The ErrorMonad in OCaml

error.ml

module ErrorMonad : sig
  type 'a t
  val ret : 'a -> 'a t
  val ( >>= ) : 'a t -> ('a -> 'b t) -> 'b t
  val fail : 'a t
  val run : 'a t -> 'a option
end = struct
  type 'a t = 'a option
  let ret x = Some x
  let fail = None
  let run x = x
  let ( >>= ) x f = match x with None -> None | Some x -> f x
end

let some_monadic_code x = ErrorMonad.(run (
    if x = 0 then fail else ret (x + 1) >>= fun y ->
    if y mod 3 = 1 then fail else ret (y * 2) >>= fun z ->
    ret z
))