Closures

Closures or lambda expressions have types which cannot be named. However, they implement special Fn, FnMut, and FnOnce traits:

fn apply_with_log(func: impl FnOnce(i32) -> i32, input: i32) -> i32 {
    println!("Calling function on {input}");
    func(input)
}

fn main() {
    let add_3 = |x| x + 3;
    println!("add_3: {}", apply_with_log(add_3, 10));
    println!("add_3: {}", apply_with_log(add_3, 20));

    let mut v = Vec::new();
    let mut accumulate = |x: i32| {
        v.push(x);
        v.iter().sum::<i32>()
    };
    println!("accumulate: {}", apply_with_log(&mut accumulate, 4));
    println!("accumulate: {}", apply_with_log(&mut accumulate, 5));

    let multiply_sum = |x| x * v.into_iter().sum::<i32>();
    println!("multiply_sum: {}", apply_with_log(multiply_sum, 3));
}
This slide should take about 20 minutes.

An Fn (e.g. add_3) neither consumes nor mutates captured values, or perhaps captures nothing at all. It can be called multiple times concurrently.

An FnMut (e.g. accumulate) might mutate captured values. You can call it multiple times, but not concurrently.

If you have an FnOnce (e.g. multiply_sum), you may only call it once. It might consume captured values.

FnMut is a subtype of FnOnce. Fn is a subtype of FnMut and FnOnce. I.e. you can use an FnMut wherever an FnOnce is called for, and you can use an Fn wherever an FnMut or FnOnce is called for.

When you define a function that takes a closure, you should take FnOnce if you can (i.e. you call it once), or FnMut else, and last Fn. This allows the most flexibility for the caller.

In contrast, when you have a closure, the most flexible you can have is Fn (it can be passed everywhere), then FnMut, and lastly FnOnce.

The compiler also infers Copy (e.g. for add_3) and Clone (e.g. multiply_sum), depending on what the closure captures.

By default, closures will capture by reference if they can. The move keyword makes them capture by value.

fn make_greeter(prefix: String) -> impl Fn(&str) {
    return move |name| println!("{} {}", prefix, name);
}

fn main() {
    let hi = make_greeter("Hi".to_string());
    hi("there");
}