one_weird_trick.ml

module type EXAMPLE = sig
  class example_class : int ->
  object
    method get_i : int
    method compare : example_class -> bool
  end
end

module Example : EXAMPLE = struct
  class example_class (i : int) =
  object
    val mutable i' = i
    method get_i = i'
    method compare (other : example_class) = other#get_i == i'
  end
end

module Experiment : sig
  type t
  class experiment_class' : int ->
  object
    method get_i : int
    method compare : t -> bool
  end
  val cast : t -> experiment_class'
  val make : int -> t
end = struct
  class experiment_class (i:int) =
  object
    val mutable i' = i
    method get_i = i'
    method compare (other: experiment_class) = other#get_i == i'
  end
  type t = experiment_class
  class experiment_class' = experiment_class
  let cast x = x
  let make i = new experiment_class' i
end

(* Commented out lines won't compile! *)
let demo () =
  let x = object                                                                                                        
    method get_i = 3
    method compare _ = false
  end in
  let y = new Example.example_class 3 in
  let z = Experiment.make 2 in
  let z'= Experiment.make 2 in
  let z'' = Experiment.make 3 in
    [ x#compare y ;
      (Experiment.cast z)#compare z ;
      y#compare x ;
      (* (Experiment.cast z)#compare y ; *)
      (* y#compare (Experiment.cast z) ; *)
      (Experiment.cast z)#compare z'' ;                                                                                 
      (Experiment.cast z)#compare z' ;                                                                                  
      (Experiment.cast z')#compare z ]