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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Operation can be dispatching in only one type
Newsgroups: comp.lang.ada
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Date: Tue, 8 Dec 2009 11:23:22 +0100
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Date: 2009-12-08T11:23:22+01:00
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On Tue, 08 Dec 2009 11:06:54 +0100, Georg Bauhaus wrote:

> Dmitry A. Kazakov schrieb:
>> In my view pre-/postconditions and
>> invariants should be static, used strictly for program correctness proofs.
>> Subtypes should complement them for dynamic run-time checks (recoverable
>> faults).
> 
> Hm. What would be your subtype based expression for
> 
>   generic
>      type E is private;
>   package Stacks  is
> 
>     type Stack is private;
> 
>     procedure push (Modified_Stack : in out Stack;
>                     Another : Element)
>          with pre => not Full (Modified_Stack),
>               post => Size (Modified_Stack'Exit) = Size (Modified_Stack);
> 
>     procedure pop (Modified_Stack : in out Stack)
>          with pre => not Empty (Modified_Stack),
>               post => Empty (Modified_Stack);

None. The above is wrong. You cannot implement this contract (if we deduced
one from the given pre- and postconditions). Proof:

   loop
      Push (Stack, X);
   end loop;

q.e.d.

Therefore the contract of a stack must always contain ideals, e.g.

1. exceptions, like Full_Error, Empty_Error;

2. blocked states, like holding the caller until the stack state is changed
from another task.

Pre- and psotconditions are to be used to prove a contract to hold. They
themselves are no contract.

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