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Scala example source code file (Endo.scala)

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

Learn more about this Scala project at its project page.

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

  /** 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 =
    def from: (A => A) => Endo[A] = endo

  val IsoFunctorEndo = new IsoFunctorTemplate[Endo, λ[α => α => α]] {
    def to[A](fa: Endo[A]): A => A =
    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 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))

Other Scala examples (source code examples)

Here is a short list of links related to this Scala Endo.scala source code file:

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