Re: [Caml-list] Smells like duck-typing

[Arnaud Spiwack <aspiwack@lix.polytechnique.fr>]

Brian Hurt a écrit :
> Dario Teixeira wrote:
>> Hi,
>>
>>   
>>> That seems backwards from the way OO inheritance is supposed to work.  
>>> You don't go from a more feature-rich case to a less feature-rich case 
>>> -- it's the other way around.
>>>     
>>
>> Of course it is -- that is precisely why inheritance is the wrong
>> formalism for my problem!  What I need is a "reverse inheritance"
>> formalism, where a fully defined data structure sits at the root,
>> and whose descendants are PRUNED versions of the parent.
>>   
> The problem with this is that it violates one of the assumptions of 
> typing, that if type A is (also) a type B, than anywhere you can use a 
> type B, you can also use a type A.  This isn't an assumption limited 
> to object oriented languages.  And this isn't true in your example- if 
> type A is lacking members type B has, then it's possible to write 
> situations where a "real" type B can be used, but not a type A- just 
> use a field of type B that type A doesn't have.
>
> I think I'd recommend rethinking your approach to the problem.
Well, what he is suggesting is to be able to derive a supertype A given 
a type B. This is not fundamentaly incorrect. However I fail to find any 
reasoning where it is made use of, in usual mathematics. So I am, 
personnally, a bit puzzled by the suggestion, unable to say if it might 
make sense or not (method exclusion is natural in the setting of 
incomplete objets or traits, but it doesn't fit the situation very much, 
since it usually produces something that requires to be completed).


Arnaud Spiwack