monadwriterexample.scala

object MonadWriterExample extends App {
  import scalaz._
  import Scalaz._

  implicit val monadWriter = EitherT.listenableMonadWriter[Writer, String, String]

  case class Person(firstName: String, age: Int, car: Option[String])

  def notEmpty(s: String) = Option(s).filter(x ⇒ !x.isEmpty && !x.forall(_.isWhitespace)) match {
    case Some(r) ⇒ monadWriter.right[String](r)
    case None    ⇒ monadWriter.left[String]("Empty String")
  }

  def majority(age: Int) =
    if (age >= 18) monadWriter.right[Int](age)
    else monadWriter.left[Int]("This person is not an adult")

  def validateData(firstName: String, age: Int, car: Option[String]) = for {
    n ← notEmpty(firstName) :++>> (x ⇒ "The first name is => " + x + "\n")
    a ← majority(age) :++>> (x ⇒ "She is an adult => " + x + "\n")
    _ ← if (car.isDefined) monadWriter.tell("Got a car") else monadWriter.tell("No car.")
  } yield Person(n, a, car)
}

Is the line that tests whether a car is defined in validateData 'a good way' of doing what its doing? I am simply logging something but proceeding anyway regardless of the value.

For me there's nothing wrong with that. Do you really need to notify "No car" ? If you don't, you could write

car.traverse(_ => monadWriter.tell("Got a car"))