OCaml implementation of a nub function

uniques.ml

let rec elem x xs =
  match xs with
  | [] -> false
  | y :: ys -> x = y || elem x ys

let rec uniques l =
  let rec helper l acc =
    match l with
    | [] -> acc
    | x :: xs ->
        if elem x acc then
          helper xs acc
        else
          helper xs (x :: acc)
  in List.rev (helper l [])