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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Preventing type extensions
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Date: Mon, 13 Sep 2010 15:05:20 +0200
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Date: 2010-09-13T15:05:20+02:00
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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).

> 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.

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.

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