Reader.scala

object Reader {
  def pure[C,A](a: A): Reader[C,A] = (_: C) => a

  def sequence[C,A](rs: List[Reader[C,A]]): Reader[C,List[A]] = (c: C) =>
    rs.foldLeft(List[A]()) { (accu,reader) => accu :+ reader(c) }

  def filter[C,A](xs: List[A])(p: A => Reader[C,Boolean]): Reader[C,List[A]] = (c: C) => {
    val bs: List[Reader[C,Boolean]] = xs.map(p)
    xs.foldLeft(List[A]()) { (accu,x) => if (p(x)(c)) accu :+ x else accu }
  }

  implicit def reader[A,B]: (A => B) => Reader[A,B] = (f: A => B) => Reader(f)
}

case class Reader[C,A](g: C => A) {
  def apply(c: C) = g(c)

  def foreach: (A => Unit) => Reader[C,Unit] = map[Unit]

  def map[B](f: A => B): Reader[C,B] = (c: C) => f(g(c))

  def flatMap[B](f: A => Reader[C,B]): Reader[C,B] = (c: C) => f(g(c))(c)

  def &&&[B](that: Reader[C,B]) : Reader[C,(A,B)] = (c: C) => (this(c),that(c))
}