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我正在使用 Tokio 框架在 Rust 中创建一个重复任务。以下代码基于完成的更改请求，以将此功能添加到 tokio-timer crate。

尝试编译时，我收到错误消息：

error[E0281]: type mismatch: the type `fn() {my_cron_func}` implements the trait `std::ops::FnMut<()>`, but the trait `std::ops::FnMut<((),)>` is required (expected tuple, found ())
  --> src/main.rs:19:36
   |
19 |     let background_tasks = wakeups.for_each(my_cron_func);
   |                                    ^^^^^^^^

error[E0281]: type mismatch: the type `fn() {my_cron_func}` implements the trait `std::ops::FnOnce<()>`, but the trait `std::ops::FnOnce<((),)>` is required (expected tuple, found ())
  --> src/main.rs:19:36
   |
19 |     let background_tasks = wakeups.for_each(my_cron_func);
   |                                    ^^^^^^^^

error[E0281]: type mismatch: the type `fn() {my_cron_func}` implements the trait `std::ops::FnMut<()>`, but the trait `std::ops::FnMut<((),)>` is required (expected tuple, found ())
  --> src/main.rs:20:10
   |
20 |     core.run(background_tasks).unwrap();
   |          ^^^
   |
   = note: required because of the requirements on the impl of `futures::Future` for `futures::stream::ForEach<tokio_timer::Interval, fn() {my_cron_func}, _>`

error[E0281]: type mismatch: the type `fn() {my_cron_func}` implements the trait `std::ops::FnOnce<()>`, but the trait `std::ops::FnOnce<((),)>` is required (expected tuple, found ())
  --> src/main.rs:20:10
   |
20 |     core.run(background_tasks).unwrap();
   |          ^^^
   |
   = note: required because of the requirements on the impl of `futures::Future` for `futures::stream::ForEach<tokio_timer::Interval, fn() {my_cron_func}, _>`

该错误表明 my_cron_func 函数的返回签名不正确。我需要更改/添加什么才能使签名正确以便编译？

extern crate futures;
extern crate tokio_core;
extern crate tokio_timer;

use std::time::*;
use futures::*;
use tokio_core::reactor::Core;
use tokio_timer::*;

pub fn main() {

    println!("The start");
    let mut core = Core::new().unwrap();
    let timer = Timer::default();
    let duration = Duration::new(2, 0); // 2 seconds
    let wakeups = timer.interval(duration);

    // issues here
    let background_tasks = wakeups.for_each(my_cron_func);
    core.run(background_tasks).unwrap();

    println!("The end???");

}

fn my_cron_func() {
    println!("Repeating");
    Ok(());
}
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1 回答 1

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我不确定错误消息的哪一部分给您带来了麻烦，但是...

类型不匹配

您提供了错误的类型

类型fn() {my_cron_func}实现了特征std::ops::FnMut<()>

使用时my_cron_func，这是一个不带参数的函数

但特质std::ops::FnMut<((),)>是必需的

但是需要一个接受单个参数的函数，即空元组。

（预期的元组，找到（））

编译器试图缩小问题的范围。

如果您查看您正在使用的库的文档，特别是tokio_timer::Interval，您可以看到它futures::Stream使用关联的类型实现Item = ()。

这会更改错误消息：

error[E0277]: the trait bound `(): futures::Future` is not satisfied
  --> src/main.rs:19:36
   |
19 |     let background_tasks = wakeups.for_each(my_cron_func);
   |                                    ^^^^^^^^ the trait `futures::Future` is not implemented for `()`
   |
   = note: required because of the requirements on the impl of `futures::IntoFuture` for `()`

error[E0277]: the trait bound `(): futures::Future` is not satisfied
  --> src/main.rs:20:10
   |
20 |     core.run(background_tasks).unwrap();
   |          ^^^ the trait `futures::Future` is not implemented for `()`
   |
   = note: required because of the requirements on the impl of `futures::IntoFuture` for `()`
   = note: required because of the requirements on the impl of `futures::Future` for `futures::stream::ForEach<tokio_timer::Interval, fn(()) {my_cron_func}, ()>`

查看文档futures::Stream，我们可以看到传递给的闭包for_each需要返回一个值，该值可以转换为将产生的未来()：

fn for_each<F, U>(self, f: F) -> ForEach<Self, F, U> 
    where F: FnMut(Self::Item) -> U,
          U: IntoFuture<Item=(), Error=Self::Error>,
          Self: Sized

您的函数尝试返回一些东西，除了没有返回类型并且您使用 a;结束函数：

fn my_cron_func(a: ()) {
    println!("Repeating");
    Ok(());
}

futures::future::ok诀窍：

fn my_cron_func(_: ()) -> futures::future::FutureResult<(), tokio_timer::TimerError> {
    println!("Repeating");
    futures::future::ok(())
}
于 2017-02-19T01:02:53.533 回答