Prologue and epilogue aspects

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

While we are at [limited] controlled interfaces why not to make them safer?

I would propose a simple aspect Composition for primitive procedures 
(including interfaces). The aspect has values: Prologue | Override 
(default) | Epilogue | Final

    procedure First (...) with Composition => Prologue;

when overridden the parent type's body is called before the override.

    procedure Last (...) with Composition => Epilogue;

when overridden the parent's body is called after successful completion 
of the override.

    procedure Always (...) with Composition => Final;

when overridden the parent's body is always called after completion of 
the override. The exceptions from the override are re-raised after 
successful execution of the body.

The aspect is inherited if overriding does not change it.

[Variant: the aspect cannot be changed if the override does not have a 
full view of the type.]

The aspect inherited from interfaces is overridden by the aspect 
inherited from a full type [diamond inheritance].

Conflicting aspects from multiple interfaces must be explicitly overridden.

----------------------------------------------------------------
Now Ada.Finalization will declare Controlled as follows:

    type <anonymous> is interface;
    procedure Initialize (Object : in out <anonymous>) is
       abstract with Composition => Prologue;
    procedure Adjust (Object : in out <anonymous>) is
       abstract with Composition => Prologue;
    procedure Finalize (Object : in out <anonymous>)
       abstract with Composition => Epilogue;
--
-- Old code will continue "enjoying" manual calls to parent
-- Initialize, Adjust, Finalize
--
    type Controlled is abstract new <anonymous> with private;
    overriding
       procedure Initialize (Object : in out Controlled) is
          null with Composition => Override;
    overriding
       procedure Adjust (Object : in out Controlled) is
          null with Composition => Override;
    overriding
       procedure Finalize (Object : in out Controlled) is
          null with Composition => Override;
--
-- New code with have things as they should have been
--
    type Controlled_Interface is new <anonymous>;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de