Control Flow

Rune supports a number of control flow expressions. We will be dedicating this section to describe the most common ones.

return expression

In the previous section we talked about functions. And one of the primary things a function does is return things. The return expression allows for returning from the current function. If used without an argument, the function will return a unit ().

The last statement in a function is known as an implicit return, and will be what the function returns by default unless a return is specified.

fn foo(n) {
    if n < 1 {
        return "less than one";
    }

    "something else"
}

#[test]
fn test_control_flow() {
    assert_eq!(foo(0), "less than one");
    assert_eq!(foo(10), "something else");
}

pub fn main() {
    println!("{}", foo(0)); // => outputs: "less than one"
    println!("{}", foo(10)); // => outputs: "something else"
}

$> cargo run --bin rune -- run scripts/book/control_flow/numbers_game.rn
less than one
something else

if expressions

If expressions allow you to provide a condition with one or more code branches. If the condition is true, the provided block of code will run.

pub fn main() {
    let number = 3;

    if number < 5 {
        println!("The number is smaller than 5");
    }
}

$> cargo run --bin rune -- run scripts/book/control_flow/conditional.rn
The number *is* smaller than 5

Optionally, we can add another branch under else, which will execute in case the condition is false.

pub fn main() {
    let number = 3;

    if number < 5 {
        println!("the number is smaller than 5");
    } else {
        println!("the number is 5 or bigger");
    }
}

$> cargo run --bin rune -- run scripts/book/control_flow/conditional_else.rn
the number is smaller than 5

We can also add an arbitrary number of else if branches, which allow us to specify many different conditions.

pub fn main() {
    let number = 3;

    if number < 5 {
        println!("the number is smaller than 5");
    } else if number == 5 {
        println!("the number is exactly 5");
    } else {
        println!("the number is bigger than 5");
    }
}

$> cargo run --bin rune -- run scripts/book/control_flow/conditional_else_ifs.rn
the number is smaller than 5

Do note however that if you have many conditions, it might be cleaner to use a match.

This will be covered in a later section, but here is a sneak peek:

pub fn main() {
    let number = 3;

    match number {
        n if n < 5 => {
            println!("the number is smaller than 5");
        }
        5 => {
            println!("the number is exactly 5");
        }
        n => {
            println!("the number is bigger than 5");
        }
    }
}

$> cargo run --bin rune -- run scripts/book/control_flow/first_match.rn
the number is smaller than 5