[Sealed class] Language feature docs

[itsjustkevin]

Summary

Sealed classes are a class modifier. Declaring a supertype class sealed means it can't be directly extended, implemented, or mixed in from outside their originating library.

From the Class Modifiers spec:

"But it doesn't prevent you from subtyping within the same library. In fact, the whole point of sealed is to define subtypes within the same library so that you can pattern match on those to cover the supertype."

Their motivating use case is exhaustiveness checking, a new feature of switch cases. They are the only class modifier that touch on patterns.

Evaluation

1. Document semantics with new Class modifiers content. Show how to use (just sealed keyword), show example of sealed superclass and subclasses, and what affect that has on subclasses.

   Content in class modifiers section shouldn't address exhaustiveness checking or patterns, but should link out to that content ("learn more about sealed classes use cases...).

   • Addressed above two in [Dart 3] Class modifiers #4754

   Class modifiers content might go under the "Libraries" umbrella (side nav), or under the "Classes" section of the "Language" umbrella (see [Class modifiers] Language feature docs #4497)

   • Tracked in [Dart 3] Document general use of class modifier by Dart core libraries in Library Tour #4730

2. In exhaustiveness checking content (which will be part of new switch content), link to semantic definition of the sealed class modifier.
   Then, go into detail on how sealed creates a closed hierarchy of classes, how that enables exhaustiveness checking that allows you to switch on object patterns.

3. IF we get to a point where we're documenting the collection of patterns features that enable functional style, then sealed classes will come up there (but that might be sufficiently covered by the description of exhaustiveness checking and switch)

4. MAYBE point to sealed content from enum content in language tour.

   Something like "you can enumerate subclasses using the sealed class modifier..."

Resources

• https://github.com/dart-lang/language/blob/master/accepted/future-releases/sealed-types/feature-specification.md