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Re: Multiple Inheritance [longish]


In message <3305ED78.1C35@mv.mv.com>, Chris Williams writes:
> Has anybody experimented with in-DB multiple inheritance?

IMO there are two valid uses for multiple inheritance :

1- self-contained classes that you have no good reason to connect at
any single place in the inheritance tree, but whose functionality you
might want in several places (feature-ish classes)

2- classes that have a genuine need to borrow code or properties from
several potential ancestors (some of which may be feature-ish classes
as described in 1)

I personally believe in making multiple inheritance explicit, as any
kind of inheritance rules frozen in language specs will either not
suit anyone's beliefs about the way it should work (except maybe its
implementer's), or attempt to do so and in the process become a hairy
mess (how many of you who program in C++ regularly can write down in
less than 1 hour all the rules about or involving inheritance, without
opening the ARM or the latest ANSI draft standard ?).

I have personally never done this in MOO, but I remember having done
similar things in Smalltalk some years back (Smalltalk also has single
inheritance). AFAIR, I implemented it in the following way :

- Feature-ish classes hang either directly below #1 or below a close
descendant (eg, $thing).

- Classes that need to hang from several points in the inheritance tree
are separated into a call core (feature-ish, contains properties and
verbs specific to the class) and one glue class for each point of
attachment in the IT.

- The glue class is responsible for calling the feature-ish core and
the ancestor. Each instance of the glue class has an instance of the
feature-ish core as a property.

- For simulating multiple inheritance, I do similar things, except that
the glue class has one property for each "simulated" ancestor and hangs
near the root. It calls the core (or doesn't, since in that case it and
the core may be merged) and calls the right verb on the right "ancestor"
as needed to achieve the desired "inheritance".

True, this requires a lot of glue code, but it may possible to abstract
it in a way similar to what #3:huh works, maybe through a multiple-
inheritance-root class, with its own :huh.

Just my $.02

Michel Lavondes (lavondes@tidtest.total.fr), speaking only for himself

"Yea, the heavens shall open and the NP-complete solution be given forth.
ATT executives shall give birth to two-headed operating systems, and 
copyrights shall be expunged. The voice of the GNU shall be heard, but
the faithless will be without transceivers." -- Steve Simmons
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