Async/Await with tokio

// Cargo.toml:
// tokio = { version = "1", features = ["full"] }

use tokio::time::{sleep, Duration};

async fn delayed_greeting(name: &str, delay_ms: u64) -> String {
    sleep(Duration::from_millis(delay_ms)).await;
    format!("Hello, {name}!")
}

#[tokio::main]
async fn main() {
    // Sequential — total time ≈ 100 + 50 = 150ms
    let a = delayed_greeting("Alice", 100).await;
    let b = delayed_greeting("Bob",    50).await;
    println!("{a}  {b}");

    // Concurrent — total time ≈ max(100, 50) = 100ms
    let (a, b) = tokio::join!(
        delayed_greeting("Alice", 100),
        delayed_greeting("Bob",    50),
    );
    println!("{a}  {b}");
}