module StringOrd = struct
  type t = string
  let compare = String.compare
end

module SMap = Map.Make (StringOrd)

let count_words text =
  let words = String.split_on_char ' ' text in
  List.fold_left (fun m w ->
    let n = match SMap.find_opt w m with
            | Some n -> n
            | None -> 0
    in
    SMap.add w (n + 1) m
  ) SMap.empty words

;;

let m = count_words "the quick brown fox jumps over the lazy dog" in
SMap.cardinal m