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`