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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Securing type extensions
Newsgroups: comp.lang.ada
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Date: Mon, 13 Sep 2010 22:41:40 +0200
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Date: 2010-09-13T22:41:39+02:00
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On Mon, 13 Sep 2010 22:22:30 +0200, Georg Bauhaus wrote:

> Yes, exactly, the design decision to declare a publicly tagged
> type (which of course will not be "final") entails mutual obligations:
> my calls on overridden subprograms will rely on profiles and further 
> assumptions about overridden ops
> (such as the LSP that has been mentioned), as do the overriding
> ops of the user defined type.  This kind of mutual
> trust is unrelated to Ada's visibility rules.

Of course it is. You have to see the operation as primitive, you have to
see the object class-wide in order to dispatch.

> Another reason, not stated, to ask about an event driven parser
> with user supplied type extensions was to learn how this can
> be done without dispatching calls on subprograms not known
> by the author of the parser (and possibly back).

This design pattern is not related to either final methods or re-dispatch.

> (Calling user supplied prim ops might become a problem if
> some bug (even a misuse) will make parties blame each
> other.

Calling any operation can. Dispatching call is no less or more safe than a
non-dispatching one [*, **]

* Assuming that dispatch may not fail
** Re-dispatching call is unsafe and relies on luck

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