Clarify the reasons for dropping existentials

Dotty documentation lists three reasons why existentials were dropped.

• Existential types violate a type soundness principle on which DOT and Dotty are constructed. That principle says that every prefix (p, respectvely S) of a type selection p.T or S#T must either come from a value constructed at runtime or refer to a type that is known to have only good bounds.

Can this be illustrated on an example? What’s a snippet of code that uses existentials and is problematic in this respect?

• Existential types create many difficult feature interactions with other Scala constructs.

This is a little too vague of an explanation for users who are going to miss existentials.

• Existential types largely overlap with path-dependent types, so the gain of having them is relatively minor.

I disagree that the gain is minor.

[hobwekiva]

@TomasMikula You can make a newtype for existentials which works pretty well in Scala2 and doesn’t box.

    type Type[+F[_]] <: (Any { type T })

    def wrap[F[_], A](value: F[A]): Type[F] =
        value.asInstanceOf[Type[F]]

    def unwrap[F[_]](value: Type[F]): F[value.T] =
        value.asInstanceOf[F[value.T]]

[RossTate]

Tate, Leung and Learner have explored some possible explanations in [11], but their treatment raises about as many questions as it answers.

Hah, I love this quote! I’d never seen it before, but I totally get it. Sorry the work doesn’t help y’all except to warn just how complicated the problem gets.

I have other work on checking/inferring existential types, but unfortunately it doesn’t work for impredicative existential types, which is the case y’all particularly care about. Nonetheless, there are probably some useful lessons to learn from the predicative case if y’all are interested.