Scala example source code file (Representable.scala)

This example Scala source code file (Representable.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

contravariant, corepresentablelaw, equal, functor, representable, representableinstances, unit

The Representable.scala Scala example source code

package scalaz

/** Representable functors, that is to say, those with isomorphisms to
  * and from `[a](X => a)`.  As such, all typeclasses and operations on
  * `[a](X => a)`, that is, fixed in `X`, can be trivially derived for
  * `F`.
  */
abstract class Representable[F[_], X](implicit val F: Functor[F]) {
  def rep[A](f: X => A): F[A]
  def unrep[A](f: F[A]): X => A

  trait RepresentableLaw {
    /** `rep compose unrep` is vacuous. */
    def repUnrep[A](f: F[A])(implicit E: Equal[F[A]]) =
      E.equal(rep(unrep(f)), f)
    /** `unrep compose rep` is vacuous. */
    def unrepRep[A](f: X => A, x: X)(implicit E: Equal[A]) =
      E.equal(unrep(rep(f))(x), f(x))
  }

  def representableLaw = new RepresentableLaw {}
}

sealed abstract class RepresentableInstances {
  import scalaz.std.function._

  /** The identity representable. */
  implicit def readerRepresentable[E]: Representable[E => ?, E] =
    new Representable[E => ?, E] {
      def rep[A](f: E => A) = f
      def unrep[A](f: E => A) = f
    }

  implicit def curryRepresentable[E]: Representable[E => ?, (E, Unit)] =
    new Representable[E => ?, (E, Unit)] {
      def rep[A](f: ((E, Unit)) => A): E => A = e => f((e, ()))
      def unrep[A](f: E => A): ((E, Unit)) => A = e => f(e._1)
    }

  implicit val f0Representable: Representable[Function0, Unit] =
    new Representable[Function0, Unit] {
      def rep[A](f: Unit => A) = () => f(())
      def unrep[A](f: () => A) = u => f()
    }
}

object Representable extends RepresentableInstances

/** Corepresentable functors */
abstract class Corepresentable[F[_], X](implicit F: Contravariant[F]) {
  def corep[A](f: A => X): F[A]
  def uncorep[A](f: F[A]): A => X

  trait CorepresentableLaw {
    def corepUncorep[A](f: F[A])(implicit E: Equal[F[A]]) =
      E.equal(corep(uncorep(f)), f)
    def uncorepCorep[A](f: A => X, a: A)(implicit E: Equal[X]) =
      E.equal(uncorep(corep(f))(a), f(a))
  }

  def corepresentableLaw = new CorepresentableLaw {}
}