Conditional Routing

[jkassis]

Is your feature request related to a problem? Please describe.
I'm always frustrated when trying to write a conditional chain of filters using or.

Describe the solution you'd like
from the examples, we have this...

pub fn assets_filter() -> BoxedFilter<(impl Reply,)> {
    warp::path("assets")
        .and(warp::fs::dir("./assets"))
        .boxed()
}

I'd like the ability to do something like this...

pub fn assets_filter() -> BoxedFilter<(impl Reply,)> {
    if let Some(optional_filter) = optional_filter {
        warp::path("assets")
            .and(warp::fs::dir("./assets"))
            .or(optional_filter)
            .boxed()
    } else {
        warp::path("assets").and(warp::fs::dir("./assets")).boxed()
    }
}

Describe alternatives you've considered
I've tried just about everything to make types work out. While it's ok to return BoxedFilter<(impl Reply,)>, it's not OK to return two different variants of this from the fn body.

Additional context
This was raised and closed in https://github.com/seanmonstar/warp/issues/699, but does not seem like a solution for the rest of us.

[seanmonstar]

Yea, that can be annoying. There's three potential options I can think of:

1. Use some Either-like enum for the impl Reply

if something {
    filters_a().map(|reply| Either::Left(reply))
} else {
    filters_b().map(|reply| Either::Right(reply))
}

2. Box the impl Reply

if something {
    filters_a().map(|reply| Box::new(reply) as Box<dyn Reply>)
} else {
    filters_b().map(|reply| Box::new(reply) as Box<dyn Reply>)
}

3. Some native filter support

This would be nice, but I don't know what exactly it would look like. It's a common request, to be able to optionally include another filter. A well thought out proposal is always welcome.