(**
 * Copyright (c) 2015, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the "hack" directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 *)


(*****************************************************************************)
(* A modified maybe monad
 * Most of the time, I prefer to use exceptions, I like things to blow up
 * if something went wrong.
 * However, in the case of dfind, exceptions are painful. We don't want things
 * to blow-up, we want to carry-on whatever happens.
 * So this monad never fails, it logs very nasty errors, for example, it will
 * log the fact that a watch couldn't be created, when the file still exists.
*)
(*****************************************************************************)

let log = ref stderr
let set_log oc = log := oc

type 'a t = 'a option

let (>>=) x f =
  match x with
  | None -> None
  | Some x -> f x

let return x = Some x

let handle_file_exn path = function
  | Fsnotify.Error (reason, 2) -> ()
      (* The file got deleted in the mean time ... we don't care *)
  | Fsnotify.Error (reason, errno) -> 
      (* This is bad ... *)
      Printf.fprintf !log 
        "Error: could not add watch to %s [%s, %d]\n" path reason errno
  | e when Sys.file_exists path ->
      (* Logging this makes the system very noisy. There are too many
       * cases where a file has been removed etc ...
       *)
      ()
  | _ -> ()

(* Calls (f path), never fails, logs the nasty exceptions *)
let call f path = 
  try f path
  with e -> handle_file_exn path e; None

let wrap f = fun x -> return (f x)

