nyan.scala

import scala.util.Try
import scala.language.reflectiveCalls

trait Validator[T] {
  def apply[O <: T](obj: O): Try[O]
}

type Username = String

object UsernameSpecification {
  type UsernameHolder = { val username: Username }

  val validator: Validator[UsernameHolder] = new Validator[UsernameHolder] {
    override def apply[O <: UsernameHolder](obj: O): Try[O] = Try {
      val username = obj.username
      // validate username...
      obj
    }
  }
}

case class User(val username: Username)
case class Group(val name: String)

val usernameValidator = UsernameSpecification.validator

usernameValidator(User("takkkun")) // => OK
usernameValidator(Group("a group")) // completion error