async fn subtask() {
println!("> > subtask"); (4)
}
async fn task() {
println!("> Before subtask"); (3)
subtask().await;
println!("> After subtask"); (5)
}
fn main() {
futures::executor::block_on(async {
println!("before future"); (1)
let future = task();
println!("future is created"); (2)
future.await;
println!("future is awaited"); (6)
});
}Async Await
Laziness in Detail
Example: JavaScript
Eager execution model
Other languages behave similarly (C#, Python, Swift, Kotlin, etc.)
JavaScript calls its futures "Promises"
Rust Futures vs JavaScript Promises
async function subtask() {
console.log("> > subtask"); (3)
}
async function task() {
console.log("> before subtask"); (2)
await subtask();
console.log("> after subtask"); (5)
}
(async function main() {
console.log("before promise"); (1)
let promise = task();
console.log("promise is created"); (4)
await promise;
console.log("promise is awaited"); (6)
})();Eager Execution (JavaScript and many other languages)
async function subtask() {
console.log("> > subtask"); (3)
}
async function task() {
console.log("> before subtask"); (2)
await subtask();
console.log("> after subtask"); (5)
}
(async function main() {
console.log("before promise"); (1)
let promise = task();
console.log("promise is created"); (4)
await promise;
console.log("promise is awaited"); (6)
})();
Eager Execution (JavaScript and many other languages)
async function subtask() {
console.log("> > subtask"); (3)
}
async function task() {
console.log("> before subtask"); (2)
await subtask();
console.log("> after subtask"); (5)
}
(async function main() {
console.log("before promise"); (1)
let promise = task();
console.log("promise is created"); (4)
await promise;
console.log("promise is awaited"); (6)
})();Output: JavaScript
before promise > before subtask > > subtask promise is created > after subtask promise is awaited
Eager Execution: Takeaways
as soon as async function is called it starts executing
runs till the first
awaitpointinner async functions run their code, too, and stop at
awaitan async function with no
awaitinside will execute its full body eagerly
Lazy Execution: Rust Futures
async fn subtask() {
println!("> > subtask"); (4)
}
async fn task() {
println!("> Before subtask"); (3)
subtask().await;
println!("> After subtask"); (5)
}
fn main() {
futures::executor::block_on(async {
println!("before future"); (1)
let future = task();
println!("future is created"); (2)
future.await;
println!("future is awaited"); (6)
});
}
Lazy Execution: Rust Futures
async fn subtask() {
println!("> > subtask"); (4)
}
async fn task() {
println!("> Before subtask"); (3)
subtask().await;
println!("> After subtask"); (5)
}
fn main() {
futures::executor::block_on(async {
println!("before future"); (1)
let future = task();
println!("future is created"); (2)
future.await;
println!("future is awaited"); (6)
});
}Output: Rust
before future future is created > Before subtask > > subtask > After subtask future is awaited
Rust Futures vs JavaScript Promises
async fn subtask() {
println!("> > subtask"); (4)
}
async fn task() {
println!("> Before subtask"); (3)
subtask().await;
println!("> After subtask"); (5)
}
fn main() {
futures::executor::block_on(async {
println!("before future"); (1)
let future = task();
println!("future is created"); (2)
future.await;
println!("future is awaited"); (6)
});
}Output: Rust
before future future is created > Before subtask > > subtask > After subtask future is awaited
Output: JavaScript
before promise > before subtask > > subtask promise is created > after subtask promise is awaited
async function subtask() {
console.log("> > subtask"); (3)
}
async function task() {
console.log("> before subtask"); (2)
await subtask();
console.log("> after subtask"); (5)
}
(async function main() {
console.log("before promise"); (1)
let promise = task();
console.log("promise is created"); (4)
await promise;
console.log("promise is awaited"); (6)
})();Lazy Future Execution: Takeaways
no code is being run until a future is
awaitedawaittriggers the whole chain to executeno "fire now,
awaitlater" workflow
Async Await Laziness in Detail Table of Contents