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Cats core

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package cats

import simulacrum.typeclass

/**
 * The combination of a Monad with a MonoidK
 */
@typeclass trait MonadCombine[F[_]] extends MonadFilter[F] with Alternative[F] {

  /**
   * Fold over the inner structure to combine all of the values with
   * our combine method inherited from MonoidK. The result is for us
   * to accumulate all of the "interesting" values of the inner G, so
   * if G is Option, we collect all the Some values, if G is Either,
   * we collect all the Right values, etc.
   */
  def unite[G[_], A](fga: F[G[A]])(implicit G: Foldable[G]): F[A] =
    flatMap(fga) { ga =>
      G.foldLeft(ga, empty[A])((acc, a) => combineK(acc, pure(a)))
    }

  /** Separate the inner foldable values into the "lefts" and "rights" */
  def separate[G[_, _], A, B](fgab: F[G[A, B]])(implicit G: Bifoldable[G]): (F[A], F[B]) = {
    val as = flatMap(fgab)(gab => G.bifoldMap(gab)(pure, _ => empty[A])(algebra[A]))
    val bs = flatMap(fgab)(gab => G.bifoldMap(gab)(_ => empty[B], pure)(algebra[B]))
    (as, bs)
  }
}