cats.effect
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Represents the exit code of an application.
Represents the exit code of an application.
code
is constrained to a range from 0 to 255, inclusive.
Attributes
- Companion:
- object
- Source:
- ExitCode.scala
- Graph
- Supertypes
Attributes
- Companion:
- class
- Source:
- ExitCode.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
- Self type
- ExitCode.type
A pure abstraction representing the intention to perform a side effect, where the result of that side effect may be obtained synchronously (via return) or asynchronously (via callback).
A pure abstraction representing the intention to perform a side effect, where the result of that side effect may be obtained synchronously (via return) or asynchronously (via callback).
IO
values are pure, immutable values and thus preserve referential transparency, being
usable in functional programming. An IO
is a data structure that represents just a
description of a side effectful computation.
IO
can describe synchronous or asynchronous computations that:
- on evaluation yield exactly one result 2. can end in either success or failure and in
case of failure
flatMap
chains get short-circuited (IO
implementing the algebra ofMonadError
) 3. can be canceled, but note this capability relies on the user to provide cancelation logic
Effects described via this abstraction are not evaluated until the "end of the world", which is to say, when one of the "unsafe" methods are used. Effectful results are not memoized, meaning that memory overhead is minimal (and no leaks), and also that a single effect may be run multiple times in a referentially-transparent manner. For example:
val ioa = IO.println("hey!")
val program = for {
_ <- ioa
_ <- ioa
} yield ()
program.unsafeRunSync()
The above will print "hey!" twice, as the effect will be re-run each time it is sequenced in the monadic chain.
IO
is trampolined in its flatMap
evaluation. This means that you can safely call
flatMap
in a recursive function of arbitrary depth, without fear of blowing the stack.
def fib(n: Int, a: Long = 0, b: Long = 1): IO[Long] =
IO.pure(a + b) flatMap { b2 =>
if (n > 0)
fib(n - 1, b, b2)
else
IO.pure(a)
}
Attributes
The primary entry point to a Cats Effect application. Extend this trait rather than defining
your own main
method. This avoids the need to run IO.unsafeRunAsync (or similar) on
your own.
The primary entry point to a Cats Effect application. Extend this trait rather than defining
your own main
method. This avoids the need to run IO.unsafeRunAsync (or similar) on
your own.
IOApp
takes care of the messy details of properly setting up (and tearing down) the
unsafe.IORuntime needed to run the IO which represents your application. All of the
associated thread pools (if relevant) will be configured with the assumption that your
application is fully contained within the IO
produced by the run method. Note that the
exact details of how the runtime will be configured are very platform-specific. Part of the
point of IOApp
is to insulate users from the details of the underlying runtime (whether JVM
or JavaScript).
object MyApplication extends IOApp {
def run(args: List[String]) =
for {
_ <- IO.print("Enter your name: ")
name <- IO.readln
_ <- IO.println("Hello, " + name)
} yield ExitCode.Success
}
In the above example, MyApplication
will be a runnable class with a main
method, visible
to Sbt, IntelliJ, or plain-old java
. When run externally, it will print, read, and print in
the obvious way, producing a final process exit code of 0. Any exceptions thrown within the
IO
will be printed to standard error and the exit code will be set to 1. In the event that
the main Fiber (represented by the IO
returned by run
) is canceled, the runtime will
produce an exit code of 1.
Note that exit codes are an implementation-specific feature of the underlying runtime, as are process arguments. Naturally, all JVMs support these functions, as does Node.js and Scala Native, but some JavaScript execution environments will be unable to replicate these features (or they simply may not make sense). In such cases, exit codes may be ignored and/or argument lists may be empty.
Note that in the case of the above example, we would actually be better off using
IOApp.Simple rather than IOApp
directly, since we are neither using args
nor are we
explicitly producing a custom ExitCode:
object MyApplication extends IOApp.Simple {
val run =
for {
_ <- IO.print("Enter your name: ")
name <- IO.readln
_ <- IO.println(s"Hello, " + name)
} yield ()
}
It is valid to define val run
rather than def run
because IO
's evaluation is lazy: it
will only run when the main
method is invoked by the runtime.
In the event that the process receives an interrupt signal (SIGINT
) due to Ctrl-C (or any
other mechanism), it will immediately cancel
the main fiber. Assuming this fiber is not
within an uncancelable
region, this will result in interrupting any current activities and
immediately invoking any finalizers (see: IO.onCancel and IO.bracket). The process
will not shut down until the finalizers have completed. For example:
object InterruptExample extends IOApp.Simple {
val run =
IO.bracket(startServer)(
_ => IO.never)(
server => IO.println("shutting down") *> server.close)
}
If we assume the startServer
function has type IO[Server]
(or similar), this kind of
pattern is very common. When this process receives a SIGINT
, it will immediately print
"shutting down" and run the server.close
effect.
One consequence of this design is it is possible to build applications which will ignore
process interrupts. For example, if server.close
runs forever, the process will ignore
interrupts and will need to be cleaned up using SIGKILL
(i.e. kill -9
). This same
phenomenon can be demonstrated by using IO.uncancelable to suppress all interruption
within the application itself:
object Zombie extends IOApp.Simple {
val run = IO.never.uncancelable
}
The above process will run forever and ignore all interrupts. The only way it will shut down
is if it receives SIGKILL
.
It is possible (though not necessary) to override various platform-specific runtime
configuration options, such as computeWorkerThreadCount
(which only exists on the JVM).
Please note that the default configurations have been extensively benchmarked and are optimal
(or close to it) in most conventional scenarios.
However, with that said, there really is no substitute to benchmarking your own application.
Every application and scenario is unique, and you will always get the absolute best results
by performing your own tuning rather than trusting someone else's defaults. IOApp
's
defaults are very ''good'', but they are not perfect in all cases. One common example of this
is applications which maintain network or file I/O worker threads which are under heavy load
in steady-state operations. In such a performance profile, it is usually better to reduce the
number of compute worker threads to "make room" for the I/O workers, such that they all sum
to the number of physical threads exposed by the kernel.
Attributes
- See also:
- Note:
While IOApp works perfectly fine for browser applications, in practice it brings little value over calling IO.unsafeRunAndForget. You can use your UI framework's preferred API for defining your application's entrypoint. On the other hand, Node.js applications must use IOApp to behave correctly.
- Companion:
- object
- Source:
- IOApp.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
- Known subtypes
- trait Simple
Attributes
- Companion:
- trait
- Source:
- IOApp.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
- Self type
- IOApp.type
IOLocal provides a handy way of manipulating a context on different scopes.
IOLocal provides a handy way of manipulating a context on different scopes.
In some scenarios, IOLocal can be considered as an alternative to cats.data.Kleisli.
IOLocal should not be treated as Ref, since the former abides different laws.
Once a fiber is forked, for example by Spawn[F].start
, the forked fiber manipulates the
copy of the parent's context. For example, two forked fibers will never see each other's
modifications to the same IOLocal, each fiber will only see its own modifications.
===Operations on IOLocal are visible to the fiber===
┌────────────┐ ┌────────────┐ ┌────────────┐
│ Fiber A │ update(_ + 1) │ Fiber A │ update(_ + 1) │ Fiber A │
│ (local 42) │──────────────►│ (local 43) │──────────────►│ (local 44) │
└────────────┘ └────────────┘ └────────────┘
def inc(name: String, local: IOLocal[Int]): IO[Unit] =
local.update(_ + 1) >> local.get.flatMap(current => IO.println(s"fiber $$name: $$current"))
for {
local <- IOLocal(42)
_ <- inc(1, local)
_ <- inc(2, local)
current <- local.get
_ <- IO.println(s"fiber A: $$current")
} yield ()
// output:
// update 1: 43
// update 2: 44
// fiber A: 44
===A forked fiber operates on a copy of the parent IOLocal===
A '''forked''' fiber (i.e. via Spawn[F].start
) operates on a '''copy''' of the parent
IOLocal
. Hence, the children operations are not reflected on the parent context.
┌────────────┐ ┌────────────┐
fork │ Fiber B │ update(_ - 1) │ Fiber B │
┌─────►│ (local 42) │──────────────►│ (local 41) │
│ └────────────┘ └────────────┘
┌────────────┐─┘ ┌────────────┐
│ Fiber A │ │ Fiber A │
│ (local 42) │────────────────────────────────────►│ (local 42) │
└────────────┘─┐ └────────────┘
│ ┌────────────┐ ┌────────────┐
│ fork │ Fiber C │ update(_ + 1) │ Fiber C │
└─────►│ (local 42) │──────────────►│ (local 43) │
└────────────┘ └────────────┘
def update(name: String, local: IOLocal[Int], f: Int => Int): IO[Unit] =
local.update(f) >> local.get.flatMap(current => IO.println(s"$$name: $$current"))
for {
local <- IOLocal(42)
fiber1 <- update("fiber B", local, _ - 1).start
fiber2 <- update("fiber C", local, _ + 1).start
_ <- fiber1.joinWithNever
_ <- fiber2.joinWithNever
current <- local.get
_ <- IO.println(s"fiber A: $$current")
} yield ()
// output:
// fiber B: 41
// fiber C: 43
// fiber A: 42
===Parent operations on IOLocal are invisible to children===
┌────────────┐ ┌────────────┐
fork │ Fiber B │ update(_ + 1) │ Fiber B │
┌─────►│ (local 42) │──────────────►│ (local 43) │
│ └────────────┘ └────────────┘
┌────────────┐─┘ ┌────────────┐
│ Fiber A │ update(_ - 1) │ Fiber A │
│ (local 42) │────────────────────────────────────►│ (local 41) │
└────────────┘─┐ └────────────┘
│ ┌────────────┐ ┌────────────┐
│ fork │ Fiber C │ update(_ + 2) │ Fiber C │
└─────►│ (local 42) │──────────────►│ (local 44) │
└────────────┘ └────────────┘
def update(name: String, local: IOLocal[Int], f: Int => Int): IO[Unit] =
IO.sleep(1.second) >> local.update(f) >> local.get.flatMap(current => IO.println(s"$$name: $$current"))
for {
local <- IOLocal(42)
fiber1 <- update("fiber B", local, _ + 1).start
fiber2 <- update("fiber C", local, _ + 2).start
_ <- fiber1.joinWithNever
_ <- fiber2.joinWithNever
_ <- update("fiber A", local, _ - 1)
} yield ()
// output:
// fiber B: 43
// fiber C: 44
// fiber A: 41
Attributes
- A
the type of the local value
- Companion:
- object
- Source:
- IOLocal.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
Attributes
- Companion:
- trait
- Source:
- IOLocal.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
- Self type
- IOLocal.type
Attributes
- Companion:
- object
- Source:
- LiftIO.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
Attributes
- Companion:
- trait
- Source:
- LiftIO.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
- Self type
- LiftIO.type
A convenience trait for defining applications which are entirely within Resource. This is implemented as a relatively straightforward wrapper around IOApp and thus inherits most of its functionality and semantics.
A convenience trait for defining applications which are entirely within Resource. This is implemented as a relatively straightforward wrapper around IOApp and thus inherits most of its functionality and semantics.
This trait should generally be used for any application which would otherwise trivially end with cats.effect.kernel.Resource!.use (or one of its variants). For example:
object HttpExample extends IOApp {
def run(args: List[String]) = {
val program = for {
config <- Resource.eval(loadConfig(args.head))
postgres <- Postgres[IO](config.jdbcUri)
endpoints <- ExampleEndpoints[IO](config, postgres)
_ <- HttpServer[IO](config.host, config.port, endpoints)
} yield ()
program.useForever.as(ExitCode.Success)
}
}
This example assumes some underlying libraries like
Skunk and Http4s, but otherwise
it represents a relatively typical example of what the main class for a realistic Cats Effect
application might look like. Notably, the whole thing is enclosed in Resource
, which is
use
d at the very end. This kind of pattern is so common that ResourceApp
defines a
special trait which represents it. We can rewrite the above example:
object HttpExample extends ResourceApp.Forever {
def run(args: List[String]) =
for {
config <- Resource.eval(loadConfig(args.head))
db <- Postgres[IO](config.jdbcUri)
endpoints <- ExampleEndpoints[IO](config, db)
_ <- HttpServer[IO](config.host, config.port, endpoints)
} yield ()
}
These two programs are equivalent.
Attributes
- See also:
- Companion:
- object
- Source:
- ResourceApp.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
- Known subtypes
- trait Simple
- Self type
Attributes
- Companion:
- trait
- Source:
- ResourceApp.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
- Self type
- ResourceApp.type
A pure abstraction representing the intention to perform a side effect, where the result of that side effect is obtained synchronously.
A pure abstraction representing the intention to perform a side effect, where the result of that side effect is obtained synchronously.
SyncIO
is similar to IO, but does not support asynchronous computations. Consequently,
a SyncIO
can be run synchronously on any platform to obtain a result via unsafeRunSync
.
This is unlike IO#unsafeRunSync
, which cannot be safely called in general -- doing so on
the JVM blocks the calling thread while the async part of the computation is run and doing so
on Scala.js is not supported.
Attributes
- Companion:
- object
- Source:
- SyncIO.scala
- Graph
- Supertypes
- trait Serializableclass Objecttrait Matchableclass Any
Attributes
- Companion:
- class
- Source:
- SyncIO.scala
- Graph
- Supertypes
- trait Sumtrait Mirrorclass Objecttrait Matchableclass Any
- Self type
- SyncIO.type
Attributes
- Source:
- Trace.scala
- Graph
- Supertypes
- class Objecttrait Matchableclass Any
Attributes
- Source:
- implicits.scala
- Graph
- Supertypes
- trait AllInstancestrait GenSpawnInstancestrait AllSyntaxtrait DispatcherSyntaxtrait AllSyntaxtrait SupervisorSyntaxtrait BackpressureSyntaxtrait AllSyntaxtrait ClockSyntaxtrait ResourceSyntaxtrait AsyncSyntaxtrait GenConcurrentSyntaxtrait GenTemporalSyntaxtrait GenSpawnSyntaxtrait MonadCancelSyntaxclass Objecttrait Matchableclass Any
- Self type
- implicits.type
Types
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Value members
Concrete fields
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala
Attributes
- Source:
- package.scala