Re: [Newbie] doubly constrained array, dumb question

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 27/02/2018 12:43, Mehdi Saada wrote:
>> For example, the "class" Integer of which
>> Positive is a "member" still has negative values, it is just so that you
>> cannot tag them Positive. E.g.
>>      A : Positive := 1;
>>      B : Positive := 100;
>>      C : Positive := (A - B) + B; -- This is OK, though A - B = -99
> Interesting, I never tried that ! Very useful indeed.
> But those types don't have classes, nor polymorphism.

They have "classes", but no *dynamic* polymorphism.

>> Not necessarily. You simply override disallowed operations with bodies
>> raising Constraint_Error. The same technique is used for multiple dispatch:
> Yes, I thought so, but it's definitely not my conception of safe and "predictable"... It's safer that "whatever is defined for the parent, won't raise exception for the descendants" holds true at any time.

It is safe because that is the defined behavior. Disallowed operation is 
defined as exception, not as never-called-thing.

Compare that with zero divide, there is nothing unsafe in that, it is 
defined to raise Constraint_Error. Unsafe is when a contract is 
violated. If a client promises not to propagate exceptions and then 
divides to zero without catching Constraint_Error, that is an unsafe 
*client*, division is still safe.

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