Coeval
Related Docs:
object Coeval
| package eval
Evaluates the underlying computation and returns the result.
Evaluates the underlying computation and returns the result.
NOTE: this can throw exceptions.
// For didactic purposes, don't do shared vars at home :-) var i = 0 val fa = Coeval { i += 1; i } fa() //=> 1 fa() //=> 2 fa() //=> 3
UNSAFE — this operation can trigger the execution of side effects, which break referential transparency and is thus not a pure function.
In FP code use with care, suspended in another Coeval
or Task, or at the edge of the FP
program.
Creates a new Coeval that will expose any triggered error from the source.
Creates a new Coeval that will expose any triggered error from the source.
val fa: Coeval[Int] = Coeval.raiseError[Int](new DummyException("dummy")) val fe: Coeval[Either[Throwable, Int]] = fa.attempt fe.flatMap { case Left(_) => Int.MaxValue case Right(v) => v }
By exposing errors by lifting the Coeval's result into an
Either value, we can handle those errors in flatMap
transformations.
Also see materialize for working with Scala's Try or transformWith for an alternative.
Returns a task that treats the source as the acquisition of a resource,
which is then exploited by the use function and then released.
Returns a task that treats the source as the acquisition of a resource,
which is then exploited by the use function and then released.
The bracket operation is the equivalent of the
try {} finally {} statements from mainstream languages, installing
the necessary exception handler to release the resource in the event of
an exception being raised during the computation. If an exception is raised,
then bracket will re-raise the exception after performing the release.
Example:
import java.io._ def readFile(file: File): Coeval[String] = { // Opening a file handle for reading text val acquire = Coeval.eval(new BufferedReader( new InputStreamReader(new FileInputStream(file), "utf-8") )) acquire.bracket { in => // Usage part Coeval.eval { // Yes, ugly Java, non-FP loop; // side-effects are suspended though var line: String = null val buff = new StringBuilder() do { line = in.readLine() if (line != null) buff.append(line) } while (line != null) buff.toString() } } { in => // The release part Coeval.eval(in.close()) } }
NOTE on error handling: one big
difference versus try {} finally {} is that, in case
both the release function and the use function throws,
the error raised by use gets signaled and the error
raised by release gets reported with System.err for
Coeval or with
Scheduler.reportFailure
for Task.
For example:
Coeval("resource").bracket { _ => // use Coeval.raiseError(new RuntimeException("Foo")) } { _ => // release Coeval.raiseError(new RuntimeException("Bar")) }
In this case the error signaled downstream is "Foo",
while the "Bar" error gets reported. This is consistent
with the behavior of Haskell's bracket operation and NOT
with try {} finally {} from Scala, Java or JavaScript.
is a function that evaluates the resource yielded by the source,
yielding a result that will get generated by the task returned
by this bracket function
is a function that gets called after use terminates,
either normally or in error, receiving as input the resource that
needs to be released
Returns a task that treats the source task as the acquisition of a resource,
which is then exploited by the use function and then released, with
the possibility of distinguishing between successful termination and
error, such that an appropriate release of resources can be executed.
Returns a task that treats the source task as the acquisition of a resource,
which is then exploited by the use function and then released, with
the possibility of distinguishing between successful termination and
error, such that an appropriate release of resources can be executed.
The bracket operation is the equivalent of the
try {} finally {} statements from mainstream languages, installing
the necessary exception handler to release the resource in the event of
an exception being raised during the computation. If an exception is raised,
then bracket will re-raise the exception after performing the release.
The release function receives as input:
Left(error) in case use terminated with an errorRight(b) in case of successNOTE on error handling: one big
difference versus try {} finally {} is that, in case
both the release function and the use function throws,
the error raised by use gets signaled and the error
raised by release gets reported with System.err for
Coeval or with
Scheduler.reportFailure
for Task.
For example:
Coeval("resource").bracket { _ => // use Coeval.raiseError(new RuntimeException("Foo")) } { _ => // release Coeval.raiseError(new RuntimeException("Bar")) }
In this case the error signaled downstream is "Foo",
while the "Bar" error gets reported. This is consistent
with the behavior of Haskell's bracket operation and NOT
with try {} finally {} from Scala, Java or JavaScript.
is a function that evaluates the resource yielded by the source, yielding a result that will get generated by this function on evaluation
is a function that gets called after use terminates,
either normally or in error, receiving as input the resource that
needs that needs release, along with the result of use
Dematerializes the source's result from a Try.
Dematerializes the source's result from a Try.
This equivalence always holds:
scala
fa.materialize.dematerialize <-> fa
Returns a new Coeval in which f is scheduled to be run on completion.
Returns a new Coeval in which f is scheduled to be run on completion.
This would typically be used to release any resources acquired by this
Coeval.
Returns a failed projection of this coeval.
Returns a failed projection of this coeval.
The failed projection is a Coeval holding a value of type Throwable,
emitting the error yielded by the source, in case the source fails,
otherwise if the source succeeds the result will fail with a
NoSuchElementException.
Creates a new Coeval by applying a function to the successful result
of the source, and returns a new instance equivalent
to the result of the function.
Creates a new Coeval by applying a function to the successful result
of the source, and returns a new instance equivalent
to the result of the function.
The application of flatMap is always lazy and because of the
implementation it is memory safe and thus it can be used in
recursive loops.
Sample:
def randomEven: Coeval[Int] = Coeval(Random.nextInt()).flatMap { x => if (x < 0 || x % 2 == 1) randomEven // retry else Coeval.now(x) }
Given a source Coeval that emits another Coeval, this function flattens the result, returning a Coeval equivalent to the emitted Coeval by the source.
Given a source Coeval that emits another Coeval, this function flattens the result, returning a Coeval equivalent to the emitted Coeval by the source.
This equivalence with flatMap always holds:
scala
fa.flatten <-> fa.flatMap(x => x)
Triggers the evaluation of the source, executing the given function for the generated element.
Triggers the evaluation of the source, executing the given function for the generated element.
The application of this function has strict behavior, as the coeval is immediately executed.
Returns a new task that upon evaluation will execute
the given function for the generated element,
transforming the source into a Coeval[Unit].
Returns a new task that upon evaluation will execute
the given function for the generated element,
transforming the source into a Coeval[Unit].
Similar in spirit with normal foreach, but lazy, as obviously nothing gets executed at this point.
Returns a new Coeval that applies the mapping function to
the element emitted by the source.
Returns a new Coeval that applies the mapping function to
the element emitted by the source.
Can be used for specifying a (lazy) transformation to the result of the source.
This equivalence with flatMap always holds:
scala
fa.map(f) <-> fa.flatMap(x => Coeval.pure(f(x)))
Creates a new Coeval that will expose any triggered error from the source.
Creates a new Coeval that will expose any triggered error from the source.
Also see attempt for working with Scala's Either or transformWith for an alternative.
Memoizes (caches) the result of the source and reuses it on
subsequent invocations of value.
Memoizes (caches) the result of the source and reuses it on
subsequent invocations of value.
The resulting coeval will be idempotent, meaning that evaluating the resulting coeval multiple times will have the same effect as evaluating it once.
UNSAFE — this operation allocates a shared, mutable reference, which can break in certain cases referential transparency, even if this operation guarantees idempotency (i.e. referential transparency implies idempotency, but idempotency does not imply referential transparency).
The allocation of a mutable reference is known to be a side effect, thus breaking referential transparency, even if calling this method does not trigger the evaluation of side effects suspended by the source.
Use with care. Sometimes it's easier to just keep a shared, memoized reference to some connection, but keep in mind it might be better to pass such a reference around as a parameter.
memoizeOnSuccess for a version that only caches successful results
Memoizes (cache) the successful result of the source
and reuses it on subsequent invocations of value.
Memoizes (cache) the successful result of the source
and reuses it on subsequent invocations of value.
Thrown exceptions are not cached.
The resulting coeval will be idempotent, but only if the result is successful.
UNSAFE — this operation allocates a shared, mutable reference, which can break in certain cases referential transparency, even if this operation guarantees idempotency (i.e. referential transparency implies idempotency, but idempotency does not imply referential transparency).
The allocation of a mutable reference is known to be a side effect, thus breaking referential transparency, even if calling this method does not trigger the evaluation of side effects suspended by the source.
Use with care. Sometimes it's easier to just keep a shared, memoized reference to some connection, but keep in mind it might be better to pass such a reference around as a parameter.
memoize for a version that caches both successful results and failures
Creates a new coeval that in case of error will fallback to the given backup coeval.
Creates a new coeval that will handle any matching throwable that this coeval might emit.
Creates a new coeval that will handle any matching throwable that this coeval might emit.
See onErrorRecover for the version that takes a partial function.
Creates a new coeval that will handle any matching throwable that this coeval might emit by executing another coeval.
Creates a new coeval that will handle any matching throwable that this coeval might emit by executing another coeval.
See onErrorRecoverWith for the version that takes a partial function.
Creates a new coeval that on error will try to map the error to another value using the provided partial function.
Creates a new coeval that on error will try to map the error to another value using the provided partial function.
See onErrorHandle for the version that takes a total function.
Creates a new coeval that will try recovering from an error by matching it with another coeval using the given partial function.
Creates a new coeval that will try recovering from an error by matching it with another coeval using the given partial function.
See onErrorHandleWith for the version that takes a total function.
Creates a new coeval that in case of error will retry executing the source again and again, until it succeeds.
Creates a new coeval that in case of error will retry executing the source again and again, until it succeeds.
In case of continuous failure the total number of executions
will be maxRetries + 1.
Creates a new coeval that in case of error will retry executing the source again and again, until it succeeds.
Creates a new coeval that in case of error will retry executing the source again and again, until it succeeds.
In case of continuous failure the total number of executions
will be maxRetries + 1.
On error restarts the source with a customizable restart loop.
On error restarts the source with a customizable restart loop.
This operation keeps an internal state, with a start value, an internal
state that gets evolved and based on which the next step gets decided,
e.g. should it restart, or should it give up and rethrow the current error.
Example that implements a simple retry policy that retries for a maximum of 10 times before giving up:
import scala.concurrent.duration._ task.onErrorRestartLoop(10) { (err, maxRetries, retry) => if (maxRetries > 0) // Do next retry please retry(maxRetries - 1) else // No retries left, rethrow the error Task.raiseError(err) }
The given function injects the following parameters:
error reference that was thrown
2. the current state, based on which a decision for the retry is made
3. retry: S => Task[B] function that schedules the next retry
is the initial state used to determine the next on error retry cycle
is a function that injects the current error, state, a function that can signal a retry is to be made and returns the next coeval
Given a predicate function, keep retrying the coeval until the function returns true.
Evaluates the underlying computation, reducing this Coeval
to a Coeval.Eager value, with successful results being
signaled with Coeval.Now and failures with Coeval.Error.
Evaluates the underlying computation, reducing this Coeval
to a Coeval.Eager value, with successful results being
signaled with Coeval.Now and failures with Coeval.Error.
val fa = Coeval.eval(10 * 2) fa.run match { case Coeval.Now(value) => println("Success: " + value) case Coeval.Error(e) => e.printStackTrace() }
See runAttempt for working with Either values and runTry for working with Try values. See apply for a partial function (that may throw exceptions in case of failure).
UNSAFE — this operation can trigger the execution of side effects, which break referential transparency and is thus not a pure function.
In FP code use with care, suspended in another Coeval
or Task, or at the edge of the FP
program.
Evaluates the underlying computation and returns the result or
any triggered errors as a Scala Either, where Right(_) is
for successful values and Left(_) is for thrown errors.
Evaluates the underlying computation and returns the result or
any triggered errors as a Scala Either, where Right(_) is
for successful values and Left(_) is for thrown errors.
val fa = Coeval(10 * 2) fa.runAttempt match { case Right(value) => println("Success: " + value) case Left(e) => e.printStackTrace() }
See run for working with Coeval.Eager values and runTry for working with Try values. See apply for a partial function (that may throw exceptions in case of failure).
UNSAFE — this operation can trigger the execution of side effects, which break referential transparency and is thus not a pure function.
In FP code use with care, suspended in another Coeval
or Task, or at the edge of the FP
program.
Evaluates the underlying computation and returns the
result or any triggered errors as a scala.util.Try.
Evaluates the underlying computation and returns the
result or any triggered errors as a scala.util.Try.
val fa = Coeval(10 * 2) fa.runTry match { case Success(value) => println("Success: " + value) case Failure(e) => e.printStackTrace() }
See run for working with Coeval.Eager values and runAttempt for working with Either values. See apply for a partial function (that may throw exceptions in case of failure).
UNSAFE — this operation can trigger the execution of side effects, which break referential transparency and is thus not a pure function.
In FP code use with care, suspended in another Coeval
or Task, or at the edge of the FP
program.
Converts the source Coeval into a cats.Eval.
Converts the source Coeval into a cats.effect.IO.
Creates a new Coeval by applying the 'fa' function to the
successful result of this future, or the 'fe' function to the
potential errors that might happen.
Creates a new Coeval by applying the 'fa' function to the
successful result of this future, or the 'fe' function to the
potential errors that might happen.
This function is similar with map, except that it can also transform errors and not just successful results.
For example attempt can be expressed in terms of transform:
source.transform(v => Right(v), e => Left(e))
And materialize too:
source.transform(v => Success(v), e => Failure(e))
function that transforms a successful result of the receiver
function that transforms an error of the receiver
Creates a new Coeval by applying the 'fa' function to the
successful result of this future, or the 'fe' function to the
potential errors that might happen.
Creates a new Coeval by applying the 'fa' function to the
successful result of this future, or the 'fe' function to the
potential errors that might happen.
This function is similar with flatMap, except that it can also transform errors and not just successful results.
For example attempt can be expressed in terms of
transformWith:
source.transformWith( v => Coeval.now(Right(v)), e => Coeval.now(Left(e)) )
function that transforms a successful result of the receiver
function that transforms an error of the receiver
Evaluates the underlying computation and returns the result.
Evaluates the underlying computation and returns the result.
NOTE: this can throw exceptions.
Alias for apply.
UNSAFE — this operation can trigger the execution of side effects, which break referential transparency and is thus not a pure function.
In FP code use with care, suspended in another Coeval
or Task, or at the edge of the FP
program.
Zips the values of this and that coeval, and creates a new coeval
that will emit the tuple of their results.
Zips the values of this and that and applies the given
mapping function on their results.
Coevalrepresents lazy computations that can execute synchronously.Word definition and origin:
Coevalis the dual of an expression that evaluates to anA.There are three evaluation strategies:
The
OnceandAlwaysare both lazy strategies whileNowandErrorare eager.OnceandAlwaysare distinguished from each other only by memoization: once evaluatedOncewill save the value to be returned immediately if it is needed again.Alwayswill run its computation every time.Both
NowandErrorare represented by the Eager trait, a sub-type of Coeval that can be used as a replacement for Scala's ownTrytype.Coevalsupports stack-safe lazy computation via the .map and .flatMap methods, which use an internal trampoline to avoid stack overflows. Computations done within.mapand.flatMapare always lazy, even when applied to a Coeval.Eager instance (e.g. Coeval.Now, Coeval.Error).Evaluation Strategies
The "now" and "raiseError" builders are building
Coevalinstances out of strict values:The "always" strategy is equivalent with a plain function:
The "once" strategy is equivalent with Scala's
lazy val(along with thread-safe idempotency guarantees):Versus Task
The other option of suspending side-effects is Task. As a quick comparison:
Coeval's execution is always immediate / synchronous, whereasTaskcan describe asynchronous computationsCoevalis not cancelable, obviously, since execution is immediate and there's nothing to cancelVersus cats.Eval
The
Coevaldata type is very similar with cats.Eval. As a quick comparison:cats.Evalis only for controlling laziness, but it doesn't handle side effects, hencecats.Evalis aComonadCoevalcan handle side effects as well and thus it implementsMonadError[Coeval, Throwable]andcats.effect.Sync, providing error-handling utilitiesIf you just want to delay the evaluation of a pure expression use
cats.Eval, but if you need to suspend side effects or you need error handling capabilities, then useCoeval.