Eager

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.

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.

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.

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 error
• Right(b) in case of success

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.

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. This would typically be used to release any resources acquired by 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.

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.

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.

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].

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

In case of continuous failure the total number of executions will be maxRetries + 1.

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:

1. 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

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.

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.

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.

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 Task.

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.

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))

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))
)

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.