cats-effect-kernel/cats.effect.kernel/Resource

Resource

sealed abstract class Resource[F[_], +A]

Resource is a data structure which encodes the idea of executing an action which has an associated finalizer that needs to be run when the action completes.

Examples include scarce resources like files, which need to be closed after use, or concurrent abstractions like locks, which need to be released after having been acquired.

There are several constructors to allocate a resource, the most common is make:

def open(file: File): Resource[IO, BufferedReader] = {
   val openFile = IO(new BufferedReader(new FileReader(file)))
   Resource.make(acquire = openFile)(release = f => IO(f.close))
}

and several methods to consume a resource, the most common is use:

 def readFile(file: BufferedReader): IO[Content]

 open(file1).use(readFile)

Finalisation (in this case file closure) happens when the action passed to use terminates. Therefore, the code above is not equivalent to:

 open(file1).use(IO.pure).flatMap(readFile)

which will instead result in an error, since the file gets closed after pure, meaning that .readFile will then fail.

Also note that a new resource is allocated every time use is called, so the following code opens and closes the resource twice:

 val file: Resource[IO, File]
 file.use(read) >> file.use(read)

If you want sharing, pass the result of allocating the resource around, and call use once.

file.use { file => read(file) >> read(file) }

The acquire and release actions passed to make are not interruptible, and release will run when the action passed to use succeeds, fails, or is interrupted. You can use makeCase to specify a different release logic depending on each of the three outcomes above.

It is also possible to specify an interruptible acquire though makeFull but be warned that this is an advanced concurrency operation, which requires some care.

Resource usage nests:

 open(file1).use { in1 =>
   open(file2).use { in2 =>
     readFiles(in1, in2)
   }
 }

However, it is more idiomatic to compose multiple resources together before use, exploiting the fact that Resource forms a Monad, and therefore that resources can be nested through flatMap. Nested resources are released in reverse order of acquisition. Outer resources are released even if an inner use or release fails.

 def mkResource(s: String) = {
   val acquire = IO(println(s"Acquiring $$s")) *> IO.pure(s)
   def release(s: String) = IO(println(s"Releasing $$s"))
   Resource.make(acquire)(release)
 }

 val r = for {
   outer <- mkResource("outer")

   inner <- mkResource("inner")
 } yield (outer, inner)

 r.use { case (a, b) =>
   IO(println(s"Using $$a and $$b"))
 }

On evaluation the above prints:

 Acquiring outer
 Acquiring inner
 Using outer and inner
 Releasing inner
 Releasing outer

A Resource can also lift arbitrary actions that don't require finalisation through eval. Actions passed to eval preserve their interruptibility.

Finally, Resource partakes in other abstractions such as MonadError, Parallel, and Monoid, so make sure to explore those instances as well as the other methods not covered here.

Resource is encoded as a data structure, an ADT, described by the following node types:

Normally users don't need to care about these node types, unless conversions from Resource into something else is needed (e.g. conversion from Resource into a streaming data type), in which case they can be interpreted through pattern matching.

Type Params

A: the type of resource
F: the effect type in which the resource is allocated and released

Companion

object

class Object

trait Matchable

class Any

class Allocate[F, A]

class Bind[F, S, A]

class Pure[F, A]

class Eval[F, A]

Value members

Concrete methods

Given a Resource, possibly built by composing multiple Resources monadically, returns the acquired resource, as well as an action that runs all the finalizers for releasing it.

If the outer F fails or is interrupted, allocated guarantees that the finalizers will be called. However, if the outer F succeeds, it's up to the user to ensure the returned F[Unit] is called once A needs to be released. If the returned F[Unit] is not called, the finalizers will not be run.

For this reason, this is an advanced and potentially unsafe api which can cause a resource leak if not used correctly, please prefer use as the standard way of running a Resource program.

Use cases include interacting with side-effectful apis that expect separate acquire and release actions (like the before and after methods of many test frameworks), or complex library code that needs to modify or move the finalizer for an existing resource.

Allocates two resources concurrently, and combines their results in a tuple.

The finalizers for the two resources are also run concurrently with each other, but within each of the two resources, nested finalizers are run in the usual reverse order of acquisition.

Note that Resource also comes with a cats.Parallel instance that offers more convenient access to the same functionality as both, for example via parMapN:

import scala.concurrent.duration._
import cats.effect.{IO, Resource}
import cats.effect.std.Random
import cats.syntax.all._

def mkResource(name: String) = {
 val acquire = for {
   n <- Random.scalaUtilRandom[IO].flatMap(_.nextIntBounded(1000))
   _ <- IO.sleep(n.millis)
   _ <- IO.println(s"Acquiring $$name")
 } yield name

 def release(name: String) =
   IO.println(s"Releasing $$name")

 Resource.make(acquire)(release)
}

val r = (mkResource("one"), mkResource("two"))
 .parMapN((s1, s2) => s"I have $s1 and $s2")
 .use(IO.println(_))

Applies an effectful transformation to the allocated resource. Like a flatMap on F[A] while maintaining the resource context

Applies an effectful transformation to the allocated resource. Like a flatTap on F[A] while maintaining the resource context

Implementation for the flatMap operation, as described via the cats.Monad type class.

Given a mapping function, transforms the resource provided by this Resource.

This is the standard Functor.map.

Given a natural transformation from F to G, transforms this Resource from effect F to effect G. The F and G constraint can also be satisfied by requiring a MonadCancelThrow[F] and MonadCancelThrow[G].

Runs finalizer when this resource is closed. Unlike the release action passed to Resource.make, this will run even if resource acquisition fails or is canceled.

Like onFinalize, but the action performed depends on the exit case.

Runs precede before this resource is allocated.

Races the evaluation of two resource allocations and returns the result of the winner, except in the case of cancelation.

Acquires the resource, runs gb and closes the resource once gb terminates, fails or gets interrupted

Creates a FunctionK that can run gb within a resource, which is then closed once gb terminates, fails or gets interrupted

Allocates a resource and supplies it to the given function. The resource is released as soon as the resulting F[B] is completed, whether normally or as a raised error.

Value Params

f: the function to apply to the allocated resource

Returns

the result of applying [F] to

Allocates a resource with a non-terminating use action. Useful to run programs that are expressed entirely in Resource.

The finalisers run when the resulting program fails or gets interrupted.

Allocates the resource and uses it to run the given Kleisli.

Creates a FunctionK that, when applied, will allocate the resource and use it to run the given Kleisli.

Allocates a resource and closes it immediately.