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From: Christopher Zimmermann <madroach@gmerlin.de>
Date: Wed, 24 Oct 2012 20:03:10 +0200
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Subject: [Caml-list] typing mutually recursive classes

Hi,

I have a problem with typing a system of mutually recursive classes.

This piece of code fails to compile:

class a =
  object end
and b =
  object
    method foo: a -> int =
      fun s -> 3
  end;;

Error: The universal type variable 'a cannot be generalized:
       it escapes its scope.


But this compiles fine:

class a =
  object end
class b =
  object
    method foo: 'a. (#a as 'a) -> int =
      fun s -> 3
  end;;


What I actually want to do is this:

class element id (registry :#registry) =
  object
    method registry = registry
  end

and registry =
  object
    val set = []
    method register :'a. (#element as 'a) -> unit =
      fun s ->
        set <- s :: set
  end


Any ideas how to do this without parametrizing the classes?

Christopher