Sending channel signal from a closure

问题

I'm trying to send a signal from within a closure, using the following code.

use std::thread;
use std::sync::mpsc::channel;

fn main() {
    let (tx, rx) = channel();

    let t1 = thread::spawn(move || {
        watch(|x| tx.send(x));
    });

    let t2 = thread::spawn(move || {
        println!("{:?}", rx.recv().unwrap());
    });

    let _ = t1.join();
    let _ = t2.join();
}

fn watch<F>(callback: F) where F : Fn(String) {
    callback("hello world".to_string());
}

However, it fails compiling raising the following error:

src/test.rs:8:19: 8:29 note: expected type `()`
src/test.rs:8:19: 8:29 note:    found type `std::result::Result<(), std::sync::mpsc::SendError<std::string::String>>`

Am I missing something?

回答1:

You have declared that your watch function receives a closure of type Fn(String). Usually a closure type includes its return type: Fn(String) -> SomeReturnType. Fn(String) is equivalent to Fn(String) -> () and means that your closure should return an empty tuple (). () is often used similar to void in C.

However, the closure you're trying to use (|x| tx.send(x)) returns std::result::Result<(), std::sync::mpsc::SendError<std::string::String>> instead. You can use unwrap() on the Result to check that the operation have succeded and to make the closure return ():

watch(|x| tx.send(x).unwrap());

Alternatively, you can declare watch function in such way so it can receive a closure returning any type:

fn watch<F, R>(callback: F)
    where F: Fn(String) -> R
{
    // ...
}

But the Result should be checked anyway.

来源：https://stackoverflow.com/questions/39216185/sending-channel-signal-from-a-closure

标签

rust

channel