OCaml split a list into fixed-size batches

let chunk size items =
  if size <= 0 then invalid_arg "chunk: size must be positive";
  let rec split count acc rest =
    if count = 0 then (List.rev acc, rest)
    else match rest with
      | [] -> (List.rev acc, [])
      | x :: xs -> split (count - 1) (x :: acc) xs
  in
  let rec loop acc rest =
    match rest with
    | [] -> List.rev acc
    | _ ->
      let batch, remaining = split size [] rest in
      loop (batch :: acc) remaining
  in
  loop [] items