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Date: Thu, 24 Jul 2014 14:36:11 -0700 (PDT)
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Subject: Re: Rationale for this not to compile?
From: Adam Beneschan <adambeneschan@gmail.com>
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Xref: number.nntp.dca.giganews.com comp.lang.ada:187844
Date: 2014-07-24T14:36:11-07:00
List-Id: <comp.lang.ada>

On Thursday, July 24, 2014 1:53:32 PM UTC-7, Victor Porton wrote:
> Is there any rationale why this does not compile?=20

A generic instantiation is equivalent to "the instance declaration ... imme=
diately followed by the instance body, both at the place of the instantiati=
on."  (12.3(12)) Since the body could contain code that depends on the repr=
esentation of X already being determined, but it hasn't yet, the language m=
akes this illegal.  At least I think that's the rationale.  AARM section 13=
.14, which discusses the freezing rules, explains in more detail why types =
have to be completely defined before they're used in certain cases (see htt=
p://www.ada-auth.org/standards/12aarm/html/AA-13-14.html).

                             -- Adam



> If it compiled, it would=20
> be probably useful in practice with a real example similar to the below t=
oy=20
> example (as to make the type T not private, I must create an *.Internal.*=
=20
> package to serve as a "private repository" for types like T, as real priv=
ate=20
> types cannot be used). It is called a hack.
>=20
> gnatmake -q -c -gnatc -u -P/home/porton/t/default.gpr test.ads
> test.ads:10:32: premature use of private type
> gnatmake: "/home/porton/t/test.ads" compilation error
>=20
> package Test is
>=20
>    generic
>       type T is limited private;
>    package Gen is
>    end Gen;
>=20
>    type X is private;
>=20
>    package Instance is new Gen(X);
>=20
> private
>    type X is null record;
> end Test;