Re: ADA Objects Help!

[Mats.Weber@matrix.ch (Mats Weber)]

In article <3fjk7t$2km@gnat.cs.nyu.edu>, dewar@cs.nyu.edu (Robert Dewar) wrote:

> "why not packages and package types"
> [...]
> Basically the conclusion is that following this line of thought leads
> (a) to excessive complexity, it is very tricky to work out all the
>  ramifications 

I can't see why unless you provide more precise information. Package
types, _provided they cannot export types or exceptions_, are basically
equivalent to record types, except that they can export subprograms (to be
used as methods).

> (b) to a confusion of concepts, packages are not happy extended in this
>  direction, and it causes a confusion (unification???) of types and packages.
>  The final decision of the Ada design is that this would have confused rather
>  than unified.

I think this is more a question of personal preference.

> (c) the building of the OOP notions on top of derived types is much more
>  powerful and general, and avoids some of the limitations that come from
>  a narrow, distinguished object approach.

Package types do not preclude the use of derivation + class-wide types as
the inheritance mechanism (I think). I built my proposal using subtyping
as the inheritance mechanism, much along what N. Cohen did in his paper on
extendible record types. If I re-did it now, I'd use another mechanism,
probably something like Modula-3's subtyping rules.

Mats