Improved StringMonoid

gistfile1.scala

object Monoid {
 implicit val StringMonoid = new Monoid[String] {
     override def zero = ""
     override def append(s1: String, s2: String) = {
             val  delimiter = if (s1 == zero || s2 == zero) "" 
                              else ", "
             s1 + delimiter + s2
     }
 }
}

Writer(5).flatMap { n => Writer(n + 3, “added three”)
           .flatMap { n => Writer(n * 5, "times five") }
           .flatMap { n => Writer(n + 2, "plusTwo") }
// => Writer(42, "added three, times 5, plus two")