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Scala example source code file (ReaderWriterStateT.scala)

This example Scala source code file (ReaderWriterStateT.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

monad, readerwriterstatet

The ReaderWriterStateT.scala Scala example source code

package scalaz

/** A monad transformer stack yielding `(R, S1) => F[(W, A, S2)]`. */
sealed abstract class IndexedReaderWriterStateT[F[_], -R, W, -S1, S2, A] {
  self =>

  def getF[S <: S1, RR <: R]: Monad[F] => F[(RR, S) => F[(W, A, S2)]]

  def run(r: R, s: S1)(implicit F: Monad[F]): F[(W, A, S2)] =
    F.join( F.map[(R, S1) => F[(W, A, S2)], F[(W, A, S2)]](getF(F))(f => f(r, s)) )

  /** Discards the writer component. */
  def state(r: R)(implicit F: Monad[F]): IndexedStateT[F, S1, S2, A] =
    IndexedStateT((s: S1) => F.map(run(r, s)) {
      case (w, a, s1) => (s1, a)
    })

  /** Calls `run` using `Monoid[S].zero` as the initial state */
  def runZero[S <: S1](r: R)(implicit F: Monad[F], S: Monoid[S]): F[(W, A, S2)] =
    run(r, S.zero)

  /** Run, discard the final state, and return the final value in the context of `F` */
  def eval(r: R, s: S1)(implicit F: Monad[F]): F[(W, A)] =
    F.map(run(r,s)) { case (w,a,s2) => (w,a) }

  /** Calls `eval` using `Monoid[S].zero` as the initial state */
  def evalZero[S <: S1](r:R)(implicit F: Monad[F], S: Monoid[S]): F[(W,A)] =
    eval(r,S.zero)

  /** Run, discard the final value, and return the final state in the context of `F` */
  def exec(r: R, s: S1)(implicit F: Monad[F]): F[(W,S2)] =
    F.map(run(r,s)){case (w,a,s2) => (w,s2)}

  /** Calls `exec` using `Monoid[S].zero` as the initial state */
  def execZero[S <: S1](r:R)(implicit F: Monad[F], S: Monoid[S]): F[(W,S2)] =
    exec(r,S.zero)

  def map[B](f: A => B)(implicit F: Functor[F]): IndexedReaderWriterStateT[F, R, W, S1, S2, B] =
    IndexedReaderWriterStateT.create[F, R, W, S1, S2, B]( (G: Monad[F]) => (r: R, s: S1) => F.map(self.run(r, s)(G))(t => (t._1, f(t._2), t._3)))

  def flatMap[B, RR <: R, S3](f: A => IndexedReaderWriterStateT[F, RR, W, S2, S3, B])(implicit F: Bind[F], W: Semigroup[W]): IndexedReaderWriterStateT[F, RR, W, S1, S3, B] =
    IndexedReaderWriterStateT.create[F, RR, W, S1, S3, B]( (G: Monad[F]) => (r: RR, s1: S1) =>
      F.bind(self.run(r, s1)(G)) {
        case (w1, a, s2) => {
          F.map(f(a).run(r, s2)(G)) {
            case (w2, b, s3) => (W.append(w1, w2), b, s3)
          }
        }
      })
}

object IndexedReaderWriterStateT extends ReaderWriterStateTInstances with ReaderWriterStateTFunctions {
  def apply[F[_], R, W, S1, S2, A](f: (R, S1) => F[(W, A, S2)]): IndexedReaderWriterStateT[F, R, W, S1, S2, A] = new IndexedReaderWriterStateT[F, R, W, S1, S2, A] {
    override def getF[S <: S1, RR <: R]: Monad[F] => F[(RR, S) => F[(W, A, S2)]] = (F: Monad[F]) => F.point((r: R, s: S) => f(r, s))
  }

  def create[F[_], R, W, S1, S2, A](f: Monad[F] => (R, S1) => F[(W,A, S2)]): IndexedReaderWriterStateT[F, R, W, S1, S2, A] = new IndexedReaderWriterStateT[F, R, W, S1, S2, A] {
    override def getF[S <: S1, RR <: R]: Monad[F] => F[(RR, S) => F[(W, A, S2)]] = (F: Monad[F]) => F.point(f(F))
  }
}

trait ReaderWriterStateTFunctions {

}

sealed abstract class IndexedReaderWriterStateTInstances1 {
  implicit def irwstFunctor[F[_], R, W, S1, S2](implicit F0: Functor[F]): Functor[IndexedReaderWriterStateT[F, R, W, S1, S2, ?]] =
    new IndexedReaderWriterStateTFunctor[F, R, W, S1, S2] {
      implicit def F = F0
    }
}

sealed abstract class IndexedReaderWriterStateTInstances0 extends IndexedReaderWriterStateTInstances1 {
  implicit def rwstBind[F[_], R, W, S](implicit F0: Bind[F], W0: Semigroup[W]): Bind[ReaderWriterStateT[F, R, W, S, ?]] =
    new ReaderWriterStateTBind[F, R, W, S] {
      def F = F0
      def W = W0
    }
}

sealed abstract class IndexedReaderWriterStateTInstances extends IndexedReaderWriterStateTInstances0 {
  implicit def irwstPlus[F[_], R, W, S1, S2](implicit F0: Plus[F]): Plus[IndexedReaderWriterStateT[F, R, W, S1, S2, ?]] =
    new IndexedReaderWriterStateTPlus[F, R, W, S1, S2] {
      override def F = F0
    }

  implicit def rwstBindRec[F[_], R, W, S](implicit F0: BindRec[F], F1: Monad[F], W0: Semigroup[W]): BindRec[ReaderWriterStateT[F, R, W, S, ?]] =
    new ReaderWriterStateTBindRec[F, R, W, S] {
      def F = F0
      def A = F1
      def W = W0
    }
}

sealed abstract class ReaderWriterStateTInstances0 extends IndexedReaderWriterStateTInstances {
  implicit def irwstPlusEmpty[F[_], R, W, S1, S2](implicit F0: PlusEmpty[F]): PlusEmpty[IndexedReaderWriterStateT[F, R, W, S1, S2, ?]] =
    new IndexedReaderWriterStateTPlusEmpty[F, R, W, S1, S2] {
      override def F = F0
    }

  implicit def rwstMonad[F[_], R, W, S](implicit W0: Monoid[W], F0: Monad[F]):
  MonadReader[ReaderWriterStateT[F, R, W, S, ?], R] with
  MonadState[ReaderWriterStateT[F, R, W, S, ?], S] with
  MonadListen[ReaderWriterStateT[F, R, W, S, ?], W] =
    new ReaderWriterStateTMonad[F, R, W, S] {
      implicit def F = F0
      implicit def W = W0
    }
}

abstract class ReaderWriterStateTInstances extends ReaderWriterStateTInstances0 {
  implicit def rwstMonadPlus[F[_], R, W, S](implicit W0: Monoid[W], F0: MonadPlus[F]): MonadPlus[ReaderWriterStateT[F, R, W, S, ?]] =
    new ReaderWriterStateTMonadPlus[F, R, W, S] {
      override def F = F0
      override def W = W0
    }

  implicit def rwstHoist[R, W, S](implicit W0: Monoid[W]): Hoist[λ[(α[_], β) => ReaderWriterStateT[α, R, W, S, β]]] =
    new ReaderWriterStateTHoist[R, W, S] {
      implicit def W = W0
    }

}

private trait IndexedReaderWriterStateTPlus[F[_], R, W, S1, S2] extends Plus[IndexedReaderWriterStateT[F, R, W, S1, S2, ?]] {
  def F: Plus[F]

  override final def plus[A](a: IRWST[F, R, W, S1, S2, A], b: => IRWST[F, R, W, S1, S2, A]) =
    IRWST.create((G: Monad[F]) => (r: R, s: S1) => F.plus(a.run(r, s)(G), b.run(r, s)(G)))
}

private trait IndexedReaderWriterStateTPlusEmpty[F[_], R, W, S1, S2]
  extends PlusEmpty[IndexedReaderWriterStateT[F, R, W, S1, S2, ?]]
  with IndexedReaderWriterStateTPlus[F, R, W, S1, S2] {
  def F: PlusEmpty[F]

  override final def empty[A] = IRWST((_, _) => F.empty)
}

private trait IndexedReaderWriterStateTFunctor[F[_], R, W, S1, S2] extends Functor[IndexedReaderWriterStateT[F, R, W, S1, S2, ?]] {
  implicit def F: Functor[F]

  override final def map[A, B](fa: IndexedReaderWriterStateT[F, R, W, S1, S2, A])(f: A => B): IndexedReaderWriterStateT[F, R, W, S1, S2, B] = fa map f
}

private trait ReaderWriterStateTBind[F[_], R, W, S] extends Bind[ReaderWriterStateT[F, R, W, S, ?]] with IndexedReaderWriterStateTFunctor[F, R, W, S, S] {
  implicit def F: Bind[F]
  implicit def W: Semigroup[W]

  override final def bind[A, B](fa: ReaderWriterStateT[F, R, W, S, A])(f: A => ReaderWriterStateT[F, R, W, S, B]) =
    fa flatMap f
}

private trait ReaderWriterStateTBindRec[F[_], R, W, S] extends BindRec[ReaderWriterStateT[F, R, W, S, ?]] with ReaderWriterStateTBind[F, R, W, S] {
  implicit def F: BindRec[F]
  implicit def A: Monad[F]

  def tailrecM[A, B](f: A => ReaderWriterStateT[F, R, W, S, A \/ B])(a: A): ReaderWriterStateT[F, R, W, S, B] = {
    def go(r: R)(t: (W, A, S)): F[(W, A, S) \/ (W, B, S)] =
      F.map(f(t._2).run(r, t._3)) {
        case (w0, e, s0) =>
          val w1 = W.append(t._1, w0)
          e.bimap((w1, _, s0), (w1, _, s0))
      }

    ReaderWriterStateT((r, s) => F.bind(f(a).run(r, s)) {
      case (w, -\/(a0), s0) => F.tailrecM(go(r))(w, a0, s0)
      case (w, \/-(b), s0) => A.point((w, b, s0))
    })
  }
}

private abstract class ReaderWriterStateTMonadPlus[F[_], R, W, S]
  extends MonadPlus[ReaderWriterStateT[F, R, W, S, ?]]
  with ReaderWriterStateTMonad[F, R, W, S]
  with IndexedReaderWriterStateTPlusEmpty[F, R, W, S, S] {
  override def F: MonadPlus[F]
}

private trait ReaderWriterStateTMonad[F[_], R, W, S]
  extends MonadReader[ReaderWriterStateT[F, R, W, S, ?], R]
  with MonadState[ReaderWriterStateT[F, R, W, S, ?], S]
  with MonadListen[ReaderWriterStateT[F, R, W, S, ?], W]
  with ReaderWriterStateTBind[F, R, W, S] {
  implicit def F: Monad[F]
  implicit def W: Monoid[W]

  def point[A](a: => A): ReaderWriterStateT[F, R, W, S, A] =
    ReaderWriterStateT((_, s) => F.point((W.zero, a, s)))
  def ask: ReaderWriterStateT[F, R, W, S, R] =
    ReaderWriterStateT((r, s) => F.point((W.zero, r, s)))
  def local[A](f: R => R)(fa: ReaderWriterStateT[F, R, W, S, A]): ReaderWriterStateT[F, R, W, S, A] =
    ReaderWriterStateT((r, s) => fa.run(f(r), s))
  override def scope[A](k: R)(fa: ReaderWriterStateT[F, R, W, S, A]): ReaderWriterStateT[F, R, W, S, A] =
    ReaderWriterStateT((_, s) => fa.run(k, s))
  override def asks[A](f: R => A): ReaderWriterStateT[F, R, W, S, A] =
    ReaderWriterStateT((r, s) => F.point((W.zero, f(r), s)))
  def init: ReaderWriterStateT[F, R, W, S, S] =
    ReaderWriterStateT((_, s) => F.point((W.zero, s, s)))
  def get = init
  def put(s: S): ReaderWriterStateT[F, R, W, S, Unit] =
    ReaderWriterStateT((r, _) => F.point((W.zero, (), s)))
  override def modify(f: S => S): ReaderWriterStateT[F, R, W, S, Unit] =
    ReaderWriterStateT((r, s) => F.point((W.zero, (), f(s))))
  override def gets[A](f: S => A): ReaderWriterStateT[F, R, W, S, A] =
    ReaderWriterStateT((_, s) => F.point((W.zero, f(s), s)))
  def writer[A](w: W, v: A): ReaderWriterStateT[F, R, W, S, A] =
    ReaderWriterStateT((_, s) => F.point((w, v, s)))
  override def tell(w: W): ReaderWriterStateT[F, R, W, S, Unit] =
    ReaderWriterStateT((_, s) => F.point((w, (), s)))
  def listen[A](ma: ReaderWriterStateT[F, R, W, S, A]): ReaderWriterStateT[F, R, W, S, (A, W)] =
    ReaderWriterStateT((r, s) => F.map(ma.run(r, s)) { case (w, a, s1) => (w, (a, w), s1)})
}

private trait ReaderWriterStateTHoist[R, W, S] extends Hoist[λ[(α[_], β) => ReaderWriterStateT[α, R, W, S, β]]] {
  implicit def W: Monoid[W]

  def hoist[M[_], N[_]](f: M ~> N)(implicit M: Monad[M]) =
    new (ReaderWriterStateT[M, R, W, S, ?] ~> ReaderWriterStateT[N, R, W, S, ?]) {
      def apply[A](ma: ReaderWriterStateT[M, R, W, S, A]): ReaderWriterStateT[N, R, W, S, A] = ReaderWriterStateT {
        case (r,s) => f.apply(ma.run(r,s))
      }
    }

  def liftM[M[_], A](ma: M[A])(implicit M: Monad[M]): ReaderWriterStateT[M, R, W, S, A] =
    ReaderWriterStateT( (r,s) => M.map(ma)((W.zero, _, s)))

  implicit def apply[M[_] : Monad]: Monad[ReaderWriterStateT[M, R, W, S, ?]] =
    IndexedReaderWriterStateT.rwstMonad
}

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