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Newsgroups: comp.lang.ada
Subject: Re: Subtype conformance... not what I was expecting.
From: Georg Bauhaus <bauhaus@futureapps.de>
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Date: Wed, 26 Jul 2006 10:06:28 +0200
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Date: 2006-07-26T10:06:01+02:00
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On Tue, 2006-07-25 at 23:02 -0400, Peter C. Chapin wrote:

> procedure Check is
>     subtype Narrow is Integer range -10..10;
>     type Function_Ptr is access function(X : Narrow) return Integer;
> 
>     function F(Para : Integer) return Narrow is
> ...
>     G      : Function_Ptr := F'Access;

> The compiler complains about the initialization of G with F'Access 
> saying that it is not "subtype conformant." ...
>  However, this seems overly restrictive.

Would you still think the same in the following variation? (I don't know
the rationale for the restriction but then I wasn't surprised :-)

procedure Check is
   subtype Above_Zero is Integer range 1 .. 95; -- from liquid to for tea
   subtype Below_Zero is Integer range -100 .. -5; -- really solid

   type Frozen_Ptr is access function(X: Below_Zero) return Integer;
   type Liquid_Ptr is access function(X: Above_Zero) return Integer;

   function Whirl(Para: Integer) return Above_Zero is
      begin
         return Para;
      end;

   function Cut(Para: Integer) return Below_Zero is
      begin
         return Para;
      end;

   G: Frozen_Ptr := Whirl'access;
   Ice: Below_Zero;
begin
   Ice := G(0);
end Check;


Above_Zero and Below_Zero let me think of different logical types, really.
The set of values in the respective subtypes is disjoint.

I won't want to deliver hot water to/from a function that needs/returns
cold ice. In my view, this will be a contract violation, in this case
at least. 

Maybe the restriction helps avoid specimens of "I'm the programmer, and
I know that the program is right, even though it's not obvious".


-- Georg