How to select or join futures with different types?

Question

I have a generic async function. I want to invoke it with different types and run resulted futures in parallel. But it seems like it creates futures of different types(although they all are impl Future), hence I can't put objects of different types in Vector, hence I can't invoke select function. Here is what I mean:

use std::fmt::Debug;

#[tokio::main]
async fn main() {
    // both arguments to test function i32. Works.
    let first = Box::pin(test(5));
    let second =  Box::pin(test(6));
    futures::future::select_all(vec![first, second]).await;
}

async fn test(x: T) {
    async {
        println!("{:?}", x);
    }.await;
}

And this doesn't work:

use std::fmt::Debug;

#[tokio::main]
async fn main() {
    // one argument to test() is i32, the second argument is &str. Doesn't work
    let first = Box::pin(test(5));
    let second =  Box::pin(test("five"));
    futures::future::select_all(vec![first, second]).await;
}

async fn test(x: T) {
    async {
        println!("{:?}", x);
    }.await;
}

In my concrete example I can use select which accepts two futures, but what if I have a lot of futures? How can I select multiple futures with different types?

user_1593196 · Answer

you just need to help the compiler a bit do detect the right type. We use dynamic dispatching with the dyn keyword here.

use std::fmt::Debug;
use std::pin::Pin;

#[tokio::main]
async fn main() {
    // one argument to test() is i32, the second argument is &str.
    let first = Box::pin(test(5));
    let second = Box::pin(test("five"));
    let v: Vec>>> = vec![first, second];
    futures::future::select_all(v).await;
}

async fn test(x: T) {
    async {
        println!("{:?}", x);
    }
    .await;
}

So all I have done was to extract the Vector into a variable and give it an explicit type.

How to select or join futures with different types?

1 Answers