Unique elements from a list

unique.ml

let unique (type s) ?(cmp = compare) (l : s list) =
  let module M = struct
    type t = s
    let compare = cmp
  end in
  let module S = Set.Make(M) in
  let s = List.fold_left (fun accu x -> S.add x accu) S.empty l in
  S.elements s