Scala example source code file (Endo.scala)

This example Scala source code file (Endo.scala) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Scala by Example" ^TM.

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Java - Scala tags/keywords

endo, endoinstances, invariantfunctor, isoendo, isofunctorendo, isofunctortemplate, isomorphism, isoset, monoid, unzip

The Endo.scala Scala example source code

package scalaz

/** Endomorphisms.  They have special properties among functions, so
  * are captured in this newtype.
  *
  * @param run The captured function.
  */
final case class Endo[A](run: A => A) {
  final def apply(a: A): A = run(a)

  /** Do `other`, than call myself with its result. */
  final def compose(other: Endo[A]): Endo[A] = Endo.endo(run compose other.run)

  /** Call `other` with my result. */
  final def andThen(other: Endo[A]): Endo[A] = other compose this
}

object Endo extends EndoInstances {
  /** Alias for `Endo.apply`. */
  final def endo[A](f: A => A): Endo[A] = Endo(f)

  /** Always yield `a`. */
  final def constantEndo[A](a: => A): Endo[A] = endo[A](_ => a)

  /** Alias for `Monoid[Endo[A]].zero`. */
  final def idEndo[A]: Endo[A] = endo[A](a => a)

  import Isomorphism.{IsoSet, IsoFunctorTemplate}

  def IsoEndo[A] = new IsoSet[Endo[A], A => A] {
    def to: (Endo[A]) => A => A = _.run
    def from: (A => A) => Endo[A] = endo
  }

  val IsoFunctorEndo = new IsoFunctorTemplate[Endo, λ[α => α => α]] {
    def to[A](fa: Endo[A]): A => A = fa.run
    def from[A](ga: A => A): Endo[A] = endo(ga)
  }
}

sealed abstract class EndoInstances {

  /** Endo forms a monoid where `zero` is the identity endomorphism
    * and `append` composes the underlying functions. */
  implicit def endoInstance[A]: Monoid[Endo[A]] = new Monoid[Endo[A]] {
    def append(f1: Endo[A], f2: => Endo[A]) = f1 compose f2
    def zero = Endo.idEndo
  }
  implicit val endoInstances: Zip[Endo] with Unzip[Endo] with InvariantFunctor[Endo] = new Zip[Endo] with Unzip[Endo] with InvariantFunctor[Endo] {
    def xmap[A, B](fa: Endo[A], f: A => B, g: B => A) =
      Endo.endo(g andThen fa.run andThen f)

    def zip[A, B](a: => Endo[A], b: => Endo[B]) =
      Endo {
        case (x, y) => (a(x), b(y))
      }

    // CAUTION: cheats with null
    def unzip[A, B](a: Endo[(A, B)]) =
      (Endo(x => a((x, null.asInstanceOf[B]))._1), Endo(x => a((null.asInstanceOf[A], x))._2))
  }
}