Rust类型推断错误

Question

我正在通过TCP编写聊天服务器作为学习项目.我受够了被修补WS箱子今天,但我遇到的一个问题.这是我编写的代码,修改了他们的服务器示例.

extern crate ws;
extern crate env_logger;

use ws::listen;

fn main() {
    // Setup logging
    env_logger::init().unwrap();

    // Listen on an address and call the closure for each connection
    if let Err(error) = listen("127.0.0.1:3012", |out| {
        let mut message: String;
        // The handler needs to take ownership of out, so we use move
        move |message| {
            message = message.trim();
            // Handle messages received on this connection
            println!("Server got message '{}'. ", message);

            // Use the out channel to send messages back
            out.send(message)
        }

    }) {
        // Inform the user of failure
        println!("Failed to create WebSocket due to {:?}", error);
    }
}

当我尝试编译它时,我收到一个错误:

error: the type of this value must be known in this context
  --> src/main.rs:15:23
   |
15 |             message = message.trim();
   |                       ^^^^^^^^^^^^^^

为什么会这样？我该如何解决这个问题？

Answer 1

move |message|隐藏message您在闭包之外声明的变量.所以在封闭中.. message据说是ws::Message......除非你这样做:

message = message.trim();

编译器会"哦不!trim()？那不存在ws::Message"..所以现在它不知道该怎么做.

选项1

第一个修复涉及将trim()调用委托给发送消息的客户端.

修复是不对此闭包内的消息做出任何假设.如果你保留这个:

move |message|

..但删除trim()调用,编译器愉快地推断它的类型ws::Message,并将构建:

if let Err(error) = listen("127.0.0.1:3012", |out| {
    // The handler needs to take ownership of out, so we use move
    move |message| {
        // --- REMOVED trim() call ---

        // Handle messages received on this connection
        println!("Server got message '{}'. ", message);

        // Use the out channel to send messages back
        out.send(message)
    }
}

这使您可以选择将trim()呼叫委派给客户端.

选项2

选项2涉及检查您收到的消息类型,并确保仅在文本时修剪它:

// The handler needs to take ownership of out, so we use move
move |mut message: ws::Message| {
    // Only do it if the Message is text
    if message.is_text() {
        message = ws::Message::Text(message.as_text().unwrap().trim().into());
    }

    // Handle messages received on this connection
    println!("Server got message '{}'. ", message);

    // Use the out channel to send messages back
    out.send(message)
}

这可能比它需要的更冗长......但希望它能告诉你原始代码片段的实际问题.