Reader.scala

case class Reader[E, A](run: E => A)

object Reader {
  def >==>[E, A, B, C](fa: A => Reader[E, B],
                       fb: B => Reader[E, C]): A => Reader[E, C] = { input: A =>
    val readerB: Reader[E, B] = fa(input)
    flatMap(readerB)(fb)
  }

  def run[E, A]: Reader[E, A] => E => A = {
    case Reader(runnable) =>
      e =>
        runnable(e)
  }

  def map[A, B, E](ra: Reader[E, A])(f: A => B): Reader[E, B] = {
    Reader(_ => f(ra.run()))
  }

  def flatMap[A, B, E](ra: Reader[E, A])(f: A => Reader[E, B]): Reader[E, B] = {
    Reader { e =>
      val a: A = ra.run(e)
      val readerB: Reader[E, B] = f(a)
      readerB.run(e)
    }
  }

  def pure[E, A](a: A): Reader[E, A] = Reader(e => a)
}