Scala example source code file (Zap.scala)

This example Scala source code file (Zap.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.

Learn more about this Scala project at its project page.

Java - Scala tags/keywords

cofree, free, functor, zap, zapinstances

The Zap.scala Scala example source code

package scalaz

import Id._

/** Functors that annihilate each other. */
trait Zap[F[_], G[_]] { self =>
  def zapWith[A, B, C](fa: F[A], gb: G[B])(f: (A, B) => C): C
  def zap[A, B](f: F[A => B], g: G[A]): B = zapWith(f, g)(_(_))
  def flip: Zap[G, F] = new Zap[G, F] {
    def zapWith[A, B, C](ga: G[A], fb: F[B])(f: (A, B) => C): C =
      self.zapWith(fb, ga)((b, a) => f(a, b))
    override def flip = self
  }
}

sealed abstract class ZapInstances {

  /** The identity functor annihilates itself. */
  implicit val identityZap: Zap[Id, Id] =
    new Zap[Id, Id] {
      def zapWith[A, B, C](a: A, b: B)(f: (A, B) => C): C = f(a, b)
    }

  /** The product of two functors annihilates their coproduct. */
  implicit def productCoproductZap[F[_], FF[_], G[_], GG[_]](implicit d1: Zap[F, FF], d2: Zap[G, GG]): Zap[λ[α => (F[α] \/ G[α])], λ[α => (FF[α], GG[α])]] =
    new Zap[λ[α => (F[α] \/ G[α])], λ[α => (FF[α], GG[α])]] {
      def zapWith[A, B, C](a: (F[A] \/ G[A]), b: (FF[B], GG[B]))(f: (A, B) => C) =
        a match {
          case -\/(fa) => d1.zapWith(fa, b._1)(f)
          case \/-(ga) => d2.zapWith(ga, b._2)(f)
        }
    }

  /** The coproduct of two functors annihilates their product. */
  implicit def coproductProductZap[F[_], FF[_], G[_], GG[_]](implicit d1: Zap[FF, F], d2: Zap[GG, G]): Zap[λ[α => (FF[α], GG[α])], λ[α => (F[α] \/ G[α])]] =
    new Zap[λ[α => (FF[α], GG[α])], λ[α => (F[α] \/ G[α])]] {
      def zapWith[A, B, C](a: (FF[A], GG[A]), b: (F[B] \/ G[B]))(f: (A, B) => C) =
        b match {
          case -\/(fb) => d1.zapWith(a._1, fb)(f)
          case \/-(gb) => d2.zapWith(a._2, gb)(f)
        }
    }

  /** A free monad and a cofree comonad annihilate each other */
  implicit def monadComonadZap[F[_], G[_]](implicit d: Zap[F, G], F: Functor[F]): Zap[Free[F, ?], Cofree[G, ?]] =
    new Zap[Free[F, ?], Cofree[G, ?]] {
      def zapWith[A, B, C](ma: Free[F, A], wb: Cofree[G, B])(f: (A, B) => C): C =
        ma.resume match {
          case \/-(a) => f(a, wb.head)
          case -\/(k) => d.zapWith(k, wb.tail)(zapWith(_, _)(f))
        }
    }

  /** A cofree comonad and a free monad annihilate each other */
  implicit def comonadMonadZap[F[_], G[_]](implicit d: Zap[F, G], G: Functor[G]): Zap[Cofree[F, ?], Free[G, ?]] =
    new Zap[Cofree[F, ?], Free[G, ?]] {
      def zapWith[A, B, C](wa: Cofree[F, A], mb: Free[G, B])(f: (A, B) => C): C =
        mb.resume match {
          case \/-(b) => f(wa.head, b)
          case -\/(k) => d.zapWith(wa.tail, k)(zapWith(_, _)(f))
        }
    }
}

object Zap extends ZapInstances