structure Dynarray: DYNARRAY = struct
  type 'a t = (int * 'a option Array.array) ref

  exception OutOfBounds

  fun create n =
    ref (0, Array.array (n, NONE))

  fun create0 () = create 10 

  fun length dynarr =
  let
    val (len, _) = !dynarr
  in
    len
  end

  fun push dynarr v =
  let
    val (len, arr) = !dynarr
  in
    if len = Array.length arr then
      let
        val arr2 = Array.array (len * 2, NONE)
      in
        Array.copy { src = arr, dst = arr2, di = 0 };
        dynarr := (len, arr2);
        push dynarr v
      end
    else
      Array.update (arr, len, SOME v);
      dynarr := (len + 1, arr)
  end

  fun get dynarr n =
  let
    val (len, arr) = !dynarr
  in
    if n >= len then
      raise OutOfBounds
    else
      valOf $ Array.sub (arr, n)
  end

  fun set dynarr n v =
  let
    val (len, arr) = !dynarr
  in
    if n >= len then
      raise OutOfBounds
    else
      Array.update (arr, n, SOME v)
  end

end