我正试图在Rust中围绕期货，但我对此代码感到困惑，该代码应该将sink的邮件发送到extern crate futures;
extern crate tokio_core;
extern crate websocket;
use websocket::message::OwnedMessage;
use websocket::server::InvalidConnection;
use websocket::async::Server;
use tokio_core::reactor::Core;
use futures::{Future, Sink, Stream};
use futures::sync::mpsc;
use std::{thread, time};
use futures::sync::mpsc::Receiver;
fn main() {
let mut core = Core::new().unwrap();
let (mut tx, rx) = mpsc::channel(5);
thread::spawn(|| worker(rx));
let mut i = 0;
loop {
let res = tx.clone().send(OwnedMessage::Text(format!("Test {}", i)));
core.run(res);
i += 1;
let period = time::Duration::from_millis(200);
thread::sleep(period);
}
}
fn worker(rx: Receiver<OwnedMessage>) {
let mut core = Core::new().unwrap();
let handle = core.handle();
// bind to the server
let server = Server::bind("127.0.0.1:9000", &handle).unwrap();
let f = server.incoming()
// we don't wanna save the stream if it drops
.map_err(|InvalidConnection { error, .. }| error)
.for_each(|(upgrade, addr)| {
// accept the request to be a ws connection if it does
let f = upgrade
.use_protocol("rust-websocket")
.accept()
.and_then(|(s, _)| {
let (sink, stream) = s.split();
rx // using stream (echoing back) works
.forward(sink)
.map_err(|error| {
error
})
.and_then(|(a, sink)| {
sink.send(OwnedMessage::Close(None))
})
});
handle.spawn(f.map_err(move |e| println!("Err"))
.map(move |_| println!("Done")));
Ok(())
});
core.run(f).expect("somerror");
}
：
stream如评论中所述，使用rx作为输入可以正常工作。使用error[E0271]: type mismatch resolving `<futures::stream::SplitSink<websocket::client::async::Framed<tokio_core::net::TcpStream, websocket::async::MessageCodec<websocket::OwnedMessage>>> as futures::Sink>::SinkError == ()`
--> src/main.rs:47:26
|
47 | .forward(sink)
| ^^^^^^^ expected enum `websocket::WebSocketError`, found ()
|
= note: expected type `websocket::WebSocketError`
found type `()`
error[E0599]: no method named `map_err` found for type `futures::stream::Forward<futures::sync::mpsc::Receiver<websocket::OwnedMessage>, futures::stream::SplitSink<websocket::client::async::Framed<tokio_core::net::TcpStream, websocket::async::MessageCodec<websocket::OwnedMessage>>>>` in the current scope
--> src/main.rs:48:26
|
48 | .map_err(|error| {
| ^^^^^^^
|
= note: the method `map_err` exists but the following trait bounds were not satisfied:
`futures::stream::Forward<futures::sync::mpsc::Receiver<websocket::OwnedMessage>, futures::stream::SplitSink<websocket::client::async::Framed<tokio_core::net::TcpStream, websocket::async::MessageCodec<websocket::OwnedMessage>>>> : futures::Future`
时，编译器会抱怨错误类型的类型不匹配（我相信）：
[dependencies]
websocket = "0.20.0"
futures = "0.1"
tokio-core = "0.1"
这些是我的依赖项：
{{1}}
我在这里缺少什么？
答案 0 :(得分：7)
error[E0271]: type mismatch resolving `<futures::stream::SplitSink< websocket::client::async::Framed< tokio_core::net::TcpStream, websocket::async::MessageCodec<websocket::OwnedMessage>>> as futures::Sink>::SinkError == ()`
我们在这里有两种类型：<futures::stream::SplitSink<...> as futures::Sink>::SinkError和()。这两种类型来自哪里？另外，第一个是未解决的关联类型;也许我们可以解决它以获得更多的洞察力？让我们一步一步地追踪它。
首先，我们需要弄清楚编译器首先尝试匹配这两种类型的原因。如果我们查看forward的签名，我们会看到约束Self::Error: From<S::SinkError>。 Self是我们正在调用forward的流的类型，而S是将forward作为参数传递给forward的接收器类型}。
我们在rx上呼叫futures::sync::mpsc::Receiver，其类型为impl<T> Stream for Receiver<T>
type Item = T
type Error = ()
。在documentation page for Receiver上，我们可以看到以下内容：
()
这向我们展示了sink来自哪里。现在让我们看看sink参数。
futures::stream::SplitSink<websocket::client::async::Framed<tokio_core::net::TcpStream, websocket::async::MessageCodec<websocket::OwnedMessage>>>的类型是impl<S: Sink> Sink for SplitSink<S>
type SinkItem = S::SinkItem
type SinkError = S::SinkError
（我们从错误消息中知道这一点; RLS也确认了这一点）。在documentation page for SplitSink上，我们有：
SplitSink
因此SinkError＆＃39; SinkError与其内部websocket::client::async::Framed<tokio_core::net::TcpStream, websocket::async::MessageCodec<websocket::OwnedMessage>>相同。内部接收器的类型是impl<T, U> Sink for Framed<T, U>
where
T: AsyncWrite,
U: Encoder,
<U as Encoder>::Error: From<Error>,
type SinkItem = <U as Encoder>::Item
type SinkError = <U as Encoder>::Error
。 documentation for Framed说什么？
Framed
websocket::async::MessageCodec<websocket::OwnedMessage>有两个类型参数，但我们只需要查看第二个参数，即SinkError，以确定websocket::codec::ws::MessageCodec类型。现在让我们来看看MessageCodec。 （注意：websocket::async::MessageCodec reexported为impl<M> Decoder for MessageCodec<M>
where
M: MessageTrait,
type Item = OwnedMessage
type Error = WebSocketError
。）
啊哈哈！接收器产生Self::Error: From<S::SinkError>
WebSocketError类型的错误。
现在我们已经确定了这些类型，让我们回过头来讨论这些类型的原因。我们试图理解为什么在调用forward时没有遇到约束(): From<WebSocketError>。我们现在知道编译器正在尝试解析impl From<WebSocketError> for ()。看起来似乎没有extern crate websocket;
fn main() {
let a = websocket::result::WebSocketError::NoDataAvailable;
let () = From::from(a);
}
。我们来验证一下：
error[E0277]: the trait bound `(): std::convert::From<websocket::WebSocketError>` is not satisfied
--> src/main.rs:5:14
|
5 | let () = From::from(a);
| ^^^^^^^^^^ the trait `std::convert::From<websocket::WebSocketError>` is not implemented for `()`
|
= note: required by `std::convert::From::from`
确实，这无法编译：
sink我们可以使用sink_map_err更改let (sink, stream) = s.split();
let sink = sink.sink_map_err(|_| ()); // <<<<<
rx
.forward(sink)
.and_then(|(a, sink)| {
sink.send(OwnedMessage::Close(None))
})
的错误类型来解决缺失的实施问题。
forward这解决了对upgrade.use_protocol("rust-websocket").accept()的调用，但现在此闭包的结果并未与WebSocketError组成，rx仍然有WebSocketError作为其错误类型。更改()的错误类型更有意义。但是我们如何从Receiver构建一个()，它没有信息？
您可能想知道，为什么poll使用!作为错误类型？如果我们查看source code，我们可以看到，实际上，WebSocketError 从不会返回错误。我认为如果错误类型是fn worker(rx: Receiver<OwnedMessage>) {
let rx = rx.map_err(|()| panic!("Receiver should never fail!"));
let mut core = Core::new().unwrap();
let handle = core.handle();
// bind to the server
let server = Server::bind("127.0.0.1:9000", &handle).unwrap();
let f = server.incoming()
// we don't wanna save the stream if it drops
.map_err(|InvalidConnection { error, .. }| error)
.for_each(|(upgrade, addr)| {
// accept the request to be a ws connection if it does
let f = upgrade
.use_protocol("rust-websocket")
.accept()
.and_then(|(s, _)| {
let (sink, stream) = s.split();
rx
.forward(sink)
.and_then(|(a, sink)| {
sink.send(OwnedMessage::Close(None))
})
});
handle.spawn(f.map_err(move |e| println!("Err"))
.map(move |_| println!("Done")));
Ok(())
});
core.run(f).expect("somerror");
}
（从不类型）或其他一些void类型更合适，以清楚地表明错误是不可能的;有一个issue open on futures要求对期货0.2进行此更改。
由于错误是不可能的，我们不需要构建error[E0507]: cannot move out of captured outer variable in an `FnMut` closure
--> src/main.rs:43:31
|
30 | let rx = rx.map_err(|()| panic!("Receiver should never fail!"));
| -- captured outer variable
...
43 | .and_then(|(s, _)| {
| ^^^^^^^^ cannot move out of captured outer variable in an `FnMut` closure
;我们可以改为分散，例如恐慌。
rx现在，还有一个错误：
forward为什么关闭试图移动self？因为FnMut按值FnOnce获取。为什么闭包是FnOnce？请注意，Future::and_then需要FnMut（它有效将值从捕获的变量移动到for_each闭包），但Stream::for_each需要{multi-producer, single-consumer 1}}。这是有道理的：{{1}}将为每个传入连接调用一次闭包！
您正在使用的频道为{{3}}（因此名称为 mpsc ），但您尝试在此处拥有多个消费者（每个连接都在尝试阅读来自接收者）。我将留给您修复程序中的此设计问题。请记住，可以有多个并发客户端连接！