Diverging Functions in Rust

Introduction

In this lab, we learn about diverging functions that are marked using ! in Rust. Diverging functions never return and their return type is an empty type. This is different from the () type which has only one possible value. Diverging functions can be useful when casting to any other type is required, such as in match branches. They are also the return type of functions that loop forever or terminate the process.

Note: If the lab does not specify a file name, you can use any file name you want. For example, you can use main.rs, compile and run it with rustc main.rs && ./main.

Diverging functions

Diverging functions never return. They are marked using !, which is an empty type.

fn foo() -> ! {
    panic!("This call never returns.");
}

As opposed to all the other types, this one cannot be instantiated, because the set of all possible values this type can have is empty. Note that, it is different from the () type, which has exactly one possible value.

For example, this function returns as usual, although there is no information in the return value.

fn some_fn() {
    ()
}

fn main() {
    let _a: () = some_fn();
    println!("This function returns and you can see this line.");
}

As opposed to this function, which will never return the control back to the caller.

#![feature(never_type)]

fn main() {
    let x: ! = panic!("This call never returns.");
    println!("You will never see this line!");
}

Although this might seem like an abstract concept, it is in fact very useful and often handy. The main advantage of this type is that it can be cast to any other one and therefore used at places where an exact type is required, for instance in match branches. This allows us to write code like this:

fn main() {
    fn sum_odd_numbers(up_to: u32) -> u32 {
        let mut acc = 0;
        for i in 0..up_to {
            // Notice that the return type of this match expression must be u32
            // because of the type of the "addition" variable.
            let addition: u32 = match i%2 == 1 {
                // The "i" variable is of type u32, which is perfectly fine.
                true => i,
                // On the other hand, the "continue" expression does not return
                // u32, but it is still fine, because it never returns and therefore
                // does not violate the type requirements of the match expression.
                false => continue,
            };
            acc += addition;
        }
        acc
    }
    println!("Sum of odd numbers up to 9 (excluding): {}", sum_odd_numbers(9));
}

It is also the return type of functions that loop forever (e.g. loop {}) like network servers or functions that terminate the process (e.g. exit()).

Summary

Congratulations! You have completed the Diverging Functions lab. You can practice more labs in LabEx to improve your skills.