Scala Immutable Builder with tight guided fluent interface

/**
 * Example of an immutable pojo with a built-in builder.
 * Still pretty ugly translation of java version at http://pastebin.com/zt8vE8Fw
 * @author twitter.com/benhardy
 */
class PersonFeatures2(val userId:Int, val age:Int, val children:Int, val dogs:Int) {

}

object PersonFeatures2 {
    // give each stage of the builder only one callable method,
    // which returns the next stage

    trait BuilderUserIdSpec {
        def userId(v:Int): BuilderAgeSpec
    }
    trait BuilderAgeSpec {
        def age(v:Int) : BuilderChildrenSpec
    }
    trait BuilderChildrenSpec {
        def  children(v:Int) : BuilderDogsSpec
    }
    trait BuilderDogsSpec {
        def  dogs(v:Int): BuilderFinal
    }
    trait BuilderFinal {
        def  build : PersonFeatures2
    }


    def builder = {
      new BuilderUserIdSpec() {
        override def userId(_userId:Int) = {
          new BuilderAgeSpec() {
            override def age(_age:Int) = {
              new BuilderChildrenSpec() {
                override def children(_children:Int) = {
                  new BuilderDogsSpec() {
                    override def dogs(_dogs:Int) = {
                      new BuilderFinal() {
                        override def build = {
                          new PersonFeatures2(_userId, _age, _children, _dogs)
                        }
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }

    /**
     * example usage. pretty obvious what is going on. only one way to do it. IDE will help.
     */
    val person = builder userId(4437) age(22) children(0) dogs(1) build
}