Sometimes a value might not exist. Your friend might not have a nickname. A search through a list might not find anything. A game character might not be holding an item. Programs need a way to say "there's nothing here", and steav2 has one: sone.

✦sone: nothing at all

sone means nothing (programmers often call this "nil" or "null"). But steav2 is careful with it: an ordinary variable can never be sone. A LekKut is always a real number, an Ahsor is always real text.

bongSlanhOun "nothing";
akthe score: LekKut = sone;

[line 2] Error at 'sone': expected LekKut but got sone

That's a good thing! It means that when you have a LekKut, you never have to worry that it's secretly empty.

✦Optionals: T?

When a value really might be missing, you say so by adding a question mark to its type. Ahsor? means "maybe some text, maybe sone". This is called an optional type. (In docs you'll see T?, where T stands for any type.)

bongSlanhOun "nickname";
akthe nickname: Ahsor? = sone;
jongyeytha nickname;
nickname = "Bo";
jongyeytha nickname;

sone
Bo

An optional box can hold either a real value or sone, and you can switch between them whenever you like.

✦Checking before you use it

Because an optional might be sone, steav2 won't let you use it as if it were definitely there:

bongSlanhOun "careless";
sampoan Pet {
    akthe name: Ahsor;
}
akthe lostPet: Pet? = sone;
jongyeytha lostPet.name;

[line 6] Error at 'name': this is Pet?, it might be sone: check it with ber (x != sone) first

The error tells you exactly what to do. Check with ber (x != sone), and inside that block, steav2 knows the value is really there, so you can use it like a normal value:

bongSlanhOun "careful";
sampoan Pet {
    akthe name: Ahsor;
}
akthe foundPet: Pet? = Pet("Mew");
ber (foundPet != sone) {
    jongyeytha "Found " + foundPet.name + "!";
} minjengte {
    jongyeytha "Still looking.";
}

Found Mew!

This also works the other way round: in the minjengte of a ber (x == sone), the value is known to be there.

bongSlanhOun "greeting";
akthe nickname: Ahsor? = "Bo";
ber (nickname == sone) {
    jongyeytha "Hello, stranger!";
} minjengte {
    jongyeytha "Hey " + nickname + "!";
}

Hey Bo!

✦Functions that might not find anything

Optionals are perfect for a function's answer when there sometimes isn't one. Here's a function that searches an array and returns the position of a word, or sone if the word isn't there:

bongSlanhOun "search";
rupamun find(words: [Ahsor], wanted: Ahsor) -> LekKut? {
    somhab (akthe i: LekKut = 0; i < words.len(); i += 1) {
        ber (words[i] == wanted) {
            morvenh i;
        }
    }
    morvenh sone;
}

akthe menu = ["rice", "noodles", "soup"];
akthe spot = find(menu, "noodles");
ber (spot != sone) {
    jongyeytha "noodles are dish number " + (spot + 1);
}
jongyeytha find(menu, "pizza");

noodles are dish number 2
sone

Notice that morvenh i; returns a plain LekKut, and that's fine: a real value can always go into a LekKut? box. And inside ber (spot != sone), spot is a plain LekKut, so spot + 1 works.

✦Your turn

1. Make an akthe favouriteColor: Ahsor? = sone; and print No favourite yet if it's sone, or I love ... with the colour otherwise. Then set it and try again. Hint: ber (favouriteColor == sone).
2. Write firstLong(words: [Ahsor]) -> Ahsor? that returns the first word that isn't "a" or "the", or sone if there isn't one.
3. Write safeDivide(a: LekKut, b: LekKut) -> LekKut? that returns sone when b is 0, and a / b otherwise.

Solution for exercise 3:

bongSlanhOun "safe divide";
rupamun safeDivide(a: LekKut, b: LekKut) -> LekKut? {
    ber (b == 0) {
        morvenh sone;
    }
    morvenh a / b;
}

jongyeytha safeDivide(10, 2);
jongyeytha safeDivide(10, 0);

5
sone