Free example

free.scala

object MTL {

  import cats._
  import cats.free._
  import cats.data._
  import cats.implicits._

  sealed trait DSL[A]

  case class Get[A]() extends DSL[A]
  case class Put[A](a: A) extends DSL[Unit]

  type PRG[A] = Free[DSL, A]

  def get[A]: PRG[A] = Free.liftF[DSL, A](Get())
  def put[A](a: A): PRG[Unit] = Free.liftF[DSL, Unit](Put(a))


  def getEither[A]: PRG[Either[Throwable, A]] = get[A].map(Right(_))


  val e = get[Int].liftT[EitherT[?[_], Throwable, ?]]

  for {
    x <- e
    y <- EitherT(getEither[Int])
  } yield x + y
}

free2.scala

object MTL {

  import cats._
  import cats.free._
  import cats.data._
  import cats.implicits._

  sealed trait DSL[+A]

  case class Get[A]() extends DSL[Either[Throwable, A]]
  case class Put[A](a: A) extends DSL[Unit]
  case class Error(t: Throwable) extends DSL[Nothing]

  type PRG[A] = Free[DSL, A]

  def get[A]: PRG[Either[Throwable, A]] = Free.liftF[DSL, Either[Throwable, A]](Get())
  def put[A](a: A): PRG[Unit] = Free.liftF[DSL, Unit](Put(a))
  def error[A](t: Throwable) : PRG[A] = Free.liftF[DSL, A](Error(t))
  
  
  for {
    result <- get[Int].flatMap{ case Right(a) => Free.pure(a); case Left(t) => error(t) }
  } yield result
}