rust - 如何解决 "the trait bound ` [closure]: tokio::prelude::Future` is not satisfied"when calling tokio::spawn?

Question

extern crate tokio; // 0.1.22

use tokio::io;
use tokio::net::TcpListener;
use tokio::prelude::*;

use bytes::Bytes; // 0.4.12

fn main() {
    let addr = "0.0.0.0:1502".parse().unwrap();
    let mut listener = TcpListener::bind(&addr).unwrap();

    let done = listener
        .incoming()
        .map_err(|e| println!("failed to accept socket; error = {:?}", e))
        .for_each(move |socket| {
            let process = move || {};

            tokio::spawn(process)
        });

    tokio::run(done);
    Ok(());

    tokio::run(done);
}

error[E0277]: the trait bound `[closure@src/main.rs:17:27: 17:37]: tokio::prelude::Future` is not satisfied
  --> src/main.rs:19:13
   |
19 |             tokio::spawn(process)
   |             ^^^^^^^^^^^^ the trait `tokio::prelude::Future` is not implemented for `[closure@src/main.rs:17:27: 17:37]`
   |
   = note: required by `tokio::spawn`

( Playground )

Answer 1

请检查definition of tokio::spawn :

pub fn spawn<F>(f: F) -> Spawn 
where
    F: Future<Item = (), Error = ()> + 'static + Send

它期望一个 Future 的实现作为它的参数。您传递给 tokio::spawn 的闭包不是 Future 的实现。 Futures-rs 没有关闭，而是有 lazy

Creates a new future which will eventually be the same as the one created by the closure provided.

简单地说，一个lazy future 拥有一个闭包来执行。这发生在执行程序第一次执行 poll 时(在本例中，您使用的是 Tokio 执行程序)。

如果你用 lazy 包装你的闭包，你的代码将按你预期的那样工作。

extern crate tokio; // 0.1.22

use tokio::net::TcpListener;
use tokio::prelude::*;

fn main() {
    let addr = "0.0.0.0:1502".parse().unwrap();
    let listener = TcpListener::bind(&addr).unwrap();

    let done = listener
        .incoming()
        .map_err(|e| println!("failed to accept socket; error = {:?}", e))
        .for_each(move |_socket| {
            let process = futures::future::lazy(move || {
                println!("My closure executed at future");
                Ok(())
            });

            tokio::spawn(process)
        });

    tokio::run(done);
}