Scala example source code file (Inject.scala)

This example Scala source code file (Inject.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

coproduct, free, functor, inject, injectinstances, option

The Inject.scala Scala example source code

package scalaz

import std.option.{none, some}

/**
 * Inject type class as described in "Data types a la carte" (Swierstra 2008).
 *
 * @see [[http://www.staff.science.uu.nl/~swier004/Publications/DataTypesALaCarte.pdf]]
 */
sealed abstract class Inject[F[_], G[_]] extends (F ~> G) {
  def apply[A](fa: F[A]): G[A] = inj(fa)
  def inj[A](fa: F[A]): G[A]
  def prj[A](ga: G[A]): Option[F[A]]
}

sealed abstract class InjectInstances {
  implicit def reflexiveInjectInstance[F[_]] =
    new Inject[F, F] {
      def inj[A](fa: F[A]) = fa
      def prj[A](ga: F[A]) = some(ga)
    }

  implicit def leftInjectInstance[F[_], G[_]] =
    new Inject[F, Coproduct[F, G, ?]] {
      def inj[A](fa: F[A]) = Coproduct.leftc(fa)
      def prj[A](ga: Coproduct[F, G, A]) = ga.run.fold(some(_), _ => none)
    }

  implicit def rightInjectInstance[F[_], G[_], H[_]](implicit I: Inject[F, G]) =
      new Inject[F, Coproduct[H, G, ?]] {
        def inj[A](fa: F[A]) = Coproduct.rightc(I.inj(fa))
        def prj[A](ga: Coproduct[H, G, A]) = ga.run.fold(_ => none, I.prj(_))
      }
}

object Inject extends InjectInstances {
  def inject[F[_], G[_], A](ga: G[Free[F, A]])(implicit I: Inject[G, F]): Free[F, A] =
    Free[F, A](I.inj(ga))

  def match_[F[_], G[_], A](fa: Free[F, A])(implicit F: Functor[F], I: Inject[G, F]): Option[G[Free[F, A]]] =
    fa.resume.fold(I.prj(_), _ => none)

  @inline def apply[F[_], G[_]](implicit I: Inject[F, G]): Inject[F, G] = I
}