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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Seeking for papers about tagged types vs access to subprograms
Date: Sat, 11 May 2013 18:51:27 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2013-05-11T18:51:27-04:00
List-Id: <comp.lang.ada>

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> The condition is moved to the post-condition. E.g.
>
> # require X >= 0.0
>    function sqrt (X : Float) return Float;
> # ensure sqrt (X)**2 = X
>
> is replaced with
>
> # require true
>    function sqrt (X : Float) return Float;
> # ensure sqrt (X)**2 = X or else Constraint_Error raised

It seems to me that this replacement loses information (the info that 
X is "supposed to be nonnegative").

For example, a body of sqrt that says "return X / 0.0;" obeys the second
contract but not the first above.  But that's obviously wrong to anyone
who thinks that "sqrt" means "approximate square root of X".

(Never mind the accuracy issues -- sqrt(X) might not be representable as
a Float. ;-) )

- Bob