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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Preventing type extensions
Date: Mon, 13 Sep 2010 23:21:28 +0300
Organization: Tidorum Ltd
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Date: 2010-09-13T23:21:28+03:00
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Dmitry A. Kazakov wrote:
> On Mon, 13 Sep 2010 13:23:10 +0300, Niklas Holsti wrote:
> 
>> Dmitry A. Kazakov wrote:
>>
>>> re-dispatch is the problem, not aggregation or delegation. Don't implement
>>> anything by means of re-dispatch and you will have no "callbacks." A type
>>> extension cannot break anything in the parent's implementation unless it
>>> re-dispatches.
>> Conversely, there are architectures (sensible ones, in my opinion) in 
>> which the extension is broken, or is difficult or cumbersome to 
>> implement, if the parent does *not* re-dispatch.
> 
> I am not sure if these case aren't kludges caused by other language
> deficiencies (e.g. lack of MI, MD, constructors etc).

I agree that re-dispatching can be used to simulate multiple dispatching 
(MD) in Ada. I have no experience of languages that allow multiple 
inheritance or multiple dispatch, so they are not in my design tool-kit. 
But in the designs where I have used Ada re-dispatching, I don't see how 
MI/MD could have replaced re-dispatching; they seem unrelated.

>> Next question: Should the language provide means to express the uses of 
>> re-dispatch, in the declaration of a tagged type and its operations? For 
>> example, "primitive operation Foo calls primitive operation Bar with 
>> re-dispatch".
> 
> In my view a better alternative would be extensible primitive operations,
> when "overriding" rather inserts prologue or epilogue than replaces all
> body.

That would not have replaced re-dispatching, as I have used it. My 
re-dispatching calls are interleaved in a more complex way with the 
logic of the calling operation.

> One could consider dispatch restricted to class-wide operations while
> allowing T'Class'Class. Then operations class-wide for T would be primitive
> for T'Class. I.e. one could override them within T'Class'Class.

I think an example is needed before I can understand that. Which objects 
are in T'Class'Class, but not in T'Class?

Re-dispatching calls from a primitive (non-class-wide) operation can 
sometimes be replaced by dispatching calls from a class-wide operation, 
and that can clarify a design. Perhaps your suggestion is an extension 
of this?

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .