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Date: Sun, 07 Jul 2002 20:25:12 -0500
From: Brian Smith <brian-l-smith@uiowa.edu>
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To: OCaml Mailing list <caml-list@inria.fr>
Subject: [Caml-list] Polymorphic methods (longest error message ever!)
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Hi everybody,

First of all, where can I find some documentation about the CVS versino 
of O'Caml? Is the documentation kept in CVS too? Anyway, on to the 
problem at hand:

I have a class like this:
    class type ['a] node_type =
      object
        method as_variant : 'a
        constraint 'a = [> `Node of 'a node_type]
      end;;
    class ['a] node : ['a] node_type =
      object (self)
        method as_variant : 'a = `Node (self :> 'a node_type)
      end;;

That works fine in O'Caml 3.04. But, I want to use the new polymorphic 
method feature in O'Caml 3.04+15. So I tried:

  # class type node_type =  object
      method as_variant : [> `Node of node_type]
    end;;
  class type node_type =
     object method as_variant : [> `Node of node_type] end

  # class node : node_type = object (self)
      method as_variant = `Node (self :> node_type)
    end;;
                                 ^^^^
  This expression cannot be coerced to type
  node_type = < as_variant : 'a. [> `Node of node_type] as 'a >;
  it has type < as_variant : 'c; .. > as 'b but is here used with type 'b

  # class node : node_type = object (self)
      method as_variant : 'a. [> `Node of node_type] as 'a
                        = `Node (self :>  node_type)
    end;;
The class type object method as_variant : [> `Node of node_type] end
is not matched by the class type node_type
The class type object method as_variant : [> `Node of node_type] end
is not matched by the class type
   object method as_variant : [> `Node of node_type] end
The method as_variant has type 'a. [> `Node of node_type] as 'a
but is expected to have type 'b. [> `Node of node_type] as 'b
The universal variable 'b would escape its scope

I have no idea what the error message means. To me, it is saying that 
class "node" doesn't match class type "node_type" even though everything 
has exactly the same type. Could somebody please explain it to me? What 
is the correct way of doing the above?

Thanks,
Brian


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