module type Number = sig
  type t
  val (+) : t -> t -> t
  val (/) : t -> t -> t
  val of_int : int -> t
  val init : unit -> t
end

module Make = functor
    (N : Number) ->
  struct
    let sum arr =
      let open N in
      let r = ref @@ init () in
      for i = 0 to (Array.length arr) - 1 do
        r := !r + arr.(i)
      done;
      !r
    let ave arr =
      let s = sum arr in
      N.(/) s (N.of_int @@ Array.length arr)
  end


module Float = struct
  type t = float
  let (+) x y = x +. y
  let (/) x y = x /. y
  let init () = 0.0
  let of_int x = float x
end


module FCal = Make(Float)

let main () =
  let arr = [|2.1; 2.2; 2.3|]
  in
  Printf.printf "%f\n" @@ FCal.ave arr