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] =
    Reader((r: T) ⇒ f(run(r)))

  def flatMap[B](f: A ⇒ Reader[T, B]): Reader[T, B] =
    Reader((r: T) ⇒ f(run(r)).run(r))

  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] = Reader(_ ⇒ a)

  def sequence[T, A](t: List[Reader[T, A]]): Reader[T, List[A]] =
    t.foldLeft(point[T, List[A]](Nil: List[A])) { (acc, v) ⇒
      for {
        x  ← v
        xs ← acc
      } yield x::xs
    }

  def filterM[T, A](p: A ⇒ Reader[T, Boolean], a: List[A]): Reader[T, List[A]] =
    a match {
      case x::xs ⇒ for {
        flg ← p(x)
        ys  ← filterM(p, xs)
      } yield if(flg) x::ys else ys
      case Nil   ⇒ point(Nil)
    }

  def fill[T, A](n: Int)(t: Reader[T, A]): Reader[T, List[A]] =
    t.map(a ⇒ List.fill(n)(a))
}