exceptions.md

title
Exceptions

In the JS world, exception could be any data, while an OCaml exception is a structured data format and supports pattern matching. Catching an OCaml exception on JS side therefore doesn't work as intended.

JS exceptions can be raised from the BuckleScript side by using the JS.Exn.raise* functions, and can be caught as a BS exception of the type Js.Exn.Error with the JS exception as its payload, typed as Js.Exn.t. The JS Exception can then either be manipulated with the accessor functions in Js.Exn, or casted to a more appropriate type.

try
  Js.Exn.raiseError "oops!"
with
| Js.Exn.Error e ->
  match Js.Exn.message e with
  | Some message -> Js.log {j|Error: $message|j}
  | None -> Js.log "An unknown error occurred"

try (
  Js.Exn.raiseError("oops!")
) {
| Js.Exn.Error(e) =>
  switch (Js.Exn.message(e)) {
  | Some(message) => Js.log({j|Error: $message|j})
  | None => Js.log("An unknown error occurred")
  }
};

Usage

Take promise for example:

exception UnhandledPromise

let handlePromiseFailure = function [@bs.open]
  | Not_found ->
    Js.log "Not found";
    Js.Promise.resolve ()

let _ =
 Js.Promise.reject Not_found
 |> Js.Promise.catch (fun error ->
      match handlePromiseFailure error with
      | Some x -> x
      | None -> raise UnhandledPromise
  )

exception UnhandledPromise;

let handlePromiseFailure =
  [@bs.open]
  (
    fun
    | Not_found => {
        Js.log("Not found");
        Js.Promise.resolve()
      }
  );

Js.Promise.reject(Not_found)
  |> Js.Promise.catch(
     (error) =>
       switch (handlePromiseFailure(error)) {
       | Some(x) => x
       | None => raise(UnhandledPromise)
       }
   );