Re: Generic abstracts

[Tucker Taft <stt@averstar.com>]

Lutz Donnerhacke wrote:
> 
> * 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

So you would like a derived type to inherit the generic
subprogrmas of its parent type, and if they are abstract,
be required to implement them?  This seems like a reasonable
request, but Ada has never supported inheritance of anything
but subprograms and enumerals.  One could imagine inheriting other 
things I suppose, such as exceptions, generics, subpackages, etc. 
However, at the moment only "overloadable" things (i.e. subprograms 
and enumerals) are inheritable, and that has some nice properties.

Be that as it may, I now understand what you were trying to do
and can safely answer that it is not supported by the Ada inheritance
model.

> 
>    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/

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA