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From: Ivan Gotovchits <ivg@ieee.org>
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Date: Thu, 25 Oct 2012 15:54:36 +0400
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Subject: [Caml-list] Polymorphic variants and inheritance


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?  

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