SharedStateAlgebra.scala

import cats.implicits._
import cats.data._
import cats.effect._
import cats.effect.concurrent._

abstract class Alg[F[_]]{
  def up: F[Unit]
  def down: F[Unit]
  def get: F[Int]
}

object Alg {

  def impl[F[_]: Sync](init: Int): F[Alg[F]] = 
    Ref.of[F, Int](init).map(new CounterAlg[F](_))
  
  private class CounterAlg[F[_]](ref: Ref[F, Int]) extends Alg[F]{
    def up: F[Unit] = ref.update(_ + 1 )
    def down: F[Unit] = ref.update(_ - 1)
    def get: F[Int] = ref.get
  }

  private class CounterAlgWithAbstractBase[F[_]](
    ref: Ref[F, Int]
  ) extends Alg[Kleisli[F, Int, ?]]{
    def up: Kleisli[F, Int, Unit] = Kleisli{i => 
      ref.update(_ + i)
    }
    def down: Kleisli[F, Int, Unit] = Kleisli{i => 
      ref.update(_ - i)
    }
    def get: Kleisli[F, Int, Int] = Kleisli{i => 
      ref.get
    }
  }
}