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Date: Wed, 26 Jul 2006 10:15:37 -0400
From: "Peter C. Chapin" <pchapin@sover.net>
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Subject: Re: Subtype conformance... not what I was expecting.
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Date: 2006-07-26T10:15:37-04:00
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Georg Bauhaus wrote:

> 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;

In the statement Ice := G(0) you are assigning an Integer to a 
Below_Zero. Naturally a runtime check would be necessary to verify the 
constraint and Constraint_Error would be raised if there was a 
violation. It's no different than

	Hot : Integer := 1000;
	...
	Ice := Hot;

> 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.

G accesses a function returning Integer. Putting Whirl'Access into G is 
safe because Whirl can't return anything that would violate G's 
contract. What you subsequently do with what G returns is another matter.

Peter