Question

在我的代码的简化版本中，我想有时执行标记的行，有时不执行，可能返回错误：

extern crate futures; // 0.1.26
extern crate hyper; // 0.12.25

use hyper::rt::{Future, Stream};
use std::str::FromStr;

struct MyStream {}

impl Stream for MyStream {
    type Item = hyper::Uri;
    type Error = ();

    fn poll(&mut self) -> Result<futures::Async<Option<Self::Item>>, Self::Error> {
        Ok(futures::Async::Ready(Some(
            hyper::Uri::from_str("http://www.google.com/").unwrap(),
        )))
    }
}

fn main() {
    let client = hyper::Client::new();
    let futs = MyStream {}
        .map(move |uri| {
            client
                .get(uri)
                .and_then(|res| {
                    res.into_body().concat2() // <----------------
                })
                .map(|body| {
                    println!("len is {}.", body.len());
                })
                .map_err(|e| {
                    println!("Error: {:?}", e);
                })
        })
        .buffer_unordered(2)
        .for_each(|_| Ok(()));

    hyper::rt::run(futs);
}

我想我可以用这样的东西代替这行：

let do_i_want_to_get_the_full_page = true;
if do_i_want_to_get_the_full_page {
    res.into_body().concat2().map_err(|_| ())
} else {
    futures::future::err(())
}

因为期货的Error部分是相同的，并且可以推断Item部分。但是，它不起作用。我该怎么办？

这是我得到的错误：

error[E0308]: if and else have incompatible types
  --> src/main.rs:31:25
   |
28 | /                     if do_i_want_to_get_the_full_page {
29 | |                         res.into_body().concat2().map_err(|_| ())
   | |                         ----------------------------------------- expected because of this
30 | |                     } else {
31 | |                         futures::future::err(())
   | |                         ^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `futures::MapErr`, found struct `futures::FutureResult`
32 | |                     }
   | |_____________________- if and else have incompatible types
   |
   = note: expected type `futures::MapErr<futures::stream::Concat2<hyper::Body>, [closure@src/main.rs:29:59: 29:65]>`
              found type `futures::FutureResult<_, ()>`

Answer 1

问题是map_err returns a struct MapErr而err returns a struct FutureResult。

解决问题的一种方法是像这样统一它们：

import timeit
code = """
s = 'Hello world'
s.endswith('d')
"""
timeit.timeit(code, number=10000)

另一种解决方法是将您的返回值装箱：

let f = if do_i_want_to_get_the_full_page {
    futures::future::ok(())
} else {
    futures::future::err(())
};
f.and_then (|_| { res.into_body().concat2().map_err(|_| ()) })

第三个解决方案是始终返回if do_i_want_to_get_the_full_page { Box::<Future<_, _>>::new (res.into_body().concat2().map_err(|_| ())) } else { Box::<Future<_, _>>::new (futures::future::err(())) }：

MapErr

但是，您遇到了一个问题，因为if do_i_want_to_get_the_full_page { res.into_body().concat2().map_err(|_| ()) } else { futures::future::err(()).map_err(|_| ()) }的错误类型必须实现client.get(…).and_then(…)：

From<hyper::Error>

如果您不关心该错误，可以将其映射到error[E0271]: type mismatch resolving `<futures::AndThen<futures::FutureResult<(), ()>, futures::MapErr<futures::stream::Concat2<hyper::Body>, [closure@src/main.rs:34:70: 34:76]>, [closure@src/main.rs:34:32: 34:77 res:_]> as futures::IntoFuture>::Error == hyper::Error` --> src/main.rs:28:18 | 28 | .and_then(|res| { | ^^^^^^^^ expected (), found struct `hyper::Error` | = note: expected type `()` found type `hyper::Error`之前：

and_then

playground

或在调用client .get(uri) .map_err (|_| ()) .and_then(|res| { let f = if do_i_want_to_get_the_full_page { futures::future::ok(()) } else { futures::future::err(()) }; f.and_then(|_| res.into_body().concat2().map_err(|_| ())) }).map(|body| { println!("len is {}.", body.len()); }).map_err(|e| { println!("Error: {:?}", e); })时使用实现From<hyper::Error>的类型。

有可能Future :: and_then有条件地返回不同的期货吗？

1 个答案: