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From: Tom Hirschowitz <Tom.Hirschowitz@ens-lyon.fr>
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Date: Wed, 26 Nov 2003 10:42:41 -0800
To: caml-list@inria.fr
Subject: [Caml-list] sum types with objects
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Hi all,

when trying to implement lists with objects and classes, I run into
the following issue.

The idea is to define a class type list, parameterized over the type
'a of elements, and containing the methods is_nil, hd, tl, map. Then,
I planned to define two classes nil and cons for building objects of
type list.

The problem arises during the typing of list:

# class type ['a] list = object
  method is_nil : bool
    method hd : 'a
    method tl : 'a list
    method map : ('a -> 'b) -> 'b list
  end
    ;;
class type ['a] list =
  object
    method hd : 'a
    method is_nil : bool
    method map : ('a -> 'a) -> 'a list
    method tl : 'a list
  end

The method map has type ('a -> 'a) -> 'a list, which is not what I
want.  Trying to explicitely quantify 'b is also wrong, because 'b
escapes its scope.

Does someone know how to make this work?

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