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Scala example source code file (MonadPlus.scala)

This example Scala source code file (MonadPlus.scala) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Scala by Example" TM.

Learn more about this Scala project at its project page.

Java - Scala tags/keywords

boolean, equal, monadlaw, monadplus, monadpluslaw, strongmonadpluslaw

The MonadPlus.scala Scala example source code

package scalaz

////
/**
 * @see [[scalaz.Monad]]
 * @see [[scalaz.PlusEmpty]]
 */
////
trait MonadPlus[F[_]] extends Monad[F] with ApplicativePlus[F] { self =>
  ////

  /** Remove `f`-failing `A`s in `fa`, by which we mean: in the
    * expression `filter(filter(fa)(f))(g)`, `g` will never be invoked
    * for any `a` where `f(a)` returns false.
    */
  def filter[A](fa: F[A])(f: A => Boolean) =
    bind(fa)(a => if (f(a)) point(a) else empty[A])

  /** Generalized version of Haskell's `catMaybes` */
  def unite[T[_], A](value: F[T[A]])(implicit T: Foldable[T]): F[A] =
    bind(value)((ta) => T.foldMap(ta)(a => point(a))(monoid[A]))

  /** Generalized version of Haskell's `partitionEithers` */
  def separate[G[_, _], A, B](value: F[G[A, B]])(implicit G: Bifoldable[G]): (F[A], F[B]) = {
    val lefts  = bind(value)((aa) => G.leftFoldable.foldMap(aa)(a => point(a))(monoid[A]))
    val rights = bind(value)((bb) => G.rightFoldable.foldMap(bb)(b => point(b))(monoid[B]))
    (lefts, rights)
  }

  /** A version of `unite` that infers the type constructor `T`. */
  final def uniteU[T](value: F[T])(implicit T: Unapply[Foldable, T]): F[T.A] =
    unite(T.leibniz.subst(value))(T.TC)

  /**The product of MonadPlus `F` and `G`, `[x](F[x], G[x]])`, is a MonadPlus */
  def product[G[_]](implicit G0: MonadPlus[G]): MonadPlus[λ[α => (F[α], G[α])]] =
    new ProductMonadPlus[F, G] {
      def F = self
      def G = G0
    }

  trait MonadPlusLaw extends EmptyLaw with MonadLaw {
    /** `empty[A]` is a polymorphic value over `A`. */
    def emptyMap[A](f1: A => A)(implicit FA: Equal[F[A]]): Boolean =
      FA.equal(map(empty[A])(f1), empty[A])

    /** `empty` short-circuits its right. */
    def leftZero[A](f: A => F[A])(implicit FA: Equal[F[A]]): Boolean = {
      FA.equal(bind(empty[A])(f), empty[A])
    }
  }
  trait StrongMonadPlusLaw extends MonadPlusLaw {
    /** `empty` short-circuits throughout its `join` tree. */
    def rightZero[A](f: F[A])(implicit FA: Equal[F[A]]): Boolean = {
      FA.equal(bind(f)(_ => empty[A]), empty[A])
    }
  }
  def monadPlusLaw = new MonadPlusLaw {}
  def strongMonadPlusLaw = new StrongMonadPlusLaw {}
  ////
  val monadPlusSyntax = new scalaz.syntax.MonadPlusSyntax[F] { def F = MonadPlus.this }
}

object MonadPlus {
  @inline def apply[F[_]](implicit F: MonadPlus[F]): MonadPlus[F] = F

  ////

  ////
}

Other Scala examples (source code examples)

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