error effect

gistfile1.scala

sealed trait Error[E, A]
object Error {
  case class ThrowError[E, A](error: E) extends Error[E, A]

  implicit def monadError[E, S[_]: Functor](implicit I: Inject[Error[E, ?], S]): MonadError[Free[S, ?], E] =
    new MonadError[Free[S, ?], E] {
      def pure[A](a: A) = Free.pure(a)
      def raiseError[A](e: E) = Free.inject[Error[E, ?], S](ThrowError(e))
      def handleErrorWith[A](fa: Free[S, A])(f: E => Free[S, A]) =
        fa.fold(
          r => fa,
          s => I.prj(s) match {
            case Some(ThrowError(e)) => f(e)
            case None => fa
          }
        )
      def flatMap[A, B](fa: Free[S, A])(f: A => Free[S, B]) = fa flatMap f
      def tailRecM[A, B](a: A)(f: A => Free[S, Either[A, B]]) = Monad[Free[S, ?]].tailRecM(a)(f)
    }
}