Writer Monad 0.1

writerMonad1.scala

case class Writer(value: Int, diary: String) {
  def flatMap(f: Int => Writer) = {
      f(value) match {
          case Writer(result, d) => Writer(result, diary + d)
      }
  }
  def map(f: Int => Int) = Writer(f(value), diary)
}
// This enables us to say:
new Writer(2, "").flatMap { new Writer(3, "three") }
                 .flatMap { new Writer(4, "four") }
// => Writer(4, "threefour")