Functions

To organize and reuse our code, we can create functions. You already know what a basic function looks like, because we’ve been using a main function as the entry point of our example programs.

A function can take arguments, which require a name and a type separated by a colon (:) and, of course, a function can also return a value. The return type is indicated by -> after the list of arguments. A function that does not have an explicit return type returns the type (), which is the unit type.

The function body is a block that returns the last expression if there is no semicolon at the end. You can also explicitly return early using the return keyword.

Roto

fn double(x: u32) -> u32 {
    2 * x    // <- no semicolon!
}

fn triple(x: u32) -> u32 {
    return 3 * x;
}

fn main() {
    let doubled = double(14);
    print(f"double(14) = {doubled}");

    let tripled = triple(14);
    print(f"triple(14) = {tripled}");
}

Output

double(14) = 28
triple(14) = 42

Hint

The order in which functions are defined does not matter; pick any order that you like!

Recursion is also supported; functions can call themselves. For example, here is a recursive definition of the factorial function. Note that the entire if is the final expression which is returned, so we do not need any return statements.

Roto

fn factorial(n: u64) -> u64 {
    if n == 0 {
        1
    } else {
        n * factorial(n - 1)
    }
}

fn main() {
    let res = factorial(5);
    print(f"factorial(5) = {res}");
}

Output

factorial(5) = 120

See also

functions in the language reference