pub hkt Functor[Self<T>]
    fn <U> map(f: fn (t: T) -> U) -> Self<U>
end

pub int Iterable<T>
    fn iter() -> Iterator<T>
end

pub int Iterator<T> = () -> Option<T>

pub enum Option<T> : Functor[Self<T>]
    Some(T),
    None;

    fn unwrap() -> T
        self is Some(t) ? t : throw Exception('Empty Option')
    end

    fn expect(msg: String) -> T
        self is Some(t) ? t : throw Exception(msg)
    end

    impl Functor
        fn <U> map(f: fn (t: T) -> U) -> Self<U>
            self is Some(t) ? Some(f(t)) : None
        end
    end

    static fn lift(t: T) -> Self<T>
        Some(t)
    end
end