CODEWITHSUNDEEP
typesasync-awaitrustfuture
xxks-kkk
xxks-kkk

Reputation: 2608

What's the type of an async fn?

I want to have a function that takes a function pointer of async fn.
What should the type of f be in fn run?

async fn foo() {}

fn run(f: /* ??? */) {}

According to the async/await RFC:

An async fn foo(args..) -> T is a function of the type fn(args..) -> impl Future<Output = T>.

However, if I write

fn run(f: fn() -> impl Future<()>)

I get the error message:

`impl Trait` not allowed outside of function and inherent method return types

Upvotes: 2

Views: 536

Answers (2)

&#214;mer Erden
&#214;mer Erden

Reputation: 8803

Using generic type parameter can help since an async fn returns an implementation of the Future trait. 

#![feature(futures_api, async_await)]

use std::future::Future;

fn main() {
    run(foo);
}

async fn foo() {}

fn run<T: Future>(f: fn() -> T) {
    f();
}

Upvotes: 2

hellow
hellow

Reputation: 13440

You have to introduce two type parameters to your function signature, one for the Fn and one for the Future, e.g.

#![feature(futures_api, async_await)]

async fn foo() {}

fn run<G: std::future::Future, F: FnOnce() -> G>(f: F) {
    f();
}

fn main() {
    bar(foo)
}

You can replace FnOnce with Fn or FnMut depending on your needs.

Upvotes: 3

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