Alternative encoding of https://gist.github.com/gvolpe/fba7efc41e924bb194e6eaabac309a09 using the Polymorphic library

existentials-alt.scala

import polymorphic._
import polymorphic.syntax.all._

class Data[A](val x: A, val f: A => String)

trait E {
  def f(ex: ∃[Data]): String = ex match { case Exists(d) => d.f(d.x) }
}

val e = new E {}

e.f(∃(new Data[Int](123, _.toString))) // res3: String = 123
e.f(∃(new Data[String]("Hello!", _.toUpperCase))) // res4: String = HELLO!

existentials-polymorphic.scala

import polymorphic._
import polymorphic.syntax.all._

class Data[A](val x: A, val f: A => String)

trait E {
  def f: String
}

def ex(implicit ev: ∃[Data]): String = ev match { case Exists(x) => x.f }

val e1 = new E {
  override def f = ex(∃(new Data[Int](123, _.toString)))
}

val e2 = new E {
  override def f = ex(∃(new Data[String]("Hello!", _.toUpperCase)))
}

e1.f // res3: String = 123
e2.f // res4: String = HELLO!