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# async-recursion macro
[](https://crates.io/crates/async-recursion)
[](https://crates.io/crates/async-recursion)
[](https://github.com/dcchut/async-recursion/actions)
Procedural macro for recursive async functions.
* [Documentation](https://docs.rs/async-recursion/)
* Cargo package: [async-recursion](https://crates.io/crates/async-recursion)
## Motivation
Consider the following recursive implementation of the fibonacci numbers:
```rust,compile_fail
async fn fib(n : u32) -> u32 {
match n {
0 | 1 => 1,
_ => fib(n-1).await + fib(n-2).await
}
}
```
The compiler helpfully tells us that:
```console
error[E0733]: recursion in an `async fn` requires boxing
--> src/main.rs:1:26
|
1 | async fn fib(n : u32) -> u32 {
| ^^^ recursive `async fn`
|
= note: a recursive `async fn` must be rewritten to return a boxed `dyn Future`
= note: consider using the `async_recursion` crate: https://crates.io/crates/async_recursion
```
This crate provides an attribute macro to automatically convert an async function
to one returning a boxed `Future`.
## Example
```rust
use async_recursion::async_recursion;
#[async_recursion]
async fn fib(n : u32) -> u32 {
match n {
0 | 1 => 1,
_ => fib(n-1).await + fib(n-2).await
}
}
```
## ?Send option
The returned `Future` has a `Send` bound to make sure it can be sent between threads.
If this is undesirable you can mark that the bound should be left out like so:
```rust
#[async_recursion(?Send)]
async fn returned_future_is_not_send() {
// ...
}
```
## Sync option
The returned `Future` doesn't have a `Sync` bound as it is usually not required.
You can include a `Sync` bound as follows:
```rust
#[async_recursion(Sync)]
async fn returned_future_is_sync() {
// ...
}
```
In detail:
- `#[async_recursion]` modifies your function to return a boxed `Future` with a `Send` bound.
- `#[async_recursion(?Send)]` modifies your function to return a boxed `Future` _without_ a `Send` bound.
- `#[async_recursion(Sync)]` modifies your function to return a boxed `Future` with a `Send` and `Sync` bound.
### License
Licensed under either of
* Apache License, Version 2.0
([LICENSE-APACHE](LICENSE-APACHE) or <http://www.apache.org/licenses/LICENSE-2.0>)
* MIT license
([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)
at your option.
|
