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From: lutz@iks-jena.de (Lutz Donnerhacke)
Subject: Re: Generic abstracts
Date: 1999/11/29
Message-ID: <slrn845ifq.18l.lutz@taranis.iks-jena.de>#1/1
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Date: 1999-11-29T00:00:00+00:00
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* Tucker Taft wrote:
>Lutz Donnerhacke wrote:
>> The abstract definition should be used to ensure a consistent
>> interface. I have two problems:
>>   - A generic procedure which can be missed on implementation.
>>     Raising an exception might be acceptable.
>>   - It is not possible to predefine a body for a generic function.
>>     No return value is available.
>>     Raising an exception does not stop the compiler from requiring
>>     a return value.
>
>I'm unclear as to what you are trying to accomplish by including
>generic subprograms that are abstract.  Generic subprograms are
>not inherited, so there is no point in declaring them on an
>abstract type.  

with Abstract_Lists;

generic
   type Item (<>) is abstract tagged limited private;
package Concrete_Lists is
   package Abstract_Template is
      new Abstract_Lists (Item => Item);

   type List          is new Abstract_Template.List          with private;
   type List_Item     is new Abstract_Template.List_Item     with private;
   type List_Iterator is new Abstract_Template.List_Iterator with private;

   -- now the compiler requires to implement all abstract functions
   -- including the generic Iterate and Delete procedures

   function Is_Empty (L : List) return Boolean;

   generic
      with procedure On_Item (E : in out Item);
   procedure For_Each        (L : in     List);

   ...
end Concrete_Lists;

An solution without (abtract) interfaces can be obtained from
ftp.iks-jena.de:/pub/mitarb/lutz/ada/types/