Ocaml class with a method that takes derived classes

Question

Consider the following code:

module Proxy = struct
  type 'a t
end

class qObject proxy = object(self : 'self)
  method proxy : 'self Proxy.t = proxy
end

class qWidget proxy = object(self : 'self)
  inherit qObject proxy
  method add : qWidget -> unit = fun w -> ()
  method as_qWidget = (self :> qWidget)
end

class qButton proxy = object(self : 'self)
  inherit qWidget proxy
  method text = "button"
end

let qObject_proxy : qObject Proxy.t = Obj.magic 0
let qWidget_proxy : qWidget Proxy.t = Obj.magic 0
let qButton_proxy : qButton Proxy.t = Obj.magic 0

let qObject = new qObject qObject_proxy
let qWidget = new qWidget qWidget_proxy
let qButton = new qButton qButton_proxy

let () = qWidget#add qWidget
let () = qWidget#add qButton#as_qWidget

This code is well typed and compiled. But qButton must be manually added to qWidget, which I want to eliminate. I want qWidget # add to accept another qWidget or any derived class (e.g. qButton). I thought #qWidget would be the right type for this, but this does not work:

class qWidget proxy = object(self : 'self)
  inherit qObject proxy
  method add : #qWidget -> unit = fun w -> ()
end

Error: Some type variables are unbound in this type: ...
The method add has type 'c -> unit where 'c is unbound

and

class qWidget proxy = object(self : 'self)
  inherit qObject proxy
  method add : 'a . (#qWidget as 'a) -> unit = fun w -> ()
end

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

Is there some way that I don't see?

Answer 1

. , add , qWidget, , qWidget, as_qWidget. , qWidget #qWidget.

class qWidget proxy = object(self : 'self)
  inherit qObject proxy
  method add : 'a . (<as_qWidget : qWidget; ..> as 'a) -> unit
             = fun w -> let w = w#as_qWidget in ()
  method as_qWidget = (self :> qWidget)
end