Re: [Caml-list] Lwt and exceptions (2015)

[Andreas Hauptmann <andreashauptmann@t-online.de>]

On Tue, 7 Jul 2015 05:40:18 +0000
Mike Lin wrote:

> I've long been observing the admonishment to avoid raising native
> exceptions in Lwt code [1,2].
[...]
> that Lwt.bind sets up a handler for native exceptions raised in
> asynchronous callbacks [3], they're usually handled just as one would
> naively expect. Actually, I spent a little time trying to come up
> with a non-terribly-contrived example where using a native exception
> breaks the intended handling logic, and did not succeed... 

Consider the following code snippet:
----
let () =
  let open Lwt.Infix in
  let test = ref "1" in
  let help () = Lwt.return_unit in
  let help2 () =
    match Random.int 2 with
    | 0 -> Lwt.return_unit
    | _ -> Lwt.pause () >>= fun () -> Lwt.return_unit
  in
  let f _ = Lwt.fail Not_found in
(*
  let f _ = raise Not_found in
  let f _ = help () >>= fun () -> raise Not_found in
  let f _ = Lwt_main.yield () >>= fun () -> raise Not_found in
  let f _ = help2 () >>= fun () -> raise Not_found in
*)
  let g () = Lwt_unix.sleep 3. >|= fun () -> test := "2" in
  let t =
    Lwt.catch ( fun () -> Lwt.join [ f 3 ; g () ] )
      (function
      | Not_found -> Lwt_io.printl !test
      | x -> Lwt.fail x)
  in
  Lwt_main.run t
----

The program will print something different, depending if you use
'Lwt.fail' or 'raise' inside f. (version one and two of f)
The third and fourth version of f will behave differently, although
both use 'raise Not_found' and help and Lwt_main.yield have the same
signature.
Finally the fifth version will behave unpredictable (help2 could be a
function that either connects to a database or returns a cached value -
it's not a terribly-contrived example).