Inheritance

[Mark L. Fussell]

Paul, 

No argument with your posting (I decided not to post a similar statement 
after rereading yours), but could you change your terms slightly?  
Although the OO terms themselves were definitely conflated during the 
eighties they have by now settled down to:

    Type:  The declaration of the interface of any set of [objects] that 
conforms to this common protocol. Any set of objects or values 
with similar behavior... [Firesmith+E 95] 

    Class:  A class is the realization of a type. [UML]  The idea of 
class is closely linked...with the description of implementation details 
of software objects [Cook+D 94].

    Type vs. Class: Types classify objects according to a common 
interface; classes classify objects according to a common implementation. 
[Firesmith+E 95] 

    Subtyping: The incremental definition of a new type in terms of one 
or more existing types, whereby the subtype conforms to all of its 
supertypes [an is-kind-of relationship] [Firesmith+E 95]

And subclassing implies implementation-inheritance (i.e. code reuse), 
exactly what you were trying to avoid implying.

So I would suggest rewriting your example to:

> > Anyhow, you can emulate OTHER using subtyping without an explicit 
> > OTHER construct.
> > 
> > <!ELEMENT CITATION (name, author, date, OTHER-CIT* )>
> > <!ELEMTYPE OTHER-CIT> <!-- no constraints on subtypes -->
> > <!ELEMTYPE ANOTHER-TYPE ISA ANY>  <!-- Be explicit about the 
automatic root -->
> > 
> > <!ELEMENT URL (#PCDATA) ISA (OTHER-CIT & ANOTHER-TYPE)>

Which makes it use the standard terminology. 

So, ELEMENTs would be the leaves of a tree/digraph of Types with ANY as
the root.  Note that ISA should formally be IS-A-KIND-OF but that is an
annoyingly long keyword.  (My dog is-a Dog which is-a-kind-of Mammal vs:
My dog is-a Dog which is-a Mammal). 

--Mark
mark.fussell at chimu.com

  i   ChiMu Corporation      Architectures for Information
 h M   info at chimu.com         Object-Oriented Information Systems
C   u    www.chimu.com         Architecture, Frameworks, and Mentoring



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