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From: Florian Weimer <fw@deneb.enyo.de>
Newsgroups: comp.lang.ada
Subject: Re: Preventing type extensions
Date: Mon, 13 Sep 2010 07:32:02 +0200
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Date: 2010-09-13T07:32:02+02:00
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* Maciej Sobczak:

> If it implements an abstract interface, then why extending it further
> is a problem? From the user point of view (that is, from the point of
> view of the subprogram that accepts Some_Interface'Class and operates
> on it), it does not matter how deep is the hierarchy.

Type extensions break encapsulation (at least if there non-abstract
dispatching subprograms which contain some non-trivial code): the
information which subprograms call each other under what circumstances
leaks to the outside of the package.  One very drastic way to
implement that is to disallow type extensions for most non-abstract
types you write, and consider the exceptions carefully.