Reader exercise (inspired by gist from @tonymorris: https://gist.github.com/3884189)

gistfile1.scala

case class Reader[T, +A](run: T ⇒ A) {
  def map[B](f: A ⇒ B): Reader[T, B] = sys.error("undefined")

  def flatMap[B](f: A ⇒ Reader[T, B]): Reader[T, B] = sys.error("undefined")

  def &&&[B](x: Reader[T, B]): Reader[T, (A, B)] =
    for {
      a ← this
      b ← x
    } yield (a, b)
}

object Reader {
  def point[T, A](a: A): Reader[T, A] = sys.error("undefined")

  def sequence[T, A](t: List[Reader[T, A]]): Reader[T, List[A]] = sys.error("undefined")

  def filterM[T, A](p: A ⇒ Reader[T, Boolean], a: List[A]): Reader[T, List[A]] = sys.error("undefined")

  def fill[T, A](n: Int)(t: Reader[T, A]): Reader[T, List[A]] = sys.error("undefined")
}