Free can be used as Eff

eff_by_free.scala

import cats.arrow.FunctionK
import cats.data.{ReaderWriterState}
import cats.free.Free

/* utilities for using free as eff */
extension [S[_], A] (free: Free[S, A]) {
  def widenEffect[R[_]](using subtype: S[A] <:< R[A]): Free[R, A] = {
    free.asInstanceOf[Free[R, A]]
  }
}

infix type :|: [S1[_], S2[_]] = [T] =>> S1[T] | S2[T]


/* effect definitions */

enum ReaderEffect[S, T] {
  case Ask[S]() extends ReaderEffect[S, S]
}

object ReaderEffect {
  def ask[S]: Free[[T] =>> ReaderEffect[S, T], S] = Free.liftF(ReaderEffect.Ask[S]())
}

enum WriterEffect[S, T] {
  case Tell[S](value: S) extends WriterEffect[S, Unit]
}

object WriterEffect {
  def tell[S](value: S): Free[[T] =>> WriterEffect[S, T], Unit] = Free.liftF(WriterEffect.Tell(value))
}


/* domain logic */

type IntReaderEffect[T] = ReaderEffect[Int, T]
type IntWriterEffect[T] = WriterEffect[Int, T]

val logic: Free[IntReaderEffect :|: IntWriterEffect, Int] = for {
  x <- ReaderEffect.ask[Int].widenEffect
  _ <- WriterEffect.tell(x + 1).widenEffect
} yield x


/* effect compiler */

val compiler: FunctionK[IntReaderEffect :|: IntWriterEffect, [T] =>> ReaderWriterState[Int, List[Int], Unit, T]] = new FunctionK[IntReaderEffect :|: IntWriterEffect, [T] =>> ReaderWriterState[Int, List[Int], Unit, T]] {
  override def apply[A](effect: ReaderEffect[Int, A] | WriterEffect[Int, A]): ReaderWriterState[Int, List[Int], Unit, A] = effect match {
    case ReaderEffect.Ask() => ReaderWriterState.ask
    case WriterEffect.Tell(value) => ReaderWriterState.tell(value :: Nil)
  }
}


/* test */

object Main {
  def main(args: Array[String]): Unit = {
    val rws: ReaderWriterState[Int, List[Int], Unit, Int] = logic.foldMap(compiler)
    val result: (List[Int], Unit, Int) = rws.runEmpty(100).value
    println(result)
  }
}