Re: [Caml-list] Polymorphic variants and inheritance

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

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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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