I'll ignore the option of returning a Vec or Option, as the article already went over why these options were rejected, and concentrate on the later options. Fi

I'll ignore the option of returning a Vec or Option, as the article already went over why these options were rejected, and concentrate on the later options. Firstly the `std::iter::empty()` attempt: The issue they had here came down to the signature of the function. What they wanted to do was return an `impl Iterator<Item=&N>`. This means that the function isn't naming the specific type it returns[1], only stating that it's an Iterator that gives some `&N`s. However, the return type of this function is still one, specific type. You cannot return two different types that are both iterators. That's why this one failed. Boxing: Instead, we can return a `Box<dyn Iterator<Item=&N>>`. This is roughly equivalent to a function in C# returning an `IEnumerabale<N>`, in that the actual type is hidden, and the work is done doing dynamic dispatch. This allows you to return different kinds of iterators, because the specific type is hidden away inside the Box. The downside here is that you're doing a heap allocation. OptionIterator: This one is actually basically just the first Option case from earlier but wrapped up in a nicer-to-use interface for the user because it implements Iterator in a way that transparently handles the Option. [1] Additionally, it's not even possible to name the type of the iterator here due to the filter closure being unnamable.