羊さんコード

sheep.scala

case class Sheep(name:String,father:Option[Sheep],mother:Option[Sheep])

object SampleBase {
  def father(sheep:Sheep):Option[Sheep] = sheep match {
    case Sheep(_, s, _) => s
    case _ => None
  }
  def mother(sheep:Sheep):Option[Sheep] = sheep match {
    case Sheep(_, _, s) => s
    case _ => None
  }
}

object Sample1 {
  import SampleBase._
  def maternalGrandfather(sheep:Sheep):Option[Sheep] = mother(sheep) match {
    case None => None
    case Some(s) => father(s)
  }
  def mothersPaternalGrandfather(sheep:Sheep):Option[Sheep] = mother(sheep) match {
    case None => None
    case Some(s) => father(s) match {
      case None => None
      case Some(ss) => father(ss)
    }
  }
}

object Sample2 {
  import SampleBase._
  case class Comb[a](x:Option[a]) {
    def comb[b](f:a => Option[b]):Option[b] = x flatMap f
  }

  implicit def sheep2Comb(s:Option[Sheep]):Comb[Sheep] = Comb(s)
  def mothersPaternalGrandfather(sheep:Sheep):Option[Sheep] =
    Option(sheep) comb mother comb father comb father
}

object Main extends App {
  val first = Sheep("1st",None,None)
  val second = Sheep("2nd",Some(first),None)
  val third = Sheep("3rd",Some(first),Some(second))
  val fourth = Sheep("4th",Some(second),Some(third))
  val fifth = Sheep("5th",Some(third),Some(fourth))

  for { s <- Sample1.maternalGrandfather(fifth) } assert(s.name == "2nd",s)
  for { s <- Sample1.mothersPaternalGrandfather(fifth) } assert(s.name == "1st",s)

  for { s <- Sample2.mothersPaternalGrandfather(fifth) } assert(s.name == "1st",s)
}

ネタ元

http://www.sampou.org/haskell/a-a-monads/html/meet.html#example1

単にこの場合に短く書くとしたら

case class Sheep(name:String,father:Option[Sheep],mother:Option[Sheep])

def maternalGrandfather(sheep:Sheep):Option[Sheep] = {
  sheep.mother.flatMap{_.father}
}

def mothersPaternalGrandfather(sheep:Sheep):Option[Sheep] = {
  sheep.mother.flatMap{_.father.flatMap{_.father}}
}

とかですかね

なるほど。その方が処理が分かりやすく、パフォーマンスの面でも有利ですね。

今思ったんですが、macroを使えばもう少しすっきり書けるような気がします。
例えば

nest"sheep.mother >> father >> father"

を

sheep.mother.flatMap{_.father.flatMap{_.father}}

に変換するのはどうでしょう。
nest型のcase classで汎用的に使えそうです。
文字リテラルなのでIDEの補完が効かないのは致命的ですが…