scalaz.Profunctor.scala Maven / Gradle / Ivy

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package scalaz

////
/**
 * Profunctors are covariant on the right and contravariant on the left.
 */
////
trait Profunctor[=>:[_, _]]  { self =>
  ////
  /** Contramap on `A`. */
  def mapfst[A, B, C](fab: (A =>: B))(f: C => A): (C =>: B)

  /** Functor map on `B`. */
  def mapsnd[A, B, C](fab: (A =>: B))(f: B => C): (A =>: C)

  /** Functor map on `A` and `B`. */
  def dimap[A, B, C, D](fab: (A =>: B))(f: C => A)(g: B => D): (C =>: D) =
    mapsnd(mapfst(fab)(f))(g)

  protected[this] trait SndCovariant[C] extends Functor[C =>: ?] {
    override def map[A, B](fa: C =>: A)(f: A => B) = mapsnd(fa)(f)
  }

  def invariantFunctor: InvariantFunctor[λ[α => α =>: α]] =
    new InvariantFunctor[λ[α => α =>: α]] {
      def xmap[A, B](ma: A =>: A, f: A => B, g: B => A) =
        mapsnd(mapfst(ma)(g))(f)
    }

  def covariantInstance[C]: Functor[C =>: ?] =
    new SndCovariant[C]{}

  def contravariantInstance[C]: Contravariant[? =>: C] =
    new Contravariant[? =>: C] {
      def contramap[A, B](fa: A =>: C)(f: B => A): (B =>: C) =
        mapfst(fa)(f)
    }

  trait ProfunctorLaw {
    def identity[A, B](gab: (A =>: B))(implicit E: Equal[A =>: B]): Boolean = E.equal(dimap(gab)((a: A) => a)((b: B) => b), gab)

    def composite[A, B, C, D, E, F](gad: (A =>: D), fac: C => B, fba: B => A, fde: D => E, fef: E => F)(implicit E: Equal[C =>: F]): Boolean = {
      E.equal(dimap(dimap(gad)(fba)(fde))(fac)(fef), dimap(gad)(fba compose fac)(fef compose fde))
    }
  }

  def profunctorLaw = new ProfunctorLaw {}

  ////
  val profunctorSyntax = new scalaz.syntax.ProfunctorSyntax[=>:] { def F = Profunctor.this }
}

object Profunctor {
  @inline def apply[F[_, _]](implicit F: Profunctor[F]): Profunctor[F] = F

  ////
  sealed trait UpStarF
  type UpStar[F[_], D, C] = (D => F[C]) @@ UpStarF
  val UpStar = Tag.of[UpStarF]

  sealed trait DownStarF
  type DownStar[F[_], D, C] = (F[D] => C) @@ DownStarF
  val DownStar = Tag.of[DownStarF]

  implicit def upStarProfunctor[F[_]: Functor]: Profunctor[UpStar[F, ?, ?]] =
    new Profunctor[UpStar[F, ?, ?]] {
      def mapfst[A, B, C](h: UpStar[F, A, B])(f: C => A): UpStar[F, C, B] =
        UpStar(Tag unwrap h compose f)
      def mapsnd[A, B, C](h: UpStar[F, A, B])(f: B => C): UpStar[F, A, C] =
        UpStar(a => Functor[F].map(Tag.unwrap(h)(a))(f))
    }

  implicit def downStarProfunctor[F[_]: Functor]: Profunctor[DownStar[F, ?, ?]] =
    new Profunctor[DownStar[F, ?, ?]] {
      def mapfst[A, B, C](h: DownStar[F, A, B])(f: C => A): DownStar[F, C, B] =
        DownStar(fa => Tag.unwrap(h)(Functor[F].map(fa)(f)))
      def mapsnd[A, B, C](h: DownStar[F, A, B])(f: B => C): DownStar[F, A, C] =
        DownStar(f compose Tag.unwrap(h))
    }

  implicit def upStarFunctor[F[_]: Functor, D]: Functor[UpStar[F, D, ?]] =
    new Functor[UpStar[F, D, ?]] {
      def map[A, B](m: UpStar[F, D, A])(f: A => B) =
        upStarProfunctor[F].mapsnd(m)(f)
    }

  implicit def downStarFunctor[F[_], D]: Functor[DownStar[F, D, ?]] =
    new Functor[DownStar[F, D, ?]] {
      def map[A, B](f: DownStar[F, D, A])(k: A => B) =
        DownStar(k compose Tag.unwrap(f))
    }
  ////
}