Скачать или смотреть How To Create Expandable Sealed Type Hierarchies In

Sealed types let us ensure that we know all implementations of an interface. Say we're modeling HTML and want to have a type for divs, another for paragraphs, another for spans, and so forth. Then it makes sense to create an interface `HtmlElement` that they all implement and that is sealed and permits exactly them as subtypes. But what if we want to allow users of our project to expand this type hierarchy? To do that, keep in mind that a type that extends a sealed type must itself be sealed, final, or explicitly non-sealed. And the latter allows arbitrary implementations, so it can act as an extension point in our hierarchy. So for custom HTML elements, we could permit a `non-sealed interface CustomElement extends HtmlElement` and then users extend from there. This creates an issue, though...