Functions can be declared with the following syntax:
"const"? "fn" ID "(" ("const"? ID),* ")" "{" stmt* "}"
For example, the following function will add two numbers:
fn add(a, b) { return a + b; }
It can be called as follows:
add(5, 6).
You can also write:
const fn add(a, b) { return a + b; }
which means add cannot be reassinged another value later, and
fn add(const a, const b) { return a + b; }
which means that a and b cannot be reassigned inside add.
YASL includes some built-in functions as well, which can be found under Built-in Functions.