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.
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
Value members
Concrete methods
Given a Resource
, possibly built by composing multiple
Resource
s monadically, returns the acquired resource, as well
as an action that runs all the finalizers for releasing it.
Given a Resource
, possibly built by composing multiple
Resource
s 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.
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
:
def mkResource(name: String) = {
val acquire =
IO(scala.util.Random.nextInt(1000).millis).flatMap(IO.sleep) *>
IO(println(s"Acquiring $$name")).as(name)
val release = IO(println(s"Releasing $$name"))
Resource.make(acquire)(release)
}
val r = (mkResource("one"), mkResource("two"))
.parMapN((s1, s2) => s"I have $s1 and $s2")
.use(msg => IO(println(msg)))
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
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
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.
Implementation for the flatMap
operation, as described via the
cats.Monad
type class.
Given a mapping function, transforms the resource provided by this Resource.
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].
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.
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.
Like onFinalize
, but the action performed depends on the exit case.
Races the evaluation of two resource allocations and returns the result of the winner, except in the case of cancelation.
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
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
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.
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
.
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.
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.
Creates a FunctionK that, when applied, will allocate the resource and use it to run the given Kleisli.