update.ml

let rec fupdate: 'a. ('a -> 'a) -> int -> 'a seq -> 'a seq =
  fun updater index digits -> match (index, digits) with
    | (index, Nil) -> raise IndexOutOfBoundsException
    | (0, One (elem, restDigits)) -> One (updater elem, restDigits)
    | (index, One (elem, restDigits)) ->
      push elem (fupdate updater (index - 1) (Zero restDigits))
    | (index, Zero restDigits) ->
    let updater' = fun (x, y) -> if index mod 2 == 0
      then (updater x, y)
      else (x, updater y)
      in Zero (fupdate updater' (index / 2) restDigits)

let rec update index element digits =
  fupdate (fun x -> element) index digits