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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: default formal parameters in generic declarations
Newsgroups: comp.lang.ada
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Date: Mon, 3 Mar 2008 14:56:31 +0100
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Date: 2008-03-03T14:56:31+01:00
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On Mon, 03 Mar 2008 12:15:45 +0100, Georg Bauhaus wrote:

> Dmitry A. Kazakov schrieb:
> 
>> IMO, it is better (as always) to get rid of generics. Default values of
>> packages are in fact inheritable interfaces. Thus inheritance should be the
>> way. I would extend this part of the language rather than generics.
> 
> Supposedly, removal of generics will also remove the
> tags from concrete types?

Only from non-by-reference ones. Actually Ada already has the keyword
"tagged" which could read "do it by reference and please keep the tag."

> How do you deal with what is now usually a generic
> swap procedure?

type Copyiable is interface ...;
    -- All the things with an assignment and copy constructor (initializer)

A. Class-wide Swap

procedure Swap (Left, Right : in out Copyiable'Class) is
   Temp : Copyiable'Class := Left;
begin
   Left := Right;
   Right := Temp;
end Swap;

B. Primitive Swap (doubly-dispatching with only the diagonal elements
allowed).

procedure Swap (Left, Right : in out Copyiable) is
   Temp : Copyiable'Class := Left;
begin
   Left := Right;
   Right := Temp;
end Swap;

P.S. One problem I didn't know how to solve was the constraints on the
combinations of the argument's types. A generic solution is
pre-constrained, so that it cannot handle some cases at all. For Swap you
might wish to detect all non-diagonal cases which would raise
Constraint_Error at compile time. Clearly, when the tags are known
statically (as with generics) and (for the variant A) the body is inlined,
for the variant B without the body, the compiler could give a warning. The
problem is how to convert warning into an error. I guess that contracted
exceptions could be a solution.

P.P.S. Another problem is adding interfaces to existing types. This is
solved by supertyping.

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