parse csv in stringmap

module StringMap = Map.Make(String);;
type xyz = {x:int;y:int;z:int;prefs:int StringMap.t};;
let prefs_str = "1134,792,1390,296,597,641,394,1480,23,551"

let parse_prefs sep str =
  let string_index_from i =
    try Some (String.index_from str i sep)
    with Not_found -> None in
    let rec aux i n x =
    match string_index_from i with
    |Some m->let m=StringMap.add (String.sub str i (m-i)) n x in aux (succ m) (succ n) m
    |None  ->let m = StringMap.add (String.sub str i (String.length str-i)) n x
    in aux 0 1 StringMap.empty

let () = let tmp = parse_prefs ',' prefs_str in
    StringMap.iter (fun k v -> Printf.printf "%s -> %d\n" k v) tmp