ValidatedT

case class ValidatedT[F[_], +E, +A](run: F[Validated[E, A]])
import cats.Monad
import cats.syntax.flatMap._
object ValidatedT {
  implicit def monad[F[_]: Monad, E]: Monad[({type l[a] = ValidatedT[F, E, a]})#l] = new Monad[({type l[a] = ValidatedT[F, E, a]})#l] {
    def pure[A](x: A) = ValidatedT(Monad[F].pure(Validated.valid(x)))

    def flatMap[A, B](fa: ValidatedT[F, E, A])(f: (A) => ValidatedT[F, E, B]) =
      ValidatedT(fa.run.flatMap(_.fold(e => Monad[F].pure(Validated.invalid[E, B](e)), i => f(i).run)))

    def tailRecM[A, B](a: A)(f: (A) => ValidatedT[F, E, Either[A, B]]) = ???
  }
}