[Feature Request] Recover a sub Exn error from children frames

[macier-pro]

Context

I'm trying to extract structured data from errors for programmatic handling, but going further by getting a sub Exn from the error tree. Currently, the error tree is lost by using downcast_ref.

The closest I can get is by walking through the error frame to extract a particular error, but it would only return a reference to the underlying error, losing the error tree.

fn find_error<T: Error + 'static>(exn: &Exn<impl Error + Send + Sync>) -> Option<&T> {
    fn walk<T: Error + 'static>(frame: &Frame) -> Option<&T> {
        if let Some(e) = frame.error().downcast_ref::<T>() {
            return Some(e);
        }
        frame.children().iter().find_map(walk)
    }
    walk(exn.frame())
}

This example comes from https://github.com/fast/exn/blob/main/examples/src/downcast.rs#L63C1-L71C2

Expectations

Let's say the error tree looks like this :

MainError -> ErrorA -> ErrorB

Using find_error, we can get a &ErrorA using a downcast_ref. However, the error tree is lost.

I would like to retrieve a Exn<ErrorA> to use it as I would with other functions I already implemented.
This way, we can always process it later and keep the info that ErrorA was caused by ErrorB.

Alternatives

I could just work with the frame tree, but it's really not user-friendly nor as convenient as having a proper Exn.

[tisonkun]

Hi @macier-pro! Thanks for reporting your use case and issue.

However, I can't get your point very clear. Could you please reorganize your report as:

1. Reproduce code.
2. What you expect to see/do.
3. What actually happens.
4. (optional) Any additional info.

[macier-pro]

Hi @tisonkun, thanks for your input.
Sorry, my question wasn't clear. I've rewritten the original message.

[andylokandy]

Hi @macier-pro, thanks for bringing this up! Sounds like you're going to pop away some layers of Exn<E> until a dedicated error frame. It maybe possible to implement such an API, but it'll make you loss the context and location of the layers being popped away. In most case, we find it sufficient to just downcast into the underlying error. Would you like share your use case where downcast to E is not sufficient and you need to downcast to an Exn<E>?