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Important Rust Topics

Rust Closures

Functions without a name that remember the variables around them.

  • Advanced
  • 3 min read
  • 1 runnable example
What you'll learn
  • |n| n * 2
  • Capturing variables
  • move and returning closures
Run code →
Closure → result
|n| n * 2apply(21) → 42
|n| n + bonusapply(5) → 15
make_adder(5)add_five(1) → 6

A function that captures its surroundings

A closure is a function without a name that you can store in a variable or pass to another function. Unlike a normal function, it can use variables from the place where it was created.

|n| n * 2 is a closure with one parameter. add_bonus reads bonus from the surrounding code. move makes the closure own what it captures, which is needed when the closure is returned from make_adder. increment changes counter, so it must be declared mut.

Key points

  1. A closure can use variables from where it was created.
  2. move makes it own what it captures.
  3. A closure that changes a variable must be mut.

Runnable Rust example

Rust
Run code →
main.rs
fn apply(value: i32, operation: impl Fn(i32) -> i32) -> i32 {
    operation(value)
}

fn make_adder(amount: i32) -> impl Fn(i32) -> i32 {
    move |value| value + amount
}

fn main() {
    let double = |n| n * 2;
    println!("{}", apply(21, double));

    let bonus = 10;
    let add_bonus = |n| n + bonus; // captures bonus from the environment
    println!("{}", apply(5, add_bonus));

    let add_five = make_adder(5);
    println!("{}", add_five(1));

    let mut counter = 0;
    let mut increment = || counter += 1;
    increment();
    increment();
    println!("counter = {counter}");
}
Get a Run button for this example Result of running the code
42
15
6
counter = 2

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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