The new feature here is typed throws, which I referred to as statically typed errors. You’ve always been able to throw a subtype of Error and then dynamically d

The new feature here is typed throws, which I referred to as statically typed errors. You’ve always been able to throw a subtype of Error and then dynamically downcast it at runtime, but function signatures were limited to “throws any subtype of Error” or “doesn’t throw at all.” Sealed classes fall into the realm of things that the Swift language designers would leave up to convention. For example, using underscored public methods for internal methods that need to have external linkage, rather than inventing a new keyword for this purpose.