### example.ml [ocaml]
open Batteries

module type N = sig
  type t
  val make: unit -> t
  val add_to_self: t -> t -> unit
  val as_float: t -> float
end

module Int: N = struct
  type t = int ref
  let make () = ref 1
  let add_to_self x y = x := !x + !y
  let as_float = (!) |- float_of_int
end

module Float: N = struct
  type t = float ref
  let make () = ref 0.5
  let add_to_self x y = x := !x +. !y
  let as_float = (!) |- identity
end

let which_module = function
  | "float" -> (module Float: N)
  | "int" -> (module Int: N)
  | _ -> invalid_arg "which_module"

let do_repetitive_work m x =
  let module C = (val m: N) in
  C.make () |> C.add_to_self x;
  C.add_to_self x x

let () =
  let module C = (val (which_module Sys.argv.(1)): N) in
  let x = C.make () in
  do_repetitive_work (module C: N) x;
  C.as_float x |> Printf.printf "%g\n"

### error.txt
File "example.ml", line 31, characters 29-30:
Error: This expression has type 'a but an expression was expected of type C.t
       The type constructor C.t would escape its scope