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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: _Type vs no _Type
Newsgroups: comp.lang.ada
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Date: Sat, 6 Nov 2010 15:53:46 +0100
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Date: 2010-11-06T15:53:44+01:00
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On Sat, 06 Nov 2010 13:34:41 +0000, Simon Wright wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> On Sat, 06 Nov 2010 12:00:38 +0100, Georg Bauhaus wrote:
>>
>>> On 11/6/10 8:12 AM, Dmitry A. Kazakov wrote:
>>> 
>>>> Hmm, I would expect you arguing for
>>>>
>>>>     function "/" (Dividend : T; Divisor : T) return T;
>>>>
>>>> For commutative operations like "+" it becomes a bit difficult:
>>> 
>>> You aren't suggesting the computer-"+" is a commutative
>>> operation, are you?
>>
>> I do. The implementation of "+" is not "strictly" commutative, but its
>> interface does not tell that. The implied, not stated contract, is
>> that "+" is indeed commutative allowing certain compiler
>> optimizations. A numeric expression denotes a set of results
>> satisfying certain conditions (accuracy constraints required by the
>> ARM). Within these limits "+" is "weakly" commutative.
> 
> One would very much _hope_ that these two would behave commutatively, of
> course.
> 
> function "+" (L : Time; R : Duration) return Time;
> function "+" (L : Duration; R : Time) return Time;

Yes, though neither of two is commutative.

It would be nice if Ada supported class-wide operations defined on tuples,
e.g. (imaginary syntax):

  function "+" (L : Time | R : Duration) return Time;

a class-wide truly commutative operation defined on the anonymous class:

  { (Time, Duration), (Duration, Time) }

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