Re: Suggestion for Ada 200x - Interface inheritance

[Craig Carey <research@ijs.co.nz>]

On 23 May 2003 21:02:26 -0400, Robert A Duff <bobduff@shell01.TheWorld.com> wrote:

>"Steve" <nospam_steved94@attbi.com> writes:
>
>> Java and C# use the concept of interface inheritance.  What do you think of
>> adding this feature to Ada?
>
>I think it's a good idea.  There is an AI on this, and I think the ARG
>is considering it one of the most urgent ones.  Look up the AI, and see
>what you think.
>

http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00251.TXT

The AI contains this comment:

: ****************************************************************
:
: From: Randy Brukardt
: Sent: Monday, December 04, 2000 10:31 PM
...
: I note that the proposal never seems to state that abstract interface
: types must not define any components. Certainly, the implementation
: model and the entire discussion seem to assume that is the case. (An
: "normal" abstract type can have components defined, they'll be part of
: any extension; but I don't think we want that here.)
:
: ****************************************************************


There seems to be inherent safety in avoiding O.O. and using subrecords.

So... Mulitple Inheritance is doing something like making the parent
record be subrecords of the new derived type.

If the aim is flatten out subrecords so that fields names can conflict
and fields can be accidentally not correctly set to a value, then 
the flattening could be made explicit with a way to rename Ada types.

Here is an example showing variable X holding 4 integers. Due to the
renaming of types, the 1st Integer is identified with the name
   X.A
instead of:
   X.R1.A

A sample extension of Ada:
------------------------------------------------------------------------
type T1 is
   tagged record
      A, B : Integer;
   end record;

type T2 is
   tagged record
      B, C : Integer;
   end record;

type Mixin is
   tagged record
      R1 : <> renames T1;     --  Flatten T1's fields into Mixin record
      R2 : T2;
   end record;

type Mixin renames Mixin.T2;  --  Flatten T2's fields into Mixin record

X  : Mixin := Mixin'(A => 1, R1.B => 2, R2.B => 3, C => 4);

------------------------------------------------------------------------

I regard the idea of X'Class as a way to make it easy to unintentionally
leave fields not-assigned-to. It would be harder to make that mistake
when there are subrecords. The "MI" gain of shifting fields of a
subrecord into the surrounding containing record must be quite slight
if renaming has made it look like that has occurred when it actually
hadn't.


Craig Carey
   http://www.ijs.co.nz/ada_95.htm