Re: [Caml-list] Polymorphic variants and inheritance

[Didier Cassirame <didier.cassirame@gmail.com>]

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type odds = [`One | `Three ]
type evens = [`Two | `Four ]
type numbers = [ odds | evens ]

class type ['a] number = object
  constraint 'a = [< numbers]
  method category: 'a
end

class type odd = object
  inherit [odds] number
  method category: odds
end

class type even = object
  inherit [evens] number
  method category: evens
end

It's probably even better like this :)

Cheers,

didier


2012/10/25 Didier Cassirame <didier.cassirame@gmail.com>

> Hi Ivan,
>
> You may use parameterized classes like this :
>
> type odds = [`One | `Three ]
> type evens = [`Two | `Four ]
> type numbers = [ odds | evens ]
>
> class type ['a] number = object
>   constraint 'a = [< numbers]
>   method category: 'a
> end
>
> class type ['a] odd = object
>   inherit ['a] number
>
>   constraint 'a = odds
>
>   method category: 'a
> end
>
>
> class type ['a] even = object
>   inherit ['a] number
>   constraint 'a = evens
>   method category: 'a
> end
>
> This passes the compilation.
>
> didier
>
>
> 2012/10/25 Ivan Gotovchits <ivg@ieee.org>
>
>>
>> Here a simple example that illustrates the problem:
>>
>>    type odds = [`One | `Three ]
>>    type evens = [`Two | `Four ]
>>    type numbers = [ odds | evens ]
>>
>>    class type number = object
>>      method category: numbers
>>    end
>>
>>    class type odd = object
>>      inherit number
>>      method category: odds
>>    end
>>
>>
>>    class type even = object
>>      inherit number
>>      method category: evens
>>    end
>>
>>
>>
>> It fails to compile, stating the following error:
>>        The method category has type odds but is expected to have type
>> numbers
>>        Type odds = [ `One | `Three ] is not compatible with type
>>          numbers = [ `Four | `One | `Three | `Two ]
>>        The first variant type does not allow tag(s) `Four, `Two
>>
>> Just by theory, odds are covariant to numbers, so this kind of subtyping
>> can be done. Is it possible to persuade compiler?
>>
>> --
>>          (__)
>>          (oo)
>>    /------\/
>>   / |    ||
>>  *  /\---/\
>>     ~~   ~~
>> ...."Have you mooed today?"...
>>
>> --
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>>
>
>

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