Proposal to remove Existential Types from the language

[dwijnand]

Hey @jsuereth!

@jsuereth

Scenario 1: Higher kinded types

SBT relies on forSome for code like the following:

def joinAny[M[_]](inits: Seq[Initialize[M[T]] forSome { type T }]): Initialize[Seq[M[_]]]

Where “Initialize” is effectively an Applicative functor / Reader-style monad. In the past, existentials were the only way to get Scala’s type inference to handle these styles of methods.

I mentioned 2 more that are present in the sbt codebase:

• Parser[Seq[ScopedKey[T]] forSome { type T }]
• Initialize[Task[T]] forSome { type T }

Mentioned in Drop `forSome`, restrict existentials to wildcards · Issue #443 · scala/scala-dev · GitHub

I’d love to know how these would be rewritten in Dotty, without losing any expressiveness.