Crate async_recursion

Procedural macro for recursive async functions

Consider the following recursive implementation of the fibonacci numbers:

async fn fib(n : u32) -> u64 {
   match n {
       0     => panic!("zero is not a valid argument to fib()!"),
       1 | 2 => 1,
       3     => 2,
       _ => fib(n-1).await + fib(n-2).await
   }
}

The compiler helpfully tells us that:

error[E0733]: recursion in an `async fn` requires boxing
--> src/main.rs:1:26
|
1 | async fn fib(n : u32) -> u64 {
|                          ^^^ recursive `async fn`
|
= note: a recursive `async fn` must be rewritten to return a boxed `dyn Future`.

This crate provides an attribute macro to automatically convert an async recursive function to one returning a boxed Future.

This crate provides an attribute macro to automatically convert async fn f(...) -> ReturnType to a fn f(...) -> Pin<Box<dyn Future<Output = ReturnType> + Send>>

Example

use async_recursion::async_recursion;

#[async_recursion]
async fn fib(n : u32) -> u64 {
   match n {
       0     => panic!("zero is not a valid argument to fib()!"),
       1 | 2 => 1,
       3     => 2,
       _ => fib(n-1).await + fib(n-2).await
   }
}

Limitations

Currently the macro doesn't consider lifetimes at all; this is something I plan to work on in the future.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT) at your option.

Attribute Macros

async_recursion