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From: "Vinzent Hoefler" <0439279208b62c95f1880bf0f8776eeb@t-domaingrabbing.de>
Newsgroups: comp.lang.ada
Subject: Re: Using Red-Black Trees
Date: Thu, 18 Nov 2010 01:13:46 +0100
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Date: 2010-11-18T01:13:46+01:00
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Yannick Duch=C3=AAne (Hibou57) wrote:

> Le Thu, 18 Nov 2010 00:15:11 +0100, Vinzent Hoefler
> <0439279208b62c95f1880bf0f8776eeb@t-domaingrabbing.de> a =C3=A9crit:
>>
>> There are still separates. Of course, it's a compile time decision an=
d it
>> would be part of the build system, not the language itself.
> You can have only one at a time. If you select an implementation, you
> select it system-wide, and you cannot select one for that usage and
> another for another usage in the same system.

Sure. I wouldn't propose it as a perfect solution. But we use that featu=
re
quite often to provide different implementations depending on the target=
.
So it doesn't seem totally useless in the given context.

> The problem is that this is a lack of orthogonality, which is surprisi=
ngly
> an Ada famous feature (Ada forget this one). This is the same issue as=
 the
> one you get with language with which classes is the only way to achiev=
e
> encapsulation. With these, you end to use classes when you do not need=
 it
> at the first place and indeed needed package instead. Here, I would li=
ke
> to be able to do something which has nothing at all to deal with
> polymorphism, and I cannot do it without polymorphism because Ada do n=
ot
> provide orthogonality here.

Well, isn't providing different implementations with a common interface
equivalent to "polymorphism"? I mean, if you don't look at the implement=
ation
how can you tell the difference? ;)

For instance, instead of using an abstract tagged type and dispatching y=
ou
could also use "delegation" (I hope that's the right term) to different
implementations under a common interface without reverting to tagged typ=
es.
I still would consider that polymorphism (actually, we do that with the
communication subsystem, the implementation decides at run-time which so=
rt of
communication is the most efficient, depending on the physical location
of sender and receiver).

> implementation. In less words, would like Ada package specification to=
 be
> more abstract; just like you can have multiple entities, all instance =
of a
> type, would be nice to be able to have multiple implementations, all
> instance of a specification.

You mean like - let me quickly think of a syntax ... - non-Ada, of cours=
e -

-- 8< spec --
package Foo
    --  Some notion to hint the compiler there may be different bodies.
    select Implementation_Choice is (Binary_Tree, Linked_List);
is
    type Bar is ...;

    procedure Implementation_Specific (X : Bar) with Implementation_Choi=
ce;
    procedure Common (X : Bar);

end Foo;
-- 8< --

-- 8< body --
package body Foo is

    procedure Foobar (X : Bar)
       case Implementation_Choice

          when Binary_Tree =3D>
             is
                ...
             begin
                ... binary tree.
             end Foobar (Binary_Tree);

         when Linked_List =3D>
             is
                ...
             begin
                ... linked list.
             end Foobar (Linked_List);
      end case;
   end Foobar;
end Foo;
-- 8< --

-- 8< usage --
with Foo; for Foo.Implementation_Choice use Foo.Binary_Tree;
-- 8< --


Vinzent.

-- =

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