iravid / managedt   0.1

Apache License 2.0 GitHub

A monad for resource acquisition and cleanup

Scala versions: 2.12 2.11

Deprecated

With cats-effect 1.0.0 out, this library is now redundant. The Resource data type provides equivalent functionality.

Build Status Maven Central

ManagedT: a monad for resource management

ManagedT[F[_], A] is a monad for constructing computations that acquire and release resources. It is a translation of Gabriel Gonzalez' managed library, with the difference that this library abstracts over the effect type.

This library only contains typeclass instances for cats. Scalaz already contains the Codensity data type which can serve the same functionality (with, albeit, some additional plumbing for cleanup handlers).

IMPORTANT CAVEATS

This library supports any F[_] that has MonadError[F, Throwable]. That includes monix.eval.Task, cats.effect.IO, scala.concurrent.Future (and others).

However, it is very likely to be broken if:

  • you use cancellation with Task or IO - MonadError is not strong enough to express bracketing with cancellation, so that'll have to wait for MonadBracket in cats-effect;
  • you use it with a monad transformer such as EitherT[IO, E, A], where there are several "layers" in which errors could be thrown;
  • you throw exceptions in the acquire or cleanup actions that are not sequenced into the F[_];
  • you pass Future values that have already been executed in the acquire or cleanup functions. The library has constructors with by-name values in most places to assist with that.

Other than that, it's all good ;-) The test suite exercises the common usecases; you are welcome to have a look.

Usage

SBT

libraryDependencies += ״com.iravid" %% "managedt" % "0.1"

In your project

First, create some resources:

import com.iravid.managedt.ManagedT
// import com.iravid.managedt.ManagedT

import monix.eval.Task
// import monix.eval.Task

val resource1 = ManagedT(Task { println("Acquiring 1"); 1 })(r => Task(println(s"Cleaning $r")))
// resource1: com.iravid.managedt.ManagedT[monix.eval.Task,Int] = com.iravid.managedt.ManagedT$$anon$4@23497a65

val resource2 = ManagedT(Task { println("Acquiring 2"); 2 })(r => Task(println(s"Cleaning $r")))
// resource2: com.iravid.managedt.ManagedT[monix.eval.Task,Int] = com.iravid.managedt.ManagedT$$anon$4@42716335

Compose them - ManagedT[F, R] is a monad in R, so you can use the usual forms of composition:

import cats.implicits._
// import cats.implicits._

val zipped = (resource1, resource2).tupled
// zipped: com.iravid.managedt.ManagedT[monix.eval.Task,(Int, Int)] = com.iravid.managedt.ManagedT$$anon$1$$anon$5@6d555633

val added = for {
  r1 <- resource1
  r2 <- resource2
} yield r1 + r2
// added: com.iravid.managedt.ManagedT[monix.eval.Task,Int] = com.iravid.managedt.ManagedT$$anon$1$$anon$5@228720d0

val resources = (1 to 10).toList traverse { r =>
  ManagedT(Task { println(s"Acquiring $r"); r})(r => Task(println(s"Cleaning $r")))
}
// resources: com.iravid.managedt.ManagedT[monix.eval.Task,List[Int]] = com.iravid.managedt.ManagedT$$anon$1$$anon$5@346d3ee3

And, finally, use your acquired resource in a function of the form R => F[A]:

val zippedUsage = zipped { case (r1, r2) => 
  Task {
    println("Using zipped"); r1 + r2 
  }
}
// zippedUsage: monix.eval.Task[Int] = Task.FlatMap$1596984844

val addedUsage = added { r => 
  Task {
    println("Using added"); r * r 
  }
}
// addedUsage: monix.eval.Task[Int] = Task.FlatMap$757626346

val resourcesUsage = resources { rs => 
  Task {
    println("Using resources")
    rs.sum 
  }
}
// resourcesUsage: monix.eval.Task[Int] = Task.FlatMap$1046714810

As we're using Task, nothing has actually run yet; we've composed programs that use the resources safely. Let's see what happens when we run them:

import monix.execution.Scheduler
implicit val scheduler = Scheduler.singleThread(name="tut")
import scala.concurrent.Await
// import scala.concurrent.Await

import scala.concurrent.duration._
// import scala.concurrent.duration._

Await.result(zippedUsage.runAsync, 5.seconds)
// Acquiring 1
// Acquiring 2
// Using zipped
// Cleaning 2
// Cleaning 1
// res0: Int = 3

The program constructed by ManagedT[F, A] properly acquires and releases resources in the right order. This also works for the traversed list of resources:

Await.result(resourcesUsage.runAsync, 5.seconds)
// Acquiring 1
// Acquiring 2
// Acquiring 3
// Acquiring 4
// Acquiring 5
// Acquiring 6
// Acquiring 7
// Acquiring 8
// Acquiring 9
// Acquiring 10
// Using resources
// Cleaning 10
// Cleaning 9
// Cleaning 8
// Cleaning 7
// Cleaning 6
// Cleaning 5
// Cleaning 4
// Cleaning 3
// Cleaning 2
// Cleaning 1
// res1: Int = 55

Should one of the acquires, clean-ups or uses fail, the clean-ups will still run properly:

val acquireFailure = for {
  _ <- ManagedT(Task.unit)(_ => Task { println("Cleaning 1") })
  _ <- ManagedT(Task.unit)(_ => Task { println("Cleaning 2")})
  _ <- ManagedT(Task.raiseError(new Exception))((_: Unit) => Task { println("Cleaning 3") })
} yield ()
// acquireFailure: com.iravid.managedt.ManagedT[monix.eval.Task,Unit] = com.iravid.managedt.ManagedT$$anon$1$$anon$5@de37d32

Await.result(acquireFailure(_ => Task.unit).attempt.runAsync, 5.seconds)
// Cleaning 2
// Cleaning 1
// res2: Either[Throwable,Unit] = Left(java.lang.Exception)

val useFailure = for {
  _ <- ManagedT(Task.unit)(_ => Task { println("Cleaning 1") })
  _ <- ManagedT(Task.unit)(_ => Task { println("Cleaning 2")})
  _ <- ManagedT(Task.unit)(_ => Task { println("Cleaning 3") })
} yield ()
// useFailure: com.iravid.managedt.ManagedT[monix.eval.Task,Unit] = com.iravid.managedt.ManagedT$$anon$1$$anon$5@6c1de753

Await.result(useFailure(_ => Task.raiseError(new Exception())).attempt.runAsync, 5.seconds)
// Cleaning 3
// Cleaning 2
// Cleaning 1
// res3: Either[Throwable,Nothing] = Left(java.lang.Exception)

val cleanupFailure = for {
  _ <- ManagedT(Task.unit)(_ => Task { println("Cleaning 1") })
  _ <- ManagedT(Task.unit)(_ => Task.raiseError(new Exception()))
  _ <- ManagedT(Task.unit)(_ => Task { println("Cleaning 3") })
} yield ()
// cleanupFailure: com.iravid.managedt.ManagedT[monix.eval.Task,Unit] = com.iravid.managedt.ManagedT$$anon$1$$anon$5@1417b95d

Await.result(cleanupFailure(_ => Task.unit).attempt.runAsync, 5.seconds)
// Cleaning 3
// Cleaning 1
// res4: Either[Throwable,Unit] = Left(java.lang.Exception)

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