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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
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 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Re: Silly and stupid post-condition or not ?
Date: Mon, 06 Feb 2012 17:12:16 +0100
Organization: Ada @ Home
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Date: 2012-02-06T17:12:16+01:00
List-Id: <comp.lang.ada>

Le Mon, 06 Feb 2012 15:39:14 +0100, Robert A Duff  =

<bobduff@shell01.theworld.com> a =C3=A9crit:

> "Yannick Duch=C3=AAne (Hibou57)" <yannick_duchene@yahoo.fr> writes:
>
>> Seems nice, but given this=E2=80=A6
>>
>>    subtype Image_Type is String
>>       with Static_Predicate =3D> Image_Type'Length in Length_Type;
>>
>>    function Image (Instance : Instance_Type) return Image_Type
>>       with Post =3D>
>>         (Parsed (Image'Result).Status =3D Parsed) and
>>         (Parsed (Image'Result).Instance =3D Instance);
>>       -- Result can always be successfully parsed and the resulting
>>       -- instance is always equal to the one whose image is returned.=

>>
>> =E2=80=A6how do I move Image's Post predicate to an Image_Type's dyna=
mic
>> predicate  ? I could not figure the syntax nor if it's at least  =

>> possible.
>
> Not possible.  Predicates can only express a property of a single
> thing, but here you're trying to express a relationship
> between two things (parameter and result).
>
> - Bob


Sorry, my wording was unclear. Here is what I wanted, and it works:

    subtype Image_Type is String
       with Dynamic_Predicate =3D>
         (Image_Type'Length in Length_Type) and
         (Parsed (Image_Type).Status =3D Parsed);

(don't know why it first failed, may be a syntax error)

The predicate =E2=80=9CParsed (Image_Type).Status =3D Parsed=E2=80=9D is=
 better on  =

Image_Type than on Image.

So thanks for your prior message which gave me this idea. Since now, wil=
l  =

better think about moving predicates on types as much as possible. Indee=
d  =

better.


-- =

=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