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Scala example source code file (Applicative.scala)

This example Scala source code file (Applicative.scala) is included in the "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

applicative, applicativelaw, boolean, equal, int, traverse

The Applicative.scala Scala example source code

package scalaz

 * Applicative Functor, described in [[ Applicative Programming with Effects]]
 * Whereas a [[scalaz.Functor]] allows application of a pure function to a value in a context, an Applicative
 * also allows application of a function in a context to a value in a context (`ap`).
 * It follows that a pure function can be applied to arguments in a context. (See `apply2`, `apply3`, ... )
 * Applicative instances come in a few flavours:
 *  - All [[scalaz.Monad]]s are also `Applicative`
 *  - Any [[scalaz.Monoid]] can be treated as an Applicative (see [[scalaz.Monoid]]#applicative)
 *  - Zipping together corresponding elements of Naperian data structures (those of of a fixed, possibly infinite shape)
 *  @see [[scalaz.Applicative.ApplicativeLaw]]
trait Applicative[F[_]] extends Apply[F] { self =>
  def point[A](a: => A): F[A]

  // alias for point
  final def pure[A](a: => A): F[A] = point(a)

  // derived functions
  override def map[A, B](fa: F[A])(f: A => B): F[B] =

  override def apply2[A, B, C](fa: => F[A], fb: => F[B])(f: (A, B) => C): F[C] =
    ap2(fa, fb)(point(f))

  // impls of sequence, traverse, etc

  def traverse[A, G[_], B](value: G[A])(f: A => F[B])(implicit G: Traverse[G]): F[G[B]] =

  def sequence[A, G[_]: Traverse](as: G[F[A]]): F[G[A]] =
    traverse(as)(a => a)

  import std.list._

  /** Performs the action `n` times, returning the list of results. */
  def replicateM[A](n: Int, fa: F[A]): F[List[A]] =

  /** Performs the action `n` times, returning nothing. */
  def replicateM_[A](n: Int, fa: F[A]): F[Unit] =

  /** Filter `l` according to an applicative predicate. */
  def filterM[A](l: List[A])(f: A => F[Boolean]): F[List[A]] =
    l match {
      case Nil => point(List())
      case h :: t => ap(filterM(t)(f))(map(f(h))(b => t => if (b) h :: t else t))

   * Returns the given argument if `cond` is `false`, otherwise, unit lifted into F.
  def unlessM[A](cond: Boolean)(f: => F[A]): F[Unit] = if (cond) point(()) else void(f)

   * Returns the given argument if `cond` is `true`, otherwise, unit lifted into F.
  def whenM[A](cond: Boolean)(f: => F[A]): F[Unit] = if (cond) void(f) else point(())

  /**The composition of Applicatives `F` and `G`, `[x]F[G[x]]`, is an Applicative */
  def compose[G[_]](implicit G0: Applicative[G]): Applicative[λ[α => F[G[α]]]] =
    new CompositionApplicative[F, G] {
      implicit def F = self
      implicit def G = G0

  /**The product of Applicatives `F` and `G`, `[x](F[x], G[x]])`, is an Applicative */
  def product[G[_]](implicit G0: Applicative[G]): Applicative[λ[α => (F[α], G[α])]] =
    new ProductApplicative[F, G] {
      implicit def F = self
      implicit def G = G0

  /** An `Applicative` for `F` in which effects happen in the opposite order. */
  override def flip: Applicative[F] =
    new Applicative[F] with FlippedApply {
      def point[A](a: => A) = Applicative.this.point(a)

  trait ApplicativeLaw extends ApplyLaw {
    /** `point(identity)` is a no-op. */
    def identityAp[A](fa: F[A])(implicit FA: Equal[F[A]]): Boolean =
      FA.equal(ap(fa)(point((a: A) => a)), fa)

    /** `point` distributes over function applications. */
    def homomorphism[A, B](ab: A => B, a: A)(implicit FB: Equal[F[B]]): Boolean =
      FB.equal(ap(point(a))(point(ab)), point(ab(a)))

    /** `point` is a left and right identity, F-wise. */
    def interchange[A, B](f: F[A => B], a: A)(implicit FB: Equal[F[B]]): Boolean =
      FB.equal(ap(point(a))(f), ap(f)(point((f: A => B) => f(a))))

    /** `map` is like the one derived from `point` and `ap`. */
    def mapLikeDerived[A, B](f: A => B, fa: F[A])(implicit FB: Equal[F[B]]): Boolean =
      FB.equal(map(fa)(f), ap(fa)(point(f)))
  def applicativeLaw = new ApplicativeLaw {}

  val applicativeSyntax = new scalaz.syntax.ApplicativeSyntax[F] { def F = Applicative.this }

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


  implicit def monoidApplicative[M:Monoid]: Applicative[λ[α => M]] = Monoid[M].applicative


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