Scala-3.x Introduce Opaque Types

[diesalbla]

Scala 3 introduces Opaque Type Aliases. Although Scala 2 already had type aliases and AnyVal, opacity is relevant in two ways:

• Opaque types should not be as fragile and restrictive as the AnyVal wrappers were.
• Unlike Scala-2 type aliases, opaque types are different symbols for implicit resolution, and thus allows us to define distinct instances of Functor or Monad for OptionT[F, A], which acts differently from that of F[Option[A]] (which is Monad[F]).

Many data types in cats can be seen as mere wrappers, interface adapters, or type renames, of data types. This include IdT, OptionT, EitherT, Kleisli, Cokleisli, ConstT, IorT, Const, Nested, ...