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From: mheaney@ni.net (Matthew Heaney)
Subject: Re: Warts was Re: The stupidity of all the Ariane 5 analysts.
Date: 1997/08/10
Message-ID: <mheaney-ya023680001008971505270001@news.ni.net>#1/1
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Organization: Estormza Software
Newsgroups: comp.lang.ada
Date: 1997-08-10T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <Pine.SGI.3.95.970807191944.13419A-100000@shellx.best.com>,
Brian Rogoff <bpr@shellx.best.com> wrote:


>generic
>package AGL.Sets.Iterators is
>    type Iterator_Type is private;
>
>    function Start ( Set : Set_Type ) return Iterator_Type;
>    function Finish ( Set : Set_Type ) return Iterator_Type;
>
>    ... etc. ...
>
>    -- ILLEGAL! These signatures can't be instantiated here!!!
>
>    package Forward_Iterators is
>      new AGL.Forward_Iterators(Value_Type, Iterator_Type, Value_Ptr,
>                                Next, Get_Value, Set_Value, Get_Pointer);
>
>    package Bidirectional_Iterators is
>      new AGL.Bidirectional_Iterators(Value_Type,Iterator_Type,Value_Ptr,
>                                      Next,Prev,Get_Value,Set_Value,
>                                      Get_Pointer,"=");
>
>end AGL.Sets.Iterators;
> 
>So what do we do about this problem? There are numerous workarounds...


Here's one, though I haven't tried to compile it.  This issue is that the
full view of the type needs to have been defined prior to instantiation of
the generics.

generic
package AGL.Sets.Iterators is

   package Iterator_Parents is

      type Iterator_Parent is private;

      <primitive ops here>
   
   private
      type Iterator_Parent is ...;

   end Iterator_Parents;

   type Iterator is new Iterator_Parents.Iterator_Parent;

   <instantiations here>

end;

Won't this solve the problem?

Yes, it seems that being able to interleave private and public sections
would be a convenient feature, because you could do this:

generic
package AGL.Sets.Iterators is

   type Iterator is private;

   <primitive ops>

private 

   type Iterator is ...;  -- full view

public

   <instantiations go here, now that full view has been seen>

end;

but you can get almost that effect using nested packages.  It's the same
trick you do in Ada 83 to be able to derive from a type declared in the
same package spec.

--------------------------------------------------------------------
Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
(818) 985-1271