unexpected output when pattern matching exception on a promise - exception

Question

I have the following piece of code:
let test =
Js.Promise.resolve("Hello") |> Js.Promise.then_(_obj => raise(Not_found));
let ker =
switch test {
| exception Not_found => Js.log("not found")
| _ => Js.log("found")
};
The output it produces is:
"found"
Why does the above code not produce the output "not found" and what should I do to make sure the first branch in the switch statement gets executed ?

Answer 1

This mostly has to do with the way JavaScript handles exceptions that get thrown inside promises. They need to be handled using https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/catch , which is bound to https://bucklescript.github.io/bucklescript/api/Js.Promise.html#VALcatch , because they are 'caught' inside the promise and stay inside the promise context.
Concretely, I would turn your example into this:
let test = Js.Promise.(resolve("Hello") |> then_(_obj => raise(Not_found)));
let ker = Js.Promise.(test
|> then_((_) => "found" |> Js.log |> resolve)
|> catch((_) => "Not found" |> Js.log |> resolve));

Answer 2

Because a promise is a promise, regardless if it's a failed one or not.
In your case, at some point it will be resolved to a failed promise, but it will never raise/throw an error. It will just be a promise whose inner value, at some point will be an error (a non thrown error).
In order to catch the error (or value), you need to use the catch function on your test binding. The catch function does with rejected promises, what the then function does to fulfilled ones.
Snippet:
let handleData = [#bs.open] (
fun
| Invalid_argument(_) => Js.Promise.resolve(0)
| Not_found => Js.Promise.resolve(1)
| Sys_error(_) => Js.Promise.resolve(2)
);
let ker =
test |> Js.Promise.catch(handleData)