Optionals are not yet fully implemented.

You use optionals in situations where a value may be absent. An optional represents two possibilities: Either there is a value, and you can unwrap the optional to access that value, or there isn’t a value at all.

Add ? after a type to make it optional.

It’s not allowed to make an optional optional. For example i64?? is invalid.


Optional variables and parameters

func foo(a: i64, b: i64? = None) -> i64?:
    # Compare to `None` to check for value existence.
    if b is not None:
        b += b
        a += b

    # Optionals can be matched.
    match b:
        case 1:
            print("Matched one.")
        case 5:
            print("Matched five.")
        case None:
            print("Matched None.")
        case _:
            print(f"Matched {b}.")

    # Clear any value.
    b = None

    # Adding `b` without a value will panic.
    # a += b

    # `None` and `a` (type i64) can be returned as optional.
    if a == 0:
        return None
        return a

func main():
    assert foo(1, None) == 1
    assert foo(1, 5) == 11
    b: i64? = 5
    assert foo(1, b) == 11
    assert foo(0) is None

    for i in range(2):
        res = foo(i, 0)

        if res is not None:
            print("res has a value.")
            print("res does not have a value.")
Matched None.
Matched 10.
Matched 10.
Matched None.
Matched 0.
res does not have a value
Matched 0.
res has a value

Optional class members

class Foo:
    a: i64
    b: string?
    c: i64?

    func get(self) -> string:
        if self.b is not None:
            return self.b
            return "not set"

    func num(self) -> i64:
        if self.c is not None:
            return self.c * self.a
            return self.a

func main():
    foo = Foo(5, None, 10)
    assert foo.get() == "not set"
    assert foo.num() == 50