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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: Design by contract and control inversion
Date: Thu, 01 Nov 2012 18:13:37 +0100
Organization: Ada @ Home
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Date: 2012-11-01T18:13:37+01:00
List-Id: <comp.lang.ada>

Le Wed, 31 Oct 2012 20:28:30 +0100, Yannick Duch=C3=AAne (Hibou57)  =

<yannick_duchene@yahoo.fr> a =C3=A9crit:
> The only thing I could imagine, is to create a second type, `U`,  =

> re=E2=80=91interfacing `T`, and passed to the handlers instead of `T`:=

>
>
>      type T is private;
>
>      type U (<>) is limited private;
>
> [=E2=80=A6]
>
> [=E2=80=A6] in the real thing I did, `U` only gets a subset of the sub=
programs  =

> applicable to `T`.

May make one feels it would be better to expression the subset relation =
 =

with a type relation, i.e. type `T` would be a derived type of `U` (henc=
e,  =

`U`'s properties, is a subset of that of `T`).

Unfortunately, doing this:


     type U (<>) is tagged limited private;

     type T is new U with private;


=E2=80=A6 leads into an issue: you want to disallow any instance of `U` =
to be  =

created (the reason of the unknown discriminant), but this also disallow=
  =

instantiation of `T` as=E2=80=91is, as `T` inherits the unknown discrimi=
nant. Can  =

have a `Create` or `Instance` function, but would be cleaner to be able =
to  =

get ride of this unknown discriminant. If `T` adds capabilities to `U`, =
it  =

should be able to get ride of it, isn't it?

That make me feel again that use of the unknown discriminant is too much=
  =

like a hack, when you just want to disallow instantiation from the clien=
t  =

side. Would be nice to have something to directly express it, not  =

involving any discriminants, which is a separate concept (indeed, the fu=
ll  =

view of the type, does not have, and does not need any).

There use to be thread on that topic, but can't remember which one it wa=
s.


-- =

=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