17.Answer

case class Id[+A](value: A):
  def map[B](f: A => B): Id[B] =
    Id(f(value))
  def flatMap[B](f: A => Id[B]): Id[B] =
    f(value)

object Id:
  given idMonad: Monad[Id] with
    def unit[A](a: => A) = Id(a)
    extension [A](fa: Id[A])
      override def flatMap[B](f: A => Id[B]) =
        fa.flatMap(f)