Count lines with async (ocaml)

lines_async.ml

open Async.Std

let t fn () =
  Reader.open_file fn
  >>| Reader.lines
  >>= Pipe.fold ~init:0 ~f:(fun n _ -> return (n + 1))
  >>= fun n -> return (Print.printf "%d\n" n)

let app = Command.(async Spec.(step (fun t fn -> t fn) +> (anon ("fn" %: string)))) ~summary:"bench" t

let () = Command.run app