Composable predicates

predicates.scala

  class SimpleFile
  class ExtendedFile extends SimpleFile


  implicit class Predicate[-A](val fn: A => Boolean) {
    def &&[B <: A](other: Predicate[B]): Predicate[B] = Predicate { (x: B) =>
      fn(x) && other.fn(x)
    }
  }

  val alwaysTrueSimple = (_: SimpleFile) => true
  val alwaysTrueAdvanced = (_: ExtendedFile) => true

  alwaysTrueSimple && alwaysTrueAdvanced
  alwaysTrueAdvanced && alwaysTrueSimple