12.Answer

trait Foldable[F[_]]:
  import Monoid.{endoMonoid, dual}

  extension [A](as: F[A])
    def foldRight[B](acc: B)(f: (A, B) => B): B =
      as.foldMap(f.curried)(using endoMonoid[B])(acc)

    def foldLeft[B](acc: B)(f: (B, A) => B): B =
      as.foldMap(a => b => f(b, a))(using dual(endoMonoid[B]))(acc)

    def foldMap[B](f: A => B)(using mb: Monoid[B]): B =
      as.foldRight(mb.empty)((a, b) => mb.combine(f(a), b))

    def combineAll(using ma: Monoid[A]): A =
      as.foldLeft(ma.empty)(ma.combine)

object Foldable:

  given Foldable[List] with
    extension [A](as: List[A])
      override def foldRight[B](acc: B)(f: (A, B) => B) =
        as.foldRight(acc)(f)
      override def foldLeft[B](acc: B)(f: (B, A) => B) =
        as.foldLeft(acc)(f)

  given Foldable[IndexedSeq] with
    extension [A](as: IndexedSeq[A])
      override def foldRight[B](acc: B)(f: (A, B) => B) =
        as.foldRight(acc)(f)
      override def foldLeft[B](acc: B)(f: (B, A) => B) =
        as.foldLeft(acc)(f)
      override def foldMap[B](f: A => B)(using mb: Monoid[B]): B =
        Monoid.foldMapV(as, mb)(f)

  given Foldable[LazyList] with
    extension [A](as: LazyList[A])
      override def foldRight[B](acc: B)(f: (A, B) => B) =
        as.foldRight(acc)(f)
      override def foldLeft[B](acc: B)(f: (B, A) => B) =
        as.foldLeft(acc)(f)