Higher-kinded types in Scala

demo.scala

package scalaInAction.chapter8ScalableExtensibleComponents

import org.scalatest.{FlatSpec, Matchers}

trait HasAtLeastOneElement[F[_]] {
  def any[A](xs: F[A]): Boolean
}

/**
  * Higher-kinded types are types that know how to create a new type from the type argument. That's why
  * higher-kinded types are also known as type constructors - they accept other types as a parameter and
  * create a new type. <code>List(+A)</code> is an example of higher-kinded type.
  */
class HigherKindedTypes extends FlatSpec with Matchers {
  "Higher-kinded types" can "abstract other types (kind of superset of types)" in {
    val anOption = Some("One value")
    val aNonEmptySeq = Seq(1, 2, 3)
    val anEmptySeq = Seq()

    val optionHandler = new HasAtLeastOneElement[Option] {
      override def any[A](xs: Option[A]) = xs.isDefined
    }

    val seqHandler = new HasAtLeastOneElement[Seq] {
      override def any[A](xs: Seq[A]) = xs.nonEmpty
    }

    optionHandler.any(anOption) should be (true)
    seqHandler.any(aNonEmptySeq) should be (true)
    seqHandler.any(anEmptySeq) should be (false)
  }
}