Re: A bad counterintuitive behaviour of Ada about OO

["J-P. Rosen" <rosen@adalog.fr>]

Le 08/08/2014 12:53, Dmitry A. Kazakov a écrit :
>> Moreover, classification is used in mathematics for the theory of
>> numbers, which is not relevant to practical usage of numbers.
> 
> Having integers ordered is unpractical? Being them additive has no usage?
> 
Yes of course, but the /theory/ of it, and especially group theory etc.
that you mentioned is not a programmer's concern.

> I don't know which relationships are "conceptual" and which are not.
> 
> generic
>    type Element_Type is private;
>    with function "<" (Left, Right : Element_Type) return Boolean is <>;
>    with function "=" (Left, Right : Element_Type) return Boolean is <>;
> package Ada.Containers.Ordered_Sets is
> 
> Isn't all types acceptable as Element_Type a "conceptual" class?
> 
I argue they are not. If you provide an operation that operates on a
class wide type, then it can be used only on types belonging to the
class, which means there is a dependency between these types.

A generic can be applied to types that are not related in any way.

This reminds me of a (bad) example of inheritance I once saw in a book:
the class "parrot" inherited from the class "human" because it needed
the method "can_speak". I'm somewhat uncomfortable with the notion that
a parrot is a special kind of human ;-). Well, it's a bad example
because inheritance should not be used just to grab any method you need.
There must be some /conceptual/ dependence, i.e. some "is-a" dependency.
That's what I meant: a generic "can_speak" function that could be
instantiated on humans or parrots would be much less disturbing - to my
taste.

-- 
J-P. Rosen
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