.eff fork+discard function for cats Arrow

ArrowForkDiscardEff.scala

//import scalaz._
//import scalaz.syntax.arrow._
//
//def eff[F[_, _]: Arrow, A, B](a: F[A, B])(effFn: F[B, Unit]): F[A, B] = {
//  val z = Arrow[F].arr(identity[B]) &&& Arrow[F].arr(identity[B])
//
//  a >>> z >>> effFn.second[B].mapsnd(_._1)
//}


import cats.arrow._
import cats.syntax.functor._
import cats.syntax.profunctor._
import cats.syntax.strong._
import cats.syntax.compose._
import scala.language.higherKinds

def effImpl[F[_, _]: Arrow, A, B](a: F[A, B])(eff: F[B, Unit]): F[A, B] = {
  val forkedA: F[A, (B, B)] =
    a.rmap(mkTuple)

  val effOnFirst: F[(B, B), (Unit, B)] =
    eff.first

  forkedA >>> effOnFirst >>> discardFirst
}

def mkTuple[A](a: A): (A, A) =
  (a, a)

def discardFirst[F[_, _]: Arrow, A, B]: F[(A, B), B] = {
  Arrow[F].lift(_._2)
}

implicit class ArrowEff[F[_, _]: Arrow, A, B](fab: F[A, B]) {
  def eff(eff: F[B, Unit]): F[A, B] = {
    effImpl(fab)(eff)
  }
}


object Abc {
  import cats.instances.all._


  def a = { x: Int => x + 1 }
    .andThen(_ * 5)
    .andThen(_ ^ 2)
    .eff(x => println(s"Current value: $x"))
    .andThen(Seq.fill(_)('Z'))
    .andThen(_.mkString)
    .eff(println(_))
    .andThen(_.length)
    .eff(println(_))
    .apply(1)


  def b = { x: Int => x + 1 }
    .>>>(_ * 5)
    .>>>(_ ^ 2)
    .eff(x => println(s"Current value: $x"))
    .>>>(Seq.fill(_)('Z'))
    .>>>(_.mkString)
    .eff(println(_))
    .>>>(_.length)
    .eff(println(_))
    .apply(1)


  def c = { x: Int => x + 1 }
    .map(_ * 5)
    .map(_ ^ 2)
    .eff(x => println(s"Current value: $x"))
    .map(Seq.fill(_)('Z'))
    .map(_.mkString)
    .eff(println(_))
    .map(_.length)
    .eff(println(_))
    .apply(1)
}

Abc.a
Abc.b
Abc.c