- Companion:
- class
Type members
Inherited classlikes
Types
Newtype encoding for an IO
datatype that has a cats.Applicative
capable of doing
parallel processing in ap
and map2
, needed for implementing cats.Parallel
.
Newtype encoding for an IO
datatype that has a cats.Applicative
capable of doing
parallel processing in ap
and map2
, needed for implementing cats.Parallel
.
For converting back and forth you can use either the Parallel[IO]
instance or the methods
cats.effect.kernel.Par.ParallelF.apply
for wrapping any IO
value and
cats.effect.kernel.Par.ParallelF.value
for unwrapping it.
The encoding is based on the "newtypes" project by Alexander Konovalov, chosen because it's devoid of boxing issues and a good choice until opaque types will land in Scala.
Value members
Concrete methods
Suspends a synchronous side effect in IO
. Use IO.apply if your side effect is not
thread-blocking; otherwise you should use IO.blocking (uncancelable) or
IO.interruptible
(cancelable).
Suspends a synchronous side effect in IO
. Use IO.apply if your side effect is not
thread-blocking; otherwise you should use IO.blocking (uncancelable) or
IO.interruptible
(cancelable).
Alias for IO.delay.
Suspends an asynchronous side effect in IO
.
Suspends an asynchronous side effect in IO
.
The given function will be invoked during evaluation of the IO
to "schedule" the
asynchronous callback, where the callback of type Either[Throwable, A] => Unit
is the
parameter passed to that function. Only the ''first'' invocation of the callback will be
effective! All subsequent invocations will be silently dropped.
The process of registering the callback itself is suspended in IO
(the outer IO
of
IO[Option[IO[Unit]]]
).
The effect returns Option[IO[Unit]]
which is an optional finalizer to be run in the event
that the fiber running async(k)
is canceled.
For example, here is a simplified version of IO.fromCompletableFuture
:
def fromCompletableFuture[A](fut: IO[CompletableFuture[A]]): IO[A] = {
fut.flatMap { cf =>
IO.async { cb =>
IO {
//Invoke the callback with the result of the completable future
val stage = cf.handle[Unit] {
case (a, null) => cb(Right(a))
case (_, e) => cb(Left(e))
}
//Cancel the completable future if the fiber is canceled
Some(IO(stage.cancel(false)).void)
}
}
}
}
- See also:
async_ for a simplified variant without a finalizer
Suspends an asynchronous side effect in IO
.
Suspends an asynchronous side effect in IO
.
The given function will be invoked during evaluation of the IO
to "schedule" the
asynchronous callback, where the callback is the parameter passed to that function. Only
the ''first'' invocation of the callback will be effective! All subsequent invocations will
be silently dropped.
As a quick example, you can use this function to perform a parallel computation given an
ExecutorService
:
def fork[A](body: => A, exc: ExecutorService): IO[A] =
IO async_ { cb =>
exc.execute(new Runnable {
def run() =
try cb(Right(body)) catch { case NonFatal(t) => cb(Left(t)) }
})
}
The fork
function will do exactly what it sounds like: take a thunk and an
ExecutorService
and run that thunk on the thread pool. Or rather, it will produce an IO
which will do those things when run; it does not schedule the thunk until the resulting
IO
is run! Note that there is no thread blocking in this implementation; the resulting
IO
encapsulates the callback in a pure and monadic fashion without using threads.
This function can be thought of as a safer, lexically-constrained version of Promise
,
where IO
is like a safer, lazy version of Future
.
- See also:
Introduces a fairness boundary that yields control back to the scheduler of the runtime system. This allows the carrier thread to resume execution of another waiting fiber.
Introduces a fairness boundary that yields control back to the scheduler of the runtime system. This allows the carrier thread to resume execution of another waiting fiber.
This function is primarily useful when performing long-running computation that is outside of the monadic context. For example:
fa.map(data => expensiveWork(data))
In the above, we're assuming that expensiveWork
is a function which is entirely
compute-bound but very long-running. A good rule of thumb is to consider a function
"expensive" when its runtime is around three or more orders of magnitude higher than the
overhead of the map
function itself (which runs in around 5 nanoseconds on modern
hardware). Thus, any expensiveWork
function which requires around 10 microseconds or
longer to execute should be considered "long-running".
The danger is that these types of long-running actions outside of the monadic context can
result in degraded fairness properties. The solution is to add an explicit cede
both
before and after the expensive operation:
(fa <* IO.cede).map(data => expensiveWork(data)) <* IO.cede
Note that extremely long-running expensiveWork
functions can still cause fairness issues,
even when used with cede
. This problem is somewhat fundamental to the nature of
scheduling such computation on carrier threads. Whenever possible, it is best to break
apart any such functions into multiple pieces invoked independently (e.g. via chained map
calls) whenever the execution time exceeds five or six orders of magnitude beyond the
overhead of map
itself (around 1 millisecond on most hardware).
This operation is not ''required'' in most applications, particularly those which are
primarily I/O bound, as IO
itself will automatically introduce fairness boundaries
without requiring user input. These automatic boundaries are controlled by the
cats.effect.unsafe.IORuntimeConfig.autoYieldThreshold configuration parameter, which in
turn may be adjusted by overriding IOApp.runtimeConfig.
This is a low-level API which is meant for implementors, please use background
, start
,
async
, or Deferred
instead, depending on the use case
This is a low-level API which is meant for implementors, please use background
, start
,
async
, or Deferred
instead, depending on the use case
Suspends a synchronous side effect which produces an IO
in IO
.
Suspends a synchronous side effect which produces an IO
in IO
.
This is useful for trampolining (i.e. when the side effect is conceptually the allocation
of a stack frame). Any exceptions thrown by the side effect will be caught and sequenced
into the IO
.
Suspends a synchronous side effect in IO
. Use IO.delay if your side effect is not
thread-blocking; otherwise you should use IO.blocking (uncancelable) or
IO.interruptible
(cancelable).
Suspends a synchronous side effect in IO
. Use IO.delay if your side effect is not
thread-blocking; otherwise you should use IO.blocking (uncancelable) or
IO.interruptible
(cancelable).
Any exceptions thrown by the effect will be caught and sequenced into the IO
.
Lifts an Eval
into IO
.
Lifts an Eval
into IO
.
This function will preserve the evaluation semantics of any actions that are lifted into
the pure IO
. Eager Eval
instances will be converted into thunk-less IO
(i.e. eager
IO
), while lazy eval and memoized will be executed as such.
Lifts an Either[Throwable, A]
into the IO[A]
context, raising the throwable if it
exists.
Lifts an Either[Throwable, A]
into the IO[A]
context, raising the throwable if it
exists.
Constructs an IO
which evaluates the given Future
and produces the result (or failure).
Constructs an IO
which evaluates the given Future
and produces the result (or failure).
Because Future
eagerly evaluates, as well as because it memoizes, this function takes its
parameter as an IO
, which could be lazily evaluated. If this laziness is appropriately
threaded back to the definition site of the Future
, it ensures that the computation is
fully managed by IO
and thus referentially transparent.
Example:
// Lazy evaluation, equivalent with by-name params
IO.fromFuture(IO(f))
// Eager evaluation, for pure futures
IO.fromFuture(IO.pure(f))
Roughly speaking, the following identities hold:
IO.fromFuture(IO(f)).unsafeToFuture() === f // true-ish (except for memoization)
IO.fromFuture(IO(ioa.unsafeToFuture())) === ioa // true
- See also:
Lifts an Option[A]
into the IO[A]
context, raising the throwable if the option is
empty.
Lifts an Option[A]
into the IO[A]
context, raising the throwable if the option is
empty.
Lifts an Try[A]
into the IO[A]
context, raising the throwable if it exists.
Lifts an Try[A]
into the IO[A]
context, raising the throwable if it exists.
An IO that contains an empty Option.
An IO that contains an empty Option.
- See also:
some for the non-empty Option variant
Like Parallel.parReplicateA
, but limits the degree of parallelism.
Like Parallel.parReplicateA
, but limits the degree of parallelism.
Like Parallel.parSequence
, but limits the degree of parallelism.
Like Parallel.parSequence
, but limits the degree of parallelism.
Like Parallel.parTraverse
, but limits the degree of parallelism.
Like Parallel.parTraverse
, but limits the degree of parallelism.
Prints a value to the standard output using the implicit cats.Show
instance.
Prints a value to the standard output using the implicit cats.Show
instance.
- Value parameters:
- a
value to be printed to the standard output
- See also:
cats.effect.std.Console
for more standard input, output and error operations
Prints a value to the standard output followed by a new line using the implicit cats.Show
instance.
Prints a value to the standard output followed by a new line using the implicit cats.Show
instance.
- Value parameters:
- a
value to be printed to the standard output
- See also:
cats.effect.std.Console
for more standard input, output and error operations
Lifts a pure value into IO
.
Lifts a pure value into IO
.
This should ''only'' be used if the value in question has "already" been computed! In other
words, something like IO.pure(readLine)
is most definitely not the right thing to do!
However, IO.pure(42)
is correct and will be more efficient (when evaluated) than
IO(42)
, due to avoiding the allocation of extra thunks.
Run two IO tasks concurrently, and return the first to finish, either in success or error. The loser of the race is canceled.
Run two IO tasks concurrently, and return the first to finish, either in success or error. The loser of the race is canceled.
The two tasks are executed in parallel, the winner being the first that signals a result.
As an example see IO.timeout and IO.timeoutTo
Also see racePair for a version that does not cancel the loser automatically on successful results.
- Value parameters:
- left
is the "left" task participating in the race
- right
is the "right" task participating in the race
Run two IO tasks concurrently, and returns a pair containing both the winner's successful value and the loser represented as a still-unfinished task.
Run two IO tasks concurrently, and returns a pair containing both the winner's successful value and the loser represented as a still-unfinished task.
On usage the user has the option of canceling the losing task, this being equivalent with plain race:
val ioA: IO[A] = ???
val ioB: IO[B] = ???
IO.racePair(ioA, ioB).flatMap {
case Left((a, fiberB)) =>
fiberB.cancel.as(a)
case Right((fiberA, b)) =>
fiberA.cancel.as(b)
}
See race for a simpler version that cancels the loser immediately.
- Value parameters:
- left
is the "left" task participating in the race
- right
is the "right" task participating in the race
Constructs an IO
which sequences the specified exception.
Constructs an IO
which sequences the specified exception.
If this IO
is run using unsafeRunSync
or unsafeRunTimed
, the exception will be
thrown. This exception can be "caught" (or rather, materialized into value-space) using the
attempt
method.
- See also:
Returns raiseError
when cond
is false, otherwise IO.unit
Returns raiseError
when cond
is false, otherwise IO.unit
- Example:
val tooMany = 5 val x: Int = ??? IO.raiseUnless(x < tooMany)(new IllegalArgumentException("Too many"))
Returns raiseError
when the cond
is true, otherwise IO.unit
Returns raiseError
when the cond
is true, otherwise IO.unit
- Example:
val tooMany = 5 val x: Int = ??? IO.raiseWhen(x >= tooMany)(new IllegalArgumentException("Too many"))
- Returns:
a randomly-generated UUID This is equivalent to
UUIDGen[IO].randomUUID
, just provided as a method for convenience
Creates an asynchronous task that on evaluation sleeps for the specified duration, emitting a notification on completion.
Creates an asynchronous task that on evaluation sleeps for the specified duration, emitting a notification on completion.
This is the pure, non-blocking equivalent to:
Thread.sleep
(JVM)ScheduledExecutorService.schedule
(JVM)setTimeout
(JavaScript)
You can combine it with flatMap
to create delayed tasks:
val timeout = IO.sleep(10.seconds).flatMap { _ =>
IO.raiseError(new TimeoutException)
}
The created task is cancelable and so it can be used safely in race conditions without resource leakage.
- Value parameters:
- delay
the time span to wait before emitting the tick
- Returns:
a new asynchronous and cancelable
IO
that will sleep for the specified duration and then finally emit a tick
An IO that contains some Option of the given value.
An IO that contains some Option of the given value.
- See also:
none for the empty Option variant
Returns the given argument if cond
is false, otherwise IO.Unit
Returns the given argument if cond
is false, otherwise IO.Unit
- See also:
IO.whenA for the inverse
IO.raiseWhen for conditionally raising an error
Returns the given argument if cond
is true, otherwise IO.Unit
Returns the given argument if cond
is true, otherwise IO.Unit
- See also:
IO.unlessA for the inverse
IO.raiseWhen for conditionally raising an error
Inherited methods
Intended for thread blocking operations. blocking
will shift the execution of the
blocking operation to a separate threadpool to avoid blocking on the main execution
context. See the thread-model documentation for more information on why this is necessary.
Note that the created effect will be uncancelable; if you need cancelation then you should
use [[interruptibleA:*]] or interruptibleMany.
Intended for thread blocking operations. blocking
will shift the execution of the
blocking operation to a separate threadpool to avoid blocking on the main execution
context. See the thread-model documentation for more information on why this is necessary.
Note that the created effect will be uncancelable; if you need cancelation then you should
use [[interruptibleA:*]] or interruptibleMany.
IO.blocking(scala.io.Source.fromFile("path").mkString)
- Value parameters:
- thunk
The side effect which is to be suspended in
IO
and evaluated on a blocking execution context Implements cats.effect.kernel.Sync.blocking.
- Inherited from:
- IOCompanionPlatform
Like blocking but will attempt to abort the blocking operation using thread interrupts in the event of cancelation. The interrupt will be attempted only once.
Like blocking but will attempt to abort the blocking operation using thread interrupts in the event of cancelation. The interrupt will be attempted only once.
Note the following tradeoffs:
- this has slightly more overhead than blocking due to the machinery necessary for the interrupt coordination,
- thread interrupts are very often poorly considered by Java (and Scala!) library authors, and it is possible for interrupts to result in resource leaks or invalid states. It is important to be certain that this will not be the case before using this mechanism.
- Value parameters:
- thunk
The side effect which is to be suspended in
IO
and evaluated on a blocking execution context Implements [[cats.effect.kernel.Sync.interruptibleA:*]]
- Inherited from:
- IOCompanionPlatform
Like blocking but will attempt to abort the blocking operation using thread interrupts in the event of cancelation. The interrupt will be attempted repeatedly until the blocking operation completes or exits.
Like blocking but will attempt to abort the blocking operation using thread interrupts in the event of cancelation. The interrupt will be attempted repeatedly until the blocking operation completes or exits.
Note the following tradeoffs:
- this has slightly more overhead than blocking due to the machinery necessary for the interrupt coordination,
- thread interrupts are very often poorly considered by Java (and Scala!) library authors, and it is possible for interrupts to result in resource leaks or invalid states. It is important to be certain that this will not be the case before using this mechanism.
- Value parameters:
- thunk
The side effect which is to be suspended in
IO
and evaluated on a blocking execution context Implements cats.effect.kernel.Sync!.interruptibleMany
- Inherited from:
- IOCompanionPlatform
Reads a line as a string from the standard input using the platform's default charset, as
per java.nio.charset.Charset.defaultCharset()
.
Reads a line as a string from the standard input using the platform's default charset, as
per java.nio.charset.Charset.defaultCharset()
.
The effect can raise a java.io.EOFException
if no input has been consumed before the EOF
is observed. This should never happen with the standard input, unless it has been replaced
with a finite java.io.InputStream
through java.lang.System#setIn
or similar.
- Returns:
an IO effect that describes reading the user's input from the standard input as a string
- See also:
cats.effect.std.Console#readLineWithCharset
for reading using a customjava.nio.charset.Charset
- Inherited from:
- IOCompanionPlatform
Deprecated and Inherited methods
- Deprecated
- Inherited from:
- IOCompanionPlatform