Functions in Sway are declared with the fn keyword. Let's take a look:

fn equals(first_param: u64, second_param: u64) -> bool {
    first_param == second_param

We have just declared a function named equals which takes two parameters: first_param and second_param. The parameters must both be 64-bit unsigned integers.

This function also returns a bool value, i.e. either true or false. This function returns true if the two given parameters are equal, and false if they are not. If we want to use this function, we can do so like this:

fn main() {
    equals(5, 5); // evaluates to `true`
    equals(5, 6); // evaluates to `false`

Mutable Parameters

We can make a function parameter mutable by adding ref mut before the parameter name. This allows mutating the argument passed into the function when the function is called.

For example:

fn increment(ref mut num: u32) {
    let prev = num;
    num = prev + 1u32;

This function is allowed to mutate its parameter num because of the mut keyword. In addition, the ref keyword instructs the function to modify the argument passed to it when the function is called, instead of modifying a local copy of it.

    let mut num: u32 = 0;
    assert(num == 1u32); // The function `increment()` modifies `num`

Note that the variable num itself has to be declared as mutable for the above to compile.

Note It is not currently allowed to use mut without ref or vice versa for a function parameter.

Similarly, ref mut can be used with more complex data types such as:

fn swap_tuple(ref mut pair: (u64, u64)) {
    let temp = pair.0;
    pair.0 = pair.1;
    pair.1 = temp;

fn update_color(ref mut color: Color, new_color: Color) {
    color = new_color;

We can then call these functions as shown below:

    let mut tuple = (42, 24);
    assert(tuple.0 == 24); // The function `swap_tuple()` modifies `tuple.0`
    assert(tuple.1 == 42); // The function `swap_tuple()` modifies `tuple.1`
    let mut color = Color::Red;
    update_color(color, Color::Blue);
    assert(match color {
        Color::Blue => true,
        _ => false,
    }); // The function `update_color()` modifies the color to Blue

Note The only place, in a Sway program, where the ref keyword is valid is before a mutable function parameter.