Writer exercise

gistfile1.scala

import scalaz._, Scalaz._

case class Writer[W: Monoid, +A](run: (A, W)) {
  def map[B](f: A ⇒ B): Writer[W, B] =
    Writer {
      val (a, w) = run
      (f(a), w)
    }

  def flatMap[B](f: A ⇒ Writer[W, B]): Writer[W, B] =
    Writer {
      val (a, w1) = run
      val (b, w2) = f(a).run
      (b, w1 |+| w2)
    }
}

object Writer {
  def point[W: Monoid, A](a: A): Writer[W, A] = Writer((a, Monoid[W].zero))

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

  def filterM[W: Monoid, A](p: A ⇒ Writer[W, Boolean], a: List[A]): Writer[W, 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 tell[W: Monoid](w: W): Writer[W, Unit] = Writer(((), w))
}