par_iter.ml

(* run like this: ocamlc unix.cma par.ml && ./a.out *)
open StdLabels

let par_iter (items: 'i list) ~(f: 'i -> unit): unit =
  let orig_pid = Unix.getpid () in
  let rec loop pids = function
  | [] -> pids
  | h :: t ->
    match Unix.fork () with
    | 0 -> f h; pids
    | pid -> loop (pid :: pids) t
  in
  let pids = loop [] items in
  if Unix.getpid () = orig_pid then begin
    List.iter pids ~f:(fun pid -> ignore @@ Unix.waitpid [] pid)
  end else
    exit 0

let () = (* usage **)
  let slow_print_int work =
      Random.self_init ();
      let wait_seconds = Random.int 3 in
      Unix.sleep @@ 1 + wait_seconds;
      Format.printf "work %d started\n" work;
      let rec loop n =
        if n < 9999999 then loop @@ n + 1

      in loop 0;
      Format.print_flush ();
      Format.printf "work %d finished in %d seconds\n" work wait_seconds;
      Format.print_flush ()
  in
  par_iter [1; 2; 3; 4; 5;] ~f:slow_print_int;
  par_iter ["hello"; "world"] ~f:print_endline;

(* example output:
  work 2 started
  work 2 finished in 0 seconds
  work 1 started
  work 1 finished in 1 seconds
  work 3 started
  work 3 finished in 2 seconds
  work 4 started
  work 4 finished in 2 seconds
  hello
  world
*)