Bifoldable

Формальное определение

Bifoldable - тип, порождающий два несвязанных Foldable.

Определение в виде кода на Scala

trait Bifoldable[F[_, _]]:
  extension [A, B](fa: F[A, B])
    /** Аккумулирование всех `A` и `B`. */
    def bifoldMap[M](f: A => M)(g: B => M)(using ma: Monoid[M]): M

    /** Аккумулирование до `C`, начиная "справа". */
    def bifoldRight[C](z: => C)(f: (A, C) => C)(g: (B, C) => C): C =
      fa.bifoldMap[C => C](f.curried)(g.curried)(using endoMonoid[C])(z)

    /** Аккумулирование до `C`, начиная "слева". */
    def bifoldLeft[C](z: C)(f: (C, A) => C)(g: (C, B) => C): C =
      fa.bifoldMap[C => C](a => c => f(c, a))(b => c => g(c, b))(using dual(endoMonoid[C]))(z)

  /** Выделение Foldable из первого параметра. */
  def leftFoldable[R]: Foldable[[X] =>> F[X, R]] =
    new Foldable[[X] =>> F[X, R]]:
      extension [A](fa: F[A, R])
        override def foldMap[B](f: A => B)(using mb: Monoid[B]): B =
          fa.bifoldMap[B](f)(_ => mb.empty)

  /** Выделение Foldable из второго параметра. */
  def rightFoldable[L]: Foldable[[X] =>> F[L, X]] =
    new Foldable[[X] =>> F[L, X]]:
      extension [A](fa: F[L, A])
        override def foldMap[B](f: A => B)(using mb: Monoid[B]): B =
          fa.bifoldMap[B](_ => mb.empty)(f)

  /** Унифицирование Foldable на обоих параметрах. */
  def uFoldable: Foldable[[X] =>> F[X, X]] =
    new Foldable[[X] =>> F[X, X]]:
      extension [A](fa: F[A, A])
        override def foldMap[B](f: A => B)(using mb: Monoid[B]): B =
          fa.bifoldMap[B](f)(f)

Примеры

Either

given eitherBifoldable: Bifoldable[Either] with
  extension [A, B](fa: Either[A, B])
    def bifoldMap[M](f: A => M)(g: B => M)(using ma: Monoid[M]): M =
      fa match
        case Right(value) => g(value)
        case Left(value)  => f(value)

Ссылки:

• Scalaz API