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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: I'm facing an issue with: call to abstract procedure must be
 dispatching
Date: Wed, 9 Dec 2015 15:23:32 +0100
Organization: cbb software GmbH
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Date: 2015-12-09T15:23:32+01:00
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On Wed, 9 Dec 2015 14:42:24 +0100, G.B. wrote:

> On 09.12.15 13:03, Dmitry A. Kazakov wrote:
>> You mean procedural types?
> 
> Not exactly procedural types. I'd like a reflection of a more
> clear cut distinction between when a(n MI-)type is needed for
> (M)dispatching versus when the (MI-)type is needed for materializing
> actual objects.

I think the problem is that you consider dispatching an interface-thing.
It is not. Dispatching is strictly an implementation.

It is important to understand that both primitive and class-wide operations
are defined on the whole class. It is the implementations of these that are
different. In particular a primitive operation has a body for each member
of the class. Dispatching is merely a mechanism of body selection. There
could be other mechanisms of body composition. For example body extension
as used in constructors, destructors, aggregates, assignment, stream I/O
operations.

>> The bottom line, you can have your lazy blocks only when not reusable and
>> weakly typed.
> 
> Put differently, at some point I can use "weakly" typed blocks
> for getting a job done that warrants just the necessary effort.

Yes, you can program C in Ada. The question is what for?

> For some perspective, dispatching can look a lot like a specially
> crafted procedural thing. I think that there are cases related
> to MD (as in the example of storing from Objects'Class into
> Databases'Class) when declaring a regular named (MI-)type would
> be going over the top.
> 
> procedure Demo is
> 
>     package Root is
>        type T is abstract tagged private;
>        procedure Op
>          (X : in out T; Val : Natural) is abstract;
>     private
>        type T is abstract tagged record
>           Identity : Natural; -- := ...;
>        end record;
>     end Root;
> 
>     package Twigs is
>        type A is new Root.T with record null; end record;
>        overriding procedure Op (X : in out A; Val : Natural);
> 
>        type B is new Root.T with record null; end record;
>        overriding procedure Op (X : in out B; Val : Natural);
>     end Twigs;
>     package body Twigs is separate;
> 
>     package X is
> 
>        procedure D12
>          (Left  : in out Root.T'Class;
>           Right : in out Twigs.B'Class);
> 
>        procedure D22
>          (Left  : in out Twigs.A'Class;
>           Right : in out Twigs.B'Class);
> 
>     end X;
>     package body X is separate;
> 
>     Va : Twigs.A;
>     Vb : Twigs.B;
> 
> begin
>     --  not Ada
>     do X.D12 with X.D2'Type (Left => V1, Right => Vb);
>     do X.D22 with X.D2'Type (Left => Va, Right => Vb);
>     do X.D22 with X.D2'Type (Left => Va, Right => Va);  -- no
> 
> end Demo;
> 
> The last lines are not really needed, but they show where
> there is a type lurking.

I don't understand the purpose of this example.

MI and MD are required for consistency. Practical use or not is irrelevant,
because consistency trumps everything else.

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