Closures

Closures are anonymous functions that capture the scope in which they are defined. They can be saved as a variable or passed into a function.

struct HelloWorld {
    msg: String
}
impl HelloWorld {
    fn test_closure(&self, name: Option<&str>) -> String {
        String::from(format!(self.msg, name.unwrap_or_else(|| self.default_name())))
    }

    fn default_name() -> String {
        return String::from("David")
    }
}

// Example with parameter and return value
let expensive_closure = |num: u32| -> u32 {
    println!("calculating slowly...");
    thread::sleep(Duration::from_secs(2));
    num
};

Ownership and References

Immutable reference

let list = vec![1, 2, 3];
println!("Before defining closure: {:?}", list);

let only_borrows = || println!("From closure: {:?}", list);

println!("Before calling closure: {:?}", list);
only_borrows();
println!("After calling closure: {:?}", list);

Mutable reference

let mut list = vec![1, 2, 3];
println!("Before defining closure: {:?}", list);

let mut borrows_mutably = || list.push(7);

borrows_mutably();
println!("After calling closure: {:?}", list);

Take ownership with move

use std::thread;

let list = vec![1, 2, 3];
println!("Before defining closure: {:?}", list);

thread::spawn(move || println!("From thread: {:?}", list))
    .join()
    .unwrap();