Control Flow

After introducing basic types and operations, it’s time to delve into more complex programs with control flow.

If-Else

An if-else expression allows you to branch based on a boolean value. In contrast with some other languages, Roto does not require parentheses around the condition, but it does require curly braces for the body. You can use the if-else expression as a statement. The else part is then optional.

Rotofn main() {
    let x = 100;
    if x % 2 == 0 {
        print("x is even");
    } else {
        print("x is odd");
    }
}

Output

x is even

An if-else expression can also evaluate to a value itself. That value is determined by the last value in each of the arms, as long as there is no semicolon at the end.

Rotofn main() {
    let x = 100;
    let sign = if x > 0 {
        print("x is positive");
        1       // <- No semicolon!
    } else {
        print("x is negative");
        -1      // <- No semicolon!
    };
    print(f"{sign}");
}

Outputx is positive
1

It’s possible to declare variables within the arms of if-else expressions, but they will only be available within that arm, and they will cease to exist on leaving that scope. The same goes for any block of statements in Roto that is delimited by {}.

Rotofn main() {
    if true {
        let x = 10;
        print(f"{x}"); // This is fine!
    }
    print(f"{x}"); // This errors during type checking as x no longer exists here!
}

Match

@todo

While Loops

We can loop in Roto using a while loop. As in many other languages, a while loop takes a condition and then a block. The body of the block is run repeatedly until the condition evaluates to false.

Roto// Euclidean algorithm for greatest common divisor
fn main() {
    let a_initial = 125;
    let b_initial = 50;

    let a = a_initial;
    let b = b_initial;
    while b != 0 {
        let t = b;
        b = a % b;
        a = t;
    }

    print(f"gcd({a_initial}, {b_initial}) = {a}")
}

Output

gcd(125, 50) = 25

For Loops

To execute some code for every element in a list, use a for loop instead of a while loop.

Rotofn main() {
    for x in [10, 20, 30] {
        let squared = x * x;
        print(f"{x} squared is {squared}");
    }
}

Output10 squared is 100
20 squared is 400
30 squared is 900

Note

A while loop is currently the best way to iterate over a range of numbers. There will be support for using for loops with ranges in the future. See this community post for more information.