populator.scala

type ID = Long
type Code = String

class User(val id: ID,
           val name: String,
           val friends: List[User])

class UserDto(val code: Code,
              val name: String,
              val friends: List[UserDto]) {
  override def toString = { 
    "UserDto(" + code + " " + name + " " + friends + ")" 
  }
}

implicit def idToCode(id: ID): Code = { 
  id.toString
}

implicit def userToUserDto(user: User): UserDto = { 
  new UserDto(user.id, user.name, user.friends)
}

implicit def usersToUserDtos(users: List[User]): List[UserDto] = { 
  users.map(userToUserDto)
}

val users = List(new User(10, "Vesa Marttila", Nil))

val userDtos: List[UserDto] = users

println(userDtos.head) // -> UserDto(10 Vesa Marttila List())