Simple Reader monad in OCaml

reader.ml

module Reader = struct
  type ('e, 'a) t = Reader of ('e -> 'a)

  let run = function
    | Reader r -> r

  let map f m = Reader (fun env -> f (run m env))

  let bind f m = Reader (fun env -> run (f (run m env)) env)

  let ( >>= ) m f = bind f m

  let return x = Reader (fun _ -> x)

  let ask () = Reader (fun env -> env)

  let local f m = Reader (fun env -> run m (f env))
end

(* Example *)
type env = {
  name: string;
  age: int;
}

let to_string age name = 
  "name: " ^ name ^ "\nage: " ^ string_of_int age

let name env = env.name

let _ =
  let open Reader in
  let r =
    return 24
    |> map (fun x -> x + 1)
    >>= (fun x -> ask () |> map name |> map (to_string x))
    |> local (fun env -> {env with name="Vincent"})
  in
  let env = {name= "Jack"; age= 85} in
  Printf.printf "%s\n" (run r env)
(* name: Vincent *)
(* age: 25 *)