The statement was made that anything that
could be written in EXPRESS could be written in OWL.
MW: I am not completely
certain of that. Almost always different languages have different capabilities.
However, it is probably true that most of what can be usefully written in
EXPRESS could be written in OWL.
However, some of the constructs in EXPRESS,
particularly those concerning the cardinality and structure of relationships
are not directly obviously expressible in OWL, such as the distinction between
a bag and a set. However, it should be possible to create a first order
interpretation of OWL such that an EXPRESS relationship is a subtype of
'thing', and the relationship constraints are then OWL properties. EXPRESS
Entity and Type also become subtypes of 'thing'
MW: Entity at least should
probably be a subtype of class rather than thing. However, I think the real
mistake in what you are doing here is trying to place one language in another.
My experience has been that doing this produces very clunky results, and that a
much better approach is to look at the new language, and what you were
originally trying to say (before you put it in EXPRESS) and find the best way
to say it in the new language. You are likely to find both things that can be
said in more elegant ways, and things that can be said in less elegant ways, as
well as some things you can now say that you could not say before, and some
things you cannot now say.
This then allows one to construct a second
order interpretation by suptyping Entity, Relationship and Type as STEP generic
entities, such as Product, Version, View, Property, Property-Representation,
Representation-Presentation etc. That is, EXPRESS entities provide an upper
level ontology for STEP in OWL.
MW: Well this is some of the
stuff that is what you really wanted to say. I would start here and look at how
best to put these into OWL directly. However, I would take the opportunity to
clean some things up along the way.
One can then create a third order interpretation,
as is done in the STEP Application Protocols, in which the STEP generic
entities are interpreted in the context of a business process, so that Product
is either a product (AP 203) a part (AP 214) or a technical data package (AP
MW: The main problem here is
the interpretation process, which means that some STEP entities in the
integrated resources are essentially treated as constructs without a particular
meaning attached to them. Again, I would go back to what you really wanted to say,
and eliminate any objects where there is interpretation that is not a subtyping
– at the very least.
Two questions arise. Firstly, one could also
describe other modelling languages such as UML and IDEF1X as first order
interpretations of OWL (in the sense above) (and even of OWL itself). Could one
then compare the expressive power of such formalisation by creating a lattice
of modelling languages? (This would also expose ambiguities in the languages).
Secondly, would the second and third order
interpretations be compatible with anybody else's use of OWL?
MW: Why would anyone’s
use of OWL be compatible with anyone else’s?
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