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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
 =?utf-8?Q?=29?=
 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Re: Seeking for papers about tagged types vs access to subprograms
Date: Sun, 12 May 2013 08:25:33 +0200
Organization: Ada @ Home
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Date: 2013-05-12T08:25:33+02:00
List-Id: <comp.lang.ada>

Le Sun, 12 May 2013 08:02:16 +0200, Dmitry A. Kazakov  =

<mailbox@dmitry-kazakov.de> a =C3=A9crit:
>>> The condition is moved to the post-condition. E.g.
>>>
>>> # require X >=3D 0.0
>>>    function sqrt (X : Float) return Float;
>>> # ensure sqrt (X)**2 =3D X
>>>
>>> is replaced with
>>>
>>> # require true
>>>    function sqrt (X : Float) return Float;
>>> # ensure sqrt (X)**2 =3D X or else Constraint_Error raised
>>
>> It seems to me that this replacement loses information (the info that=

>> X is "supposed to be nonnegative").
>
> # ensure
>     (X >=3D 0.0 and then sqrt (X)**2 =3D X)
> or else
>     (X < 0.0 and then Constraint_Error raised)

But what's the benefit of moving the precondition to the postcondition? =
Or  =

more simply, why dropping the idea of precondition for sub=E2=80=91progr=
am  =

declarations?

Do you have to re=E2=80=91invent or re=E2=80=91derive the precondition a=
t each call place,  =

 from the postcondition? Isn't it kind of bloat?

It makes more me feel like losing something than the opposite. And how d=
o  =

you clearly connect valid invocations into sequence? You can't connect a=
ny  =

more, the post condition of A to precondition of B (when B comes next to=
  =

A), or else, less clearly. There may be an issue with separation of  =

concern here (the God postcondition?). I won't enjoy a variant of Ada  =

doing so.

-- =

=E2=80=9CSyntactic sugar causes cancer of the semi-colons.=E2=80=9D [1]
=E2=80=9CStructured Programming supports the law of the excluded muddle.=
=E2=80=9D [1]
[1]: Epigrams on Programming =E2=80=94 Alan J. =E2=80=94 P. Yale Univers=
ity