# Async / await

```cplus
import "stdlib/future" as future;
import "stdlib/executor" as executor;

async fn inner() -> i32 { return 7; }

async fn outer() -> i32 {
    let x: i32 = await inner();
    return x +% 1;
}

fn main() -> i32 {
    let f: future::Future[i32] = outer();
    return executor::block_on::[i32](f);
}
```

## Signature rules

- An `async fn` returns `Future[T]`, written as the bare `T` in the signature.
- `await EXPR` suspends until `EXPR` (a future) resolves and yields its value.
- Borrow-shaped parameters (`str`, `T[]`, `mut x: NonCopyT`) are rejected in an `async fn` signature (**E0900**), because a suspension can outlive the caller's frame. Pass `string` and `Vec[T]` instead.

## Reactor: concurrent I/O

The reactor (kqueue on macOS) makes `async` actually concurrent rather than just structured. It provides cooperative primitives over `stdlib/executor`, `stdlib/reactor`, and `stdlib/time`, so awaiting a timer or socket yields the executor to other ready tasks instead of blocking the thread.