focusListAt :: Int -> [a] -> Maybe (ListZipper a)
focusListAt = go []
  where
    go _   _ []        = Nothing
    go acc 0 (hd : tl) = Just (acc, hd, tl)
    go acc n (hd : tl) = go (hd : acc) (n - 1) tl

OCaml

let focuscell celllist n =
 let rec loop acc n l =
  match l,n with
    | hd :: tl,n when n > 0 -> loop (hd :: acc) (n - 1) tl
    | [],_  -> None
    | hd :: tl,0 -> Some (acc, hd, tl)
 in loop  [] 0 celllist
;;