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Q: about type inclusion
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Date: 1997-04-10 (08:51)
From: Christian Boos <boos@a...>
Subject: Q: about type inclusion

[Summary: it seems that module type inclusion doesn't use the notion of
          class subtyping. Is there a theoretical reason for that or
	  could it be done in the future ?]


  I was at first puzzled by an error message saying "type x is not included
in type y", because type x was a class inherited from class y, until I realized
that there is two distinct notions of type inclusion in the langage : 
module type inclusion and class subtyping. 
  However, I don't see why they couldn't mix, in particular on the example
given at the end of this message.

Any comments welcomed !


[Resume: il semblerait que l'inclusion entre types de modules n'exploite
         pas les relations de sous-typage entre classes. Y a-t-il une
         impossibilite quelque part ou bien est-ce realisable ?]


  Bien que presentant de nombreuses similitudes, l'inclusion entre types
de modules et le sous-typage entre classes sont deux choses distinctes dans
le langage. Cependant je n'entrevois pas les raisons d'une incompatibilite de 
fond entre les deux, notamment celle conduisant a produire une erreur sur 
l'exemple ci-apres.


    module type A =
	  sig    class a (unit) =  method a : int  end     end
    module type B =
	  sig    class a (unit) =  method a : int  method b : int  end     end
    module Bimpl = 
	  struct    class a () =  method a = 1  method b = 2  end   end
    module B = (Bimpl : B)  (* of course *)
    (* but *)
    module A = (Bimpl : A)  

Characters 16-21:
Signature mismatch:
Modules do not match:
  sig class a (unit) = method a : int method b : int end end
is not included in
Class types do not match:
  class a (unit) = method a : int method b : int end
is not included in
  class a (unit) = method a : int end

-- Christian