Functions: Reusable Code

The bigger a program gets, the more important it is not to write the same code over and over. Functions solve this: a named piece of code that can be called anytime.

Creating Your First Function

fn sapa() {
    print_str("Halo, dunia!\n");
}

sapa();
sapa();

Output:

Halo, dunia!
Halo, dunia!

Structure:

fn <function name>(<optional parameters>) {
    ...function body...
}

Parameters: Input for a Function

A function can accept parameters — values given when it's called:

fn sapa_nama(nama) {
    print_str("Halo, " + nama + "!\n");
}

sapa_nama("Budi");
sapa_nama("Dina");

Output:

Halo, Budi!
Halo, Dina!

The parameters in the sapa_nama example work like variables that are automatically filled with the values from the call.

Return Values

A function can return a value. Here's how: write an expression (without a semicolon) at the end of the function — the value of that expression is what gets returned:

fn tambah(a, b) {
    a + b
}

let hasil = tambah(3, 4);
print(hasil); // 7

print(tambah(10, 20)); // 30

Note: a + b on the last line has no semicolon — that's how AHA! Lang says "this is the return value".

The return value can be used directly in another expression — exactly like an ordinary value.

return to Exit Early

Sometimes you want to stop in the middle of a function because of some condition. Use return:

fn harga_ongkir(total) {
    if total >= 100000 {
        return 0;    // free shipping, exit immediately
    }
    15000           // otherwise, the shipping cost stays
}

print(harga_ongkir(200000)); // 0
print(harga_ongkir(5000));   // 15000

Functions Can Use Other Functions

A function can call another function — even itself (recursion) or in both directions (mutual recursion):

fn is_even(n) {
    n % 2 == 0
}

fn is_odd(n) {
    if is_even(n) {
        0
    } else {
        1
    }
}

print(is_even(10)); // 1
print(is_odd(10));  // 0

A function can be called before it's defined — AHA! Lang handles the ordering. This lets code be read from top to bottom naturally.

Combining All the Tricks

A complete program that combines what you've learned:

fn kelipatan(angka, k) {
    angka % k == 0
}

fn label_angka(n) {
    if kelipatan(n, 15) {
        "FizzBuzz"
    } else {
        if kelipatan(n, 3) {
            "Fizz"
        } else {
            if kelipatan(n, 5) {
                "Buzz"
            } else {
                "Angka biasa"
            }
        }
    }
}

for i 1..16 {
    print_str(label_angka(i) + "\n");
}

Output:

Angka biasa
Angka biasa
Fizz
...
FizzBuzz

Exercises

1. Create a function kubik(x) that returns x * x * x.
2. Create a function maksimal(a, b) that returns the largest value — use if as an expression.
3. Create a function hitung_mundur(mulai) that prints from mulai down to 1 with while, then print_str("Go!\n").
4. Create a function bilangan_prima(n) that returns 1 if n is prime and 0 if not. Hint: divide n by all numbers from 2 to n - 1; if any gives n % i == 0, it's not prime. (Challenge!)

On to the final lesson: A Small Project 🎉