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From: jsa@alexandria (Jon S Anthony)
Subject: Re: Generic children, instances and visibility...
Date: 1997/04/09
Message-ID: <JSA.97Apr9151107@alexandria>#1/1
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References: <JSA.97Apr8212412@alexandria> <334BA5F2.4111@gsfc.nasa.gov>
Organization: PSI Public Usenet Link
Newsgroups: comp.lang.ada
Date: 1997-04-09T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <334BA5F2.4111@gsfc.nasa.gov> Stephen Leake <Stephen.Leake@gsfc.nasa.gov> writes:

> with Parent.Child;
> with Inst_Types;
> package Parent.PC_Inst is new Parent.Child (Inst_Types.It);
> 
> gnat says neither case allows visibility:

True.  But that may or may not be accurate (though it probably is...)


> So, hoping that a private child would have visibility, I tried a third
> variation:
> 
> with Parent.Child;
> with Inst_Types;
> private package Parent.Test is
>    type It is new Parent.T with null record;
> 
>    package PC_Inst is new Parent.Child (Inst_Types.It);

Since PC_Inst is not a child of Parent, 12.3(11) seems to say that
this can't work.  The case where the instance is a child of the parent
is the interesting one.  What is more, this case "works" (according to
gnat) if you declare the IT type in a child package of Parent:

package Parent.Inst_Types is
    type IT is new Parent.T with null record;
end Parent.Inst_Types;

with Parent.Child;
with Parent.Inst_Types;
package Parent.PC_Inst is new Parent.Child(Parent.Inst_Types.IT);

Hmmm, just checked and it also "works" if you just dump the
instantiation in Parent.Inst_Types:

with Parent.Child;
package Parent.Inst_Types is
    type IT is new Parent.T with null record;
    package PC_Inst is new Parent.Child(IT);
end Parent.Inst_Types;

Now this last case is really odd as it does not seem different (in the
relevant point - properties of IT visible at instantiation point) than
the case where Inst_Types is not a child and simply withs Parent to
derive the IT type.  Now, I _am_ beginning to think that GNAT has a
bug here somewhere.


> It would seem that no generic can see inside type It; it is a limited
> private type, period.

That may well be true (actually the limited part is irrelevant), but
as the above shows, you can't use GNAT as a verification of this
statement.  I was wondering about this from a "language lawyer" point
of view, but now am wondering if GNAT has a bug in it as well.

Tucker, Bob, Norman - any comments appreciated.

/Jon
-- 
Jon Anthony
Organon Motives, Inc.
Belmont, MA 02178
617.484.3383
jsa@organon.com