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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
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Newsgroups: comp.lang.ada
Subject: Re: Overloading attributes
Date: Sun, 27 Nov 2011 21:34:42 +0100
Organization: Ada @ Home
Message-ID: <op.v5mgr4x1ule2fv@douda-yannick>
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Date: 2011-11-27T21:34:42+01:00
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Le Sun, 27 Nov 2011 21:00:39 +0100, <anon@att.net> a =C3=A9crit:

> From time to time this forum has discussed the concept of the
> "attribute definition clause" (Representation Attributes). Some have
> even say it would cause an increase of 4_000 lines of code to be
> written to include this feature in a compiler.
>
>
> From Ada 95 RM
>
> 13.3 Representation Attributes
>
> 2   attribute_definition_clause ::=3D
>           for local_name'attribute_designator use expression;
>         | for local_name'attribute_designator use name;
>
>
>                                Legality Rules
>
> 5     (95)
> 5/1 (2005)
>
> An attribute_designator is allowed in an attribute_definition_clause o=
nly
> if this International Standard explicitly allows it, or for an
> implementation-defined attribute if the implementation allows it.  Eac=
h
> specifiable attribute constitutes an aspect of representation.

You should have clearly marked where the quotation starts and where it  =

ends. Actually, it ends here, and all of the following, is your personal=
  =

wordings, not part at all of the standard.

> This clause does not forbid the creation of new attribute it only stat=
es
> that it is up to the implementation to allow it if the Standard does n=
ot
> explicitly allows it.
>
> For instant, the implementation could say that the programmer can crea=
te
> new attributes for types created by the programmer only. Aka like the
> attribute "Image" (which is define for scalar objects), for a programm=
er
> defined record. Or the implementation could predefine a set of "attrib=
ute
> designators" that could be used by a programmer.
>
>
> And for Ada 83, the complete syntax and implementation of must be
> documented in Appendix F for the allowed representation clauses.
>
> So, Ada 83 allows the creation with just documentation. As for Standar=
ds
> Ada 95, Ada 2005 and Ada 2012 they started to limit the usage, but sti=
ll
> allow with the implementation permission.
>
> Which means that Ada can allow a programmer to create "Representation
> Attributes" aka "Attributes" with some limitations if the Standard
> or the Implementation allows it.

You could go far this way, even up to an Ada compiler understanding MS  =

Quick Basic. Unfortunately for you, the standard also clearly state this=
:

> 1.1.3 Conformity of an Implementation with the Standard
> Implementation Requirements
> A conforming implementation shall:
> [=E2=80=A6]
> * Contain no variations except those explicitly permitted by this  =

> International Standard,
> or those that are impossible or impractical to avoid given the  =

> implementation's
> execution environment;

So you can forget about user defined attributes, unless you can quote th=
e  =

RM where it =E2=80=9Cexplicitly permit=E2=80=9D user defined attributes.=
 At least, user  =

defined attributes does not fall in the =E2=80=9Cimpossible or impractic=
al to  =

avoid=E2=80=9D case.

-- =

=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]