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	(return-path <lpw25@cam.ac.uk>); Thu, 30 Oct 2014 15:19:39 +0000
From: Leo White <lpw25@cam.ac.uk>
To: Philippe Veber <philippe.veber@gmail.com>
Cc: caml users <caml-list@inria.fr>
References: <CAOOOohQ=KhAxx0jXZ9v1F1xfu-ddVXEz8O=oOqG7uo5O67FNSA@mail.gmail.com>
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Date: Thu, 30 Oct 2014 15:19:39 +0000
In-Reply-To: <CAOOOohQ=KhAxx0jXZ9v1F1xfu-ddVXEz8O=oOqG7uo5O67FNSA@mail.gmail.com>
	(Philippe Veber's message of "Thu, 30 Oct 2014 15:51:35 +0100")
Message-ID: <86y4rxfuus.fsf@cam.ac.uk>
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Subject: Re: [Caml-list] Typing of a parameterized class where the type
 parameter is the type of self.

> # class type ['a] c = object ('a) method m : 'b end;;
> class type ['a] c = object ('a) constraint 'a = < m : 'b. 'b; .. > method m : 'b end
>
> In particular I don't understand why the method m gets a polymorphic
> type.

To allow shorter type annotations for class types, OCaml tries to
automatically turn methods with polymorphic types into polymorphic
methods. In this case it is being a bit over eager and making your
method polymorphic even though it doesn't need to.

You can fix this by making it more obvious that your method's
polymorphism is contained within the class parameter 'a. You can do this
by writing in the constraint by hand:

    class type ['a] c = object ('a)
      constraint 'a = < m : 'b; ..>
      method m : 'b
    end

Which now allows your type definition to work:

    # type 'a t = (< m : int ; .. > as 'a) c;;
    type 'a t = 'a c constraint 'a = < m : int >

Regards,

Leo