StringBuilder Monoid

gistfile1.scala

object Monoid {
    implicit val StringBuilderMonoid = new Monoid[StringBuilder] {
        override def zero = new StringBuilder("")
        override def append(sb1: StringBuilder, sb2: StringBuilder) = {
            val  delimiter = if (sb1.toString == "" || 
                                 sb2.toString == "") "" 
                             else ", "
            sb1.append(delimiter).append(sb2)
        }
    }
}

Writer(22, new StringBuilder("")).flatMap { n => 
    Writer(n + 2, new StringBuilder("plus two")) 
}.flatMap { n => 
    Writer(n * 4, new StringBuilder("times four")) 
})

// => Writer(96, StringBuilder("plus two, times four"))