case.scala

object comonadbuilder {
  import scalaz._
  import Scalaz._
  case class Config(a: Option[String] = None, b: Option[Int] = None)

  type Builder[A] = Config => A
  
  def setFieldA(a: String) = (b:Builder[Config]) => b(Config(a=Some(a)))
                                                  //> setFieldA: (a: String)(comonadbuilder.Config => comonadbuilder.Config) => co
                                                  //| monadbuilder.Config
  def setFieldB(a: Int) = (b:Builder[Config]) => b(Config(b=Some(a)))
                                                  //> setFieldB: (a: Int)(comonadbuilder.Config => comonadbuilder.Config) => comon
                                                  //| adbuilder.Config
  

  val builder: Builder[Config] = c => c           //> builder  : comonadbuilder.Config => comonadbuilder.Config = <function1>

  implicit val mo = new Monoid[Config] {
		  def zero: Config = Config()

  // Members declared in scalaz.Semigroup
  def append(f1: Config,f2: => Config): Config = Config(f1.a orElse f2.a, f1.b orElse f2.b)
	}                                         //> mo  : scalaz.Monoid[comonadbuilder.Config] = comonadbuilder$$anonfun$main$1$
                                                  //| $anon$1@5b980c78
  builder.cobind(setFieldA("aa")).cobind(setFieldB(3)).cobind(setFieldA("bb")).copoint
                                                  //> res0: comonadbuilder.Config = Config(Some(bb),Some(3))

}

gist.scala

object comonadbuilder {
  import scalaz._
  import Scalaz._
  
  def withConfig = setter("config")               //> withConfig: => (List[String] => comonadbuilder.Config) => comonadbuilder.Con
                                                  //| fig
  def withStuff = setter("stuff")                 //> withStuff: => (List[String] => comonadbuilder.Config) => comonadbuilder.Conf
                                                  //| ig
  type Builder[A] = List[String] => A
  type Config = List[String]
  
  def setter(opt: String) = (builder: Builder[Config]) => {
    builder(opt :: Nil)
  }                                               //> setter: (opt: String)(List[String] => comonadbuilder.Config) => comonadbuild
                                                  //| er.Config

  val builder: Builder[Config] = ls => Config(ls) //> builder  : List[String] => comonadbuilder.Config = <function1>

  builder.cobind(withConfig).cobind(withStuff).copoint
                                                  //> res0: comonadbuilder.Config = Config(List(stuff, config))
}