open Core

module File : sig
  type readonly
  type readwrite
  type 'a t
  val open_readwrite : string -> readwrite t
  val open_readonly : string -> readonly t
  val read: 'a t -> string
  val write : 'readwrite t -> string -> unit
end = struct
  type readonly
  type readwrite
  type 'a t = int
  let open_readwrite filename : (readwrite t) =
    ((String.length filename) : (readwrite t))

  let open_readonly filename =
    String.length filename

  let read f = "reading" ^ (string_of_int f)

  let write f (_ : string) =
    let (_ : unit) = print_string ("writing" ^ (string_of_int f) ^ "\n") in ()
end

let () =
  let f = File.open_readonly "file" in
  File.write f "blah"