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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
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Newsgroups: comp.lang.ada
Subject: Re: Why constant components are not permitted ?
Date: Fri, 25 Nov 2011 10:53:47 +0100
Organization: Ada @ Home
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Date: 2011-11-25T10:53:47+01:00
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Le Fri, 25 Nov 2011 07:56:03 +0100, Niklas Holsti  =

<niklas.holsti@tidorum.invalid> a =C3=A9crit:

> On 11-11-24 19:14 , David Sauvage wrote:
>> Hello,
>>
>> given the following :
>>
>> type Object is record
>>     Tag : constant String :=3D "key"; -- GNAT compilation failed :
>> constant components are not permitted
>> end record;
>>
>> In my case I find it useful to declare my Object Tag component as a
>> constant, but it is not permitted, I would be interested to learn
>> why ?
>
> It seems no earlier responder has understood why you want to have a  =

> constant component, nor do I, so I'll ask directly: Why do you think a=
  =

> constant component would be  useful?
The same question for me. A record, is not an interface, that's an  =

implementation solution. One want to use a record to store some aggregat=
ed  =

stuff, thus its name, =E2=80=9Crecord=E2=80=9D.

If a type T is implemented as a record, the interface should not expose =
 =

the record (as with any implementation details, except if part of an  =

expected specification). If T is to be associated with a constant, the  =

constant could be implemented as a function getting a T as parameter, an=
d  =

returning that constant.

Unless the OP was to reinvent a wheel already provided by Ada, which is =
 =

record discriminant. If that constant is a kind of characterization of  =

instances of T, then a discriminant is the obvious choice.

-- =

=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 Univer=
sity]