Dear John, (01)
> I agree that the distinction is essential, but that approach
> makes it difficult to express all necessary constraints: (02)
MW: Yes. What Chris neglected to mention is that this was a
pre-4-dimensionalist approach for me (c1993), and this is indeed
a problem if you take this approach to combine roles.
>
> > MW: Actually it was a modelling principle, that entity
> types should be
> > based on the underlying nature of a thing rather than a role that it
> > plays.
> >
> > MW: Following this approach relations were also modelled as
> entity types
> > and roles were modelled on the relationships (E-R sense) linking the
> > relations to the domains of the things they related.
>
> But I put all types in a single hierarchy, in which all roles are
> under the type Role. Since the hierarchy is a lattice, it allows
> all possible combinations, such as MaleEmployee or PregnantEmployee. (03)
MW: This is also what we now do with a 4-dimensionalist approach.
However, it is this approach that also demonstrates, as I explained
to Duane that employee is not a subtype of person, taking person
as the person-for-the-whole-of-their-life. However, both employee
and person-for-the-whole-of-their-life are subtypes of what I
tend to call state-of-person, which is any temporal part, or
aggregate of temporal part of a person-for-the-whole-of-their-life. (04)
MW: Let us look at a couple of possibilities here: (05)
1. We could decide that the ordinary sense of person points at not at
person-for-the-whole-of-their-life, but at state-of-person. However,
I think we expect to be able to count people, and get one for each
person-for-the-whole-of-their-life, so this does not seem to be
correct. (06)
2. Accept that, at least under 4 dimensionalism, employee is not a
subtype of person (meaning person-for-the-whole-of-their-life) but
that both person and employee are subtypes of state-of-person. (07)
MW: Note this does not stop roles being subtypes of role (we use the
term participation). We are just being more precise about the
relationship between e.g. employee and person, that is, that it is
a temporal part of a person that is an employee, not the whole of
their life. (08)
Regards (09)
Matthew West
Reference Data Architecture and Standards Manager
Shell International Petroleum Company Limited
Registered in England and Wales
Registered number: 621148
Registered office: Shell Centre, London SE1 7NA, United Kingdom (010)
Tel: +44 20 7934 4490 Mobile: +44 7796 336538
Email: matthew.west@xxxxxxxxx
http://www.shell.com
http://www.matthew-west.org.uk/ (011)
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