Typeclassed Writer Monad

gistfile1.scala

case class Writer[A, D](value: A, diary: D)(implicit m: Monoid[D]) {
  def flatMap[B](f: A => Writer2[B, D]) =  f(value) match {
      case Writer(result, d) => Writer(result, m.append(diary, d)
  }
  def map[B](f: A => B) = Writer[B, D](f(value), diary)
}
trait Monoid[T] {
   def zero: T
   def append(a: T, b: T): T
}
object Monoid {
   implicit val StringMonoid = new Monoid[St ring] {
       override def zero = ""
       override def append(s1: String, s2: String) = s1 + s2
   }
}