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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: How come Ada isn't more popular?
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Date: Sat, 3 Feb 2007 10:45:26 +0100
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Date: 2007-02-03T10:45:10+01:00
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On Fri, 02 Feb 2007 13:34:21 +0100, Markus E Leypold wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> Sure you can:
>>
>> type Additive is abstract;  -- Should be enough for an interface
>> function "+" (A, B : Additive) return Additive is abstract;
>>
>> Here I am not sure about "increment" variable, because it is not
>> initialized. Anyway:
>>
>> function Increment return Additive is abstract;
>>    -- Better be getter/setter pair. Unfortunately Ada does not have
>>    -- "abstract variable interface" it should have
>>
>> function Inc (Arg : Additive'Class) return Additive'Class is
>> begin
>>    return Increment + Arg;
>> end Inc;
>>
>> There is a very simple rule for all this:
>>
>> 1. Formal generic subroutines -> abstract primitive operations
>> 2. Subroutines in the body -> class-wide operations.
> 
> So you derive from Additive to get a specific implementation?
> 
> Like
> 
>   type Vector   is Additive with ...;
>   type CountVal is Additive with ...;
> 
> Right? But then, you should note that with
> 
>   C: Countval;
> 
>   Inc(C).
> 
> returns an 'Additive', not a 'CountVal'.

No, it does Additive'Class! Thus, no problem.

(note that the goal was polymorphic Inc, if you wanted it covariant that
would automatically exclude polymorphism)

> That is one problem. The
> other is of course that Increment is not initialized. This is exactly
> what generics are here to solve.

As it was in Georg's example. But with inheritance covariant Increment
would be required to be overridden (praised be Ada). So an initialization
will be enforced anyway.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de