Re: [Caml-list] C++/C# inheritance is bad?

[Kuba Ober <ober.14@osu.edu>]

On Jan 17, 2009, at 8:27 AM, Jon Harrop wrote:

> On Friday 16 January 2009 15:18:50 Kuba Ober wrote:
>> So, when correctly applied, what's so disturbing about inheritance?
>
> Inheritance is disturbing because, in general, it cannot be applied  
> correctly.
> That is why there are so many workarounds, many of which do not even  
> exist in
> C++ like final methods, sealed classes, multimethods, extractors and  
> so on.
> OO is not good in all circumstances and other tools can be  
> preferable. IME,
> OO is only very rarely the best solution and it is only chosen so  
> often
> because it is typically the only solution available (e.g. in C++ or  
> Java).
>
>> You inherit only where it makes sense, and if it makes sense then  
>> you don't
>> care about which particular method is called: it's supposed to be  
>> safe.
>
> Safety is quite different from easy of use, of course.

I know I'm perhaps overusing Qt as an example, but it's just handy.
In Qt, the basic "instrumented" class is QObject. It implements  
retrospection,
signal/slot mechanism, etc. QWidget (a GUI building block) derives  
from it.
(and from a QPaintDevice, since you can paint on it).
Then, you have various widgets that derive from QWidget. Some of those
are abstract widgets (interfaces), like QAbstractButton, where a  
concrete
class faces the end user, like QPushButton.

It is quite a straightforward model -- as long as you think of it in  
terms of "ISA"
relationship between classes.

I wonder how such a hierarchy would be implemented in a natural way
in OCaml? By "natural" I mean being good practice and not feeling like  
a hack.
One should remember that users must extend the framework, so it should
be easy to extend say QWidget to create your own "class".

Cheers, Kuba