module Duck_tales =
  struct
    class little_duck (name : string) =
      object
        method quack = name ^ " quacks"
      end
 
    let huey  = new little_duck "Huey"
    let dewey = new little_duck "Dewey"
    let louie = new little_duck "Louie"
  end
 
module Disney =
  struct
    let donald_duck =
      object
        method quack = "Donald Duck quacks"
      end
  end
 
module Ducks_client =
  struct
    type duck = <
      quack : string
    >
 
    let sound (d : duck) = print_endline d#quack
  end
 
let _ =
  let ducks = [
    Duck_tales.huey ;
    Duck_tales.dewey ;
    Duck_tales.louie ;
    Disney.donald_duck
  ] in
  List.iter Ducks_client.sound ducks
 

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