The failchain's discuss from cristicbz

failchain's Issues

Story with futures

It's real nice having result.chain_err(|| ...) for your results but then when using code involving futures you end up having to go back to

future.map_err(|e| -> MyError {
    let kind = ...;
    e.context(kind).into()
})

In order to get around this, I made a variant of ResultExt that works for futures (0.1 for now):

pub trait FutureExt<S, E: Fail>: Sized + Future<Item = S, Error = E> {
    fn chain_inspect_err_fut<ErrorKindT: ChainErrorKind>(
        self,
        fn_in: impl FnOnce(&mut E) -> ErrorKindT + Send + Sync + 'static,
    ) -> MapErr<Self, Box<dyn FnOnce(E) -> ErrorKindT::Error + Send + Sync>> {
        self.map_err(Box::new(|mut e| {
            let kind = fn_in(&mut e);
            e.context(kind).into()
        }))
    }

    fn chain_err_fut<ErrorKindT: ChainErrorKind>(
        self,
        fn_in: impl FnOnce() -> ErrorKindT + Send + Sync + 'static,
    ) -> MapErr<Self, Box<dyn FnOnce(E) -> ErrorKindT::Error + Send + Sync>> {
        self.chain_inspect_err_fut(|_| fn_in())
    }
}

impl<S, E: Fail, F: Future<Item = S, Error = E>> FutureExt<S, E> for F {}

// used like:
    client
        .request(request)
        .chain_err_fut(|| ErrorKind::OtherError("foo".to_owned()))

Some immediate issues:

I need Send + Sync + 'static and need to enforce them, maybe some use cases don't
Hard coded to futures-0.1, but I think we wouldn't be able to make it more general until HKT
Boxing because you can't use impl Fn in traits.

Is there any interest here? I'm using it and it seems handy. Maybe there's a better discussion to be had in failure's ResultExt which could have the same pattern?

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