Scala example source code file (RegionT.scala)

This example Scala source code file (RegionT.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

ioref, kleisli, liftio, monad, regiont, regiontliftio, regiontmonad

The RegionT.scala Scala example source code

package scalaz
package effect

import Kleisli._

// An implementation of "Lightweight Monadic Regions" by Kiselyov and Shan
// http://okmij.org/ftp/Haskell/regions.html#light-weight
// Based on a Haskell library by Bas van Dijk

/**
 * A monad transformer in which scarce resources can be opened. When the region
 * terminates, all opened resources will be closed automatically. It's a type error
 * to return an opened resource from the region, and no I/O with closed
 * resources is possible.
 */
sealed abstract class RegionT[S, P[_], A] {
  def value: Kleisli[P, IORef[List[RefCountedFinalizer]], A]

  def runT(r: IORef[List[RefCountedFinalizer]]): P[A] =
    value.run(r)
}

object RegionT extends RegionTInstances {
  def apply[S, P[_], A](k: Kleisli[P, IORef[List[RefCountedFinalizer]], A]): RegionT[S, P, A] = new RegionT[S, P, A] {
    val value = k
  }

  def regionT[S, P[_], A](k: Kleisli[P, IORef[List[RefCountedFinalizer]], A]): RegionT[S, P, A] = RegionT(k)
}

sealed abstract class RegionTInstances1 {
  implicit def RegionTLiftIO[S, M[_]](implicit M: LiftIO[M]): LiftIO[RegionT[S, M, ?]] = new RegionTLiftIO[S, M] {
    implicit def L = M
  }

  implicit def RegionTMonad[S, M[_]](implicit M0: Monad[M]): Monad[RegionT[S, M, ?]] = new RegionTMonad[S, M] {
    implicit def M = M0
  }
}

sealed abstract class RegionTInstances extends RegionTInstances1 {
}

trait RegionTMonad[S, M[_]] extends Monad[RegionT[S, M, ?]] {
  implicit def M: Monad[M]

  def point[A](a: => A): RegionT[S, M, A] = RegionT(kleisli(s => M.point(a)))
  def bind[A, B](fa: RegionT[S, M, A])(f: A => RegionT[S, M, B]): RegionT[S, M, B] =
    RegionT(kleisli(s => M.bind(fa.value.run(s))((a: A) => f(a).value.run(s))))
}

trait RegionTLiftIO[S, M[_]] extends LiftIO[RegionT[S, M, ?]] {
  implicit def L: LiftIO[M]

  def liftIO[A](ioa: IO[A]) = RegionT.regionT(kleisli(_ => L.liftIO(ioa)))
}