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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Ada.Strings.Bounded
Newsgroups: comp.lang.ada
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Date: Mon, 14 Apr 2008 11:07:47 +0200
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Date: 2008-04-14T11:07:47+02:00
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On Sun, 13 Apr 2008 16:20:24 -0700 (PDT), Eric Hughes wrote:

> Why
> shouldn't you derive from a universal type?  Because it's not
> implementable!

One does not imply another. If you are aiming at Liskov substitutability
principle, you should also remember how it is defined. It is in terms of
*provable* propositions. Pragmatically, the program is correct even if
there exist unreachable states where it could crash. However, LSP does not
work anyway.

> So I can now outline what the relationship ought to be:
> -- The set of values of an implemented type is
>       a _subset_ of the terms of the universal type.

Counterexample: NaN of Float.

(The set of values of a derived type is neither subset or superset. It is a
different set. There could be mappings defined between these two sets in
order to achieve substitutability. Such mappings could be injective, or
not)

> -- The set of operators on an implemented type
>       _satisfy_ the axioms of the universal type.

This cannot work because any implementation is necessarily a DFA.

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