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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Copy vector in Ada
Date: Wed, 7 Nov 2012 21:04:27 +0100
Organization: cbb software GmbH
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Date: 2012-11-07T21:04:27+01:00
List-Id: <comp.lang.ada>

On Wed, 07 Nov 2012 19:06:11 +0100, Georg Bauhaus wrote:

> Well, a basic principle about derived types was stated,
> 
>  "a derived type has to cover a base type, and will probably
>  do more, but in anyway, not less",

which evidently does not hold in two major cases used in Ada:

1. specialization:

1.1 Ada's subtypes
1.2 Ada's in-mode arguments
1.3 Ada's constants
1.4 Ada's discriminated types
1.5 Ada's specific types as related to the class-wide counterparts, e.g. T
is a specialization of T'Class.

2. extension = Cartesian product, Ada's tagged types

> The commandment can be called a basic principle in the light of
> the substitution principle,

Generalization, the kind of type deviation you are talking about, does not
save LSP. In particular, it breaks in-operations. This is the reason why,
for example, extensible enumerations (which would be a generalization) pose
such a big problem.

It is important to remember that any non-trivial modification of a type
always breaks something. There is no rule expressed in terms of type values
which could ensure LSP. Bijection of domain sets the only mapping which
would unconditionally satisfy the requirement of weakening pre- and
strengthening post-conditions of all operations.

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