied second role that would make any
> sense.

No, because there is no second player, hence no second role.

> | This is not the only place where the issue comes up, as topic name,
> | for example, is mentioned as a specialized kind of occurrence (TMDM,
> | page 11) and it is later noted that an occurrence is a specialized
> | form of an association (TMDM, page 13).
> 
> Well, you can't have a "unary name" or "unary occurrence", so this is
> not a problem. If you have a name or an occurrence they both *must*
> have a topic they are attached to.

Actually, "unary name" and "unary occurrence" do sound a bit strange,
but are accurate descriptions from an ontological modeling perspective.
Unary associations are also called monadic predicates, which are by
some called "properties" (e.g., Sowa's "Knowledge Representation", p469).
Topic names and occurrences are what we call "Topic characteristics"
but what in the KR/OntEngr field would often call properties of the
Topic. They are related to the Topic via a monadic predicate. Had we
decided not to provide the syntactic sugar in XTM, they could have been
modeled using typed <association> elements.

> | NOTE: I am NOT suggesting any change in syntax, processing, etc.,
> | but am asking if the "unary" association in syntax should be modeled
> | as though it were an association with two (or more) roles, role
> | players, etc. 
> 
> Well, this would be a change in processing, but if it were the right
> thing to do we might change it anyway. So far I've seen nothing to
> indicate that it is the right thing to do.
> 
> | Some parts of which are implied, for example with topic name if it
> | is considered ultimately to be a form of an association (I take it
> | the roles are implied even though the role players are known).
> 
> Yes, you could think of them as being implied, but in practice that's
> not a consideration as in the structure there aren't actually any
> roles, and so what their types might be is purely an academic
> question.

In the structure there certainly are implied roles, and form the
semantics of the association. The roles are "entity" and "name",
or some such labels. I don't think of this as purely academic (and
I've demonstrated over the past few years I'm hardly purely academic).
It's a matter of what in ontological engineering is your common,
everyday modeling. I can't think of an instance where n-adic relations
have no roles -- there's *always* a role, even in a monadic relation.

> | PS: For those of you who are interested, the example that Lars' gives
> | above:
> | 
> | is-inquisitive(jan : person)
> | 
> | Can be  seen  as  two roles: characteristic/person, with two role
> | players: is-inquisitive/jan.
> 
> You mean that it would be equivalent to
> 
>   is-inquisitive(jan : person, is-inquisitive : characteristic)?
> 
> That seems like a bizarre interpretation to me. Why would we do this?

Well, that's precisely what we did in ISO 13250 and XTM 1.0. We have
been calling these things "Topic characteristics" and the relation
they have with the Topic is a monadic, property relation. As Patrick
points out, the role is basically characteristic/entity or character-
istic/Topic (in our case).

[I happen to have expanded on the property relation by adding the
semantics to the entire affair to mimic Faceted Classification,
where the properties all fit within their own taxonomy. In my
extension of FC, they fit into their own ontology (since a
taxonomy is just a poor cousin to an ontology).]

Murray

......................................................................
Murray Altheim                          http://www.altheim.com/murray/
Strategic and Services Development
The Open University Library
The Open University, Milton Keynes, Bucks, MK7 6AA, UK               .

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