predicate.scala

object Predicates {
  type Pred[-T] = (T => Boolean)

  implicitly[Seq[Int] >:> List[Int]]
  implicitly[Pred[Seq[Int]] <:< Pred[List[Int]]] // predicate of seq is a subtype of predicate of list

  val hasLength3: Pred[Seq[Int]] = _.size == 3

  hasLength3(Vector(1,2,3)) // we can pass any seq to hasLength3
  hasLength3(List(1,2,3))                                                                                                 

  val listOfLength3: Pred[List[Int]] = hasLength3 // this ascription is safe because of the subtype relationship
  listOfLength3(List(1,2,3))
  // listOfLength3(Vector(1,2,3)) // fails to compile, listOfLength3 accepts only lists
}