hphp/hack/src/dfind/dfindMaybe.ml

   1 (*
   2  * Copyright (c) 2015, Facebook, Inc.
   3  * All rights reserved.
   4  *
   5  * This source code is licensed under the MIT license found in the
   6  * LICENSE file in the "hack" directory of this source tree.
   7  *
   8  *)
   9
  10 (*****************************************************************************)
  11 (* A modified maybe monad
  12  * Most of the time, I prefer to use exceptions, I like things to blow up
  13  * if something went wrong.
  14  * However, in the case of dfind, exceptions are painful. We don't want things
  15  * to blow-up, we want to carry-on whatever happens.
  16  * So this monad never fails, it logs very nasty errors, for example, it will
  17  * log the fact that a watch couldn't be created, when the file still exists.
  18  *)
  19 (*****************************************************************************)
  20
  21 let log = ref stderr
  22
  23 let set_log oc = log := oc
  24
  25 type 'a t = 'a option
  26
  27 let ( >>= ) x f =
  28   match x with
  29   | None -> None
  30   | Some x -> f x
  31
  32 let return x = Some x
  33
  34 let handle_file_exn path = function
  35   | Fsnotify.Error (_, Unix.ENOENT) ->
  36     () (* The file got deleted in the mean time ... we don't care *)
  37   | Fsnotify.Error (reason, _) ->
  38     (* This is bad ... *)
  39     Printf.fprintf !log "Error: could not add watch to %s [%s]\n" path reason
  40   | _ when Sys.file_exists path ->
  41     (* Logging this makes the system very noisy. There are too many
  42      * cases where a file has been removed etc ...
  43      *)
  44     ()
  45   | _ -> ()
  46
  47 (* Calls (f path), never fails, logs the nasty exceptions *)
  48 let call f path =
  49   try f path with
  50   | e ->
  51     handle_file_exn path e;
  52     None
  53
  54 let wrap f x = return (f x)