Add example showing how to customize rendering errors about warp

[jxs]

Hi there,
I tried running this example:

#![deny(warnings)]
extern crate warp;
extern crate hyper;

use warp::Filter;
use hyper::{Response, StatusCode};


fn main() {
    let simple = warp::get(warp::path::index()).map(warp::reply);

    let errors = simple.or_else(|rejection: warp::Rejection| match rejection.status() {
        StatusCode::NOT_FOUND => {
            Ok(Response::builder()
                .status(404)
                .body("Not Found"))
        },
        _ => {
            // convert all?? others into just 400s
            Ok(Response::builder()
                .status(400)
                .body("i swallowed an error!"))
        }
    });

    warp::serve(simple).run(([127, 0, 0, 1], 3030));
}

and after having to annotate the closure parameter I get

error[E0271]: type mismatch resolving `<std::result::Result<std::result::Result<hyper::Response<&str>, warp::http::Error>, warp::Rejection> as futures::future::IntoFuture>::Item == (impl warp::Reply,)`
  --> examples/custom_errors.rs:12:25
   |
12 |     let errors = simple.or_else(|rejection: warp::Rejection| match rejection.status() {
   |                         ^^^^^^^ expected enum `std::result::Result`, found tuple
   |
   = note: expected type `std::result::Result<hyper::Response<&str>, warp::http::Error>`
              found type `(impl warp::Reply,)`

if I change the or_else return to match (impl warp::Reply,) I get

error[E0277]: the trait bound `(std::result::Result<hyper::Response<&str>, warp::http::Error>,): warp::Future` is not satisfied
  --> examples/custom_errors.rs:12:25
   |
12 |     let errors = simple.or_else(|rejection: warp::Rejection| match rejection.status() {
   |                         ^^^^^^^ the trait `warp::Future` is not implemented for `(std::result::Result<hyper::Response<&str>, warp::http::Error>,)`
   |
   = note: required because of the requirements on the impl of `futures::future::IntoFuture` for `(std::result::Result<hyper::Response<&str>, warp::http::Error>,)`

shouldn't or_else's output be just an IntoFuture type yielding the same item and error types. ?

[seanmonstar]

Yep, I started working on this just now, and ran into exactly what you noticed too. I may have goofed on the or_else type signature... Well, I guess it works as long as you return in the Ok case the exact same type, it just fails in this first example because the first type is erased (impl Reply), and I don't think you could make a new one in separate function. It'd work if this example were changed to return a concrete type.

Still, I think this means or_else is not actually the easiest way to catch errors and simply return a new reply. That means probably a new combinator is needed that allows returning a new type (and so the the filter would end up returning Either<A, B>). I'm leaning towards recover as the combinator...

[caolan]

I also ran into this recently and ended up nesting my custom error handler somewhere to simplify the types. If you do implement a new combinator, I'd really like to see an example of handling e.g. 404 differently for different sub-paths. I couldn't figure that out using or_else.