The Mun Programming Language

Control flow

Executing or repeating a block of code only under specific conditions are common constructs that allow developers to control the flow of execution. Mun provides if/else expressions and loops.

if expressions

An if expression allows you to branch your code depending on conditions.

fn main() {
    let number = 3;

    if number < 5 {
        number = 4;
    } else {
        number = 6;
    }
}

All if expressions start with the keyword if, followed by a condition. As opposed to many C-like languages, Mun omits parentheses around the condition. Only when the condition is true - in the example, whether the number variable is less than 5 - the consecutive code block (or arm) is executed.

Optionally, an else expression can be added that will be executed when the condition evaluates to false. You can also have multiple conditions by combining if and else in an else if expression. For example:

fn main() {
    let number = 6;
    if number > 10 {
        // The number if larger than 10
    } else if number > 8 {
        // The number is larger than 8 but smaller or equal to 10
    } else if number > 2 {
        // The number is larger than 2 but smaller or equal to 8
    } else {
        // The number is smaller than- or equal to 2.
    }
}

Using if in a let statement

The if expression can be used on the right side of a let statement just like a block:

fn main() {
    let condition = true;
    let number = if condition {
        5
    } else {
        6
    };

Depending on the condition, the number variable will be bound to the value of the if block or the else block. This means that both the if and else arms need to evaluate to the same type. If the types are mismatched the compiler will report an error.

loop expressions

A loop expression can be used to create an infinite loop. Breaking out of the loop is done using the break statement.

fn main() {
    let i = 0;
    loop {
        if i > 5 {
            break;
        }

        i += 1;
    }
}

Similar to if/else expressions, loop blocks can have a return value that can be returned through the use of a break statement.

fn count(i: i32, n: i32) -> i32 {
    let loop_count = 0;
    loop {
        if i >= n {
            break loop_count;
        }

        loop_count += 1;
    }
}

All break statements in a loop must have the same return type.

let a = loop {
    break 3;
    break; // expected `{integer}`, found `nothing`
};

while expressions

while loops execute a block of code as long as a condition holds. A while loop starts with the keyword while followed by a condition expression and a block of code to execute upon each iteration. Just like with the if expression, no parentheses are required around the condition expression.

fn main() {
    let i = 0;
    while i <= 5 {
        i += 1;
    }
}

A break statement inside the while loop immediately exits the loop.

Unlike a loop expression, a break in a while loop cannot return a value because a while loop can exit both through the use of a break statement and because the condition no longer holds. Although we could explicitly return a value from the while loop through the use of a break statement it is unclear which value should be returned if the loop exits because the condition no longer holds.