[Caml-list] Error: In this definition, a type variable cannot be deduced from the type parameters.

[Goswin von Brederlow <goswin-v-b@web.de>]

Hi,

if I use the type aliases in the commented out lines then I get:

    Error: In this definition, a type variable cannot be deduced
    from the type parameters.

To me the types sure do look alike and nothing is private or hidden my
a signature. So what am I missing?

MfG
	Goswin
-------------------------------------------------------------------------
module Proxy = struct
  type +'a t
end

module QObject = struct
  type +'a qObject'
  type +'a t = 'a qObject' Proxy.t

  class ['a] qObject proxy = object
    constraint 'a = 'b qObject' as 'a
    method proxy : 'a Proxy.t = proxy
  end
end

module QWidget = struct
  type +'a qWidget'
  type +'a t = 'a qWidget' QObject.t

  external add : 'a qWidget' QObject.qObject' Proxy.t -> 'b qWidget' QObject.qObject' Proxy.t -> unit = "add"
  (*
    external add : 'a t -> 'b qWidget' QObject.t -> unit = "add"
  *)
  class ['a] qWidget proxy = object
    constraint 'a = 'b qWidget' QObject.qObject' as 'a
    inherit ['a] QObject.qObject proxy
    method add : 'c 'd . ('d qWidget' QObject.qObject' #QObject.qObject as 'c) -> unit = fun w -> add proxy w#proxy
  end
end

module QButton = struct
  type +'a qButton'
  type +'a t = 'a qButton' QWidget.t

  external set : 'a qButton' QWidget.qWidget' QObject.qObject' Proxy.t -> 'b QWidget.qWidget' QObject.qObject' Proxy.t -> unit = "set"
  (*
    external set : 'a t -> 'b QWidget.t -> unit = "set"
  *)
  class ['a] qButton proxy = object
    constraint 'a = 'b qButton' QWidget.qWidget' QObject.qObject' as 'a
    inherit ['a] QWidget.qWidget proxy
    method set : 'c 'd . ('c QWidget.qWidget' QObject.qObject' #QWidget.qWidget as 'd) -> unit = fun w -> set proxy w#proxy
    method text = "button"
  end
end

let qObject_proxy : unit QObject.t = Obj.magic 0
let qWidget_proxy : unit QWidget.t = Obj.magic 0
let qButton_proxy : unit QButton.t = Obj.magic 0
let qObject = new QObject.qObject qObject_proxy
let qWidget = new QWidget.qWidget qWidget_proxy
let qButton = new QButton.qButton qButton_proxy
let () = qWidget#add qWidget
let () = qWidget#add qButton
let () = qButton#set qWidget
let () = qButton#set qButton
let () = qButton#add qWidget
let () = qButton#add qButton