Make a "guessing game" where there is a target number, and as the user makes guesses, the output returns higher or lower until the user is correct.

guesser.ml

open Stdio

let parse_input inp =
  match inp with
  | None -> raise (Failure "No input recorded! Exiting")
  | Some x -> int_of_string x
;;

let rec guesser secret num_of_guesses =
  let input = Stdio.In_channel.input_line Stdio.stdin in
  let guess = parse_input input in
  match Int.compare guess secret with
  | 0 -> printf "Correct! You won in %d guesses!\n" num_of_guesses
  | 1 ->
    print_endline "Lower";
    guesser secret (num_of_guesses + 1)
  | _ ->
    print_endline "Higher";
    guesser secret (num_of_guesses + 1)
;;

let () =
  print_endline "Guess the number!";
  let secret = Random.int 100 in
  guesser secret 1
;;