Stuff.scala

import scala.language.higherKinds

sealed abstract class Stuff[F[_], +A]

case class Suspend[F[_], A](a: () => F[A])
  extends Stuff[F, A]
                                      
def extract[F[_], A](source: Stuff[F, A]): Suspend[F, A] =
  source match {
    case ref @ Suspend(_) => ref.asInstanceOf[Suspend[F,A]]
}


import scala.collection.mutable

val s = mutable.Set.empty[String]
val stuff: Stuff[mutable.Set,Any] = Suspend(() => s)
val s2 = extract(stuff).a()
s2 += 123
s.head

//java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
//  ... 29 elided