Packages

  • package root

    This is the documentation for the Scala standard library.

    This is the documentation for the Scala standard library.

    Package structure

    The scala package contains core types like Int, Float, Array or Option which are accessible in all Scala compilation units without explicit qualification or imports.

    Notable packages include:

    Other packages exist. See the complete list on the right.

    Additional parts of the standard library are shipped as separate libraries. These include:

    Automatic imports

    Identifiers in the scala package and the scala.Predef object are always in scope by default.

    Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example, List is an alias for scala.collection.immutable.List.

    Other aliases refer to classes provided by the underlying platform. For example, on the JVM, String is an alias for java.lang.String.

    Definition Classes
    root
  • package scala

    Core Scala types.

    Core Scala types. They are always available without an explicit import.

    Definition Classes
    root
  • package concurrent

    This package object contains primitives for concurrent and parallel programming.

    This package object contains primitives for concurrent and parallel programming.

    Guide

    A more detailed guide to Futures and Promises, including discussion and examples can be found at https://docs.scala-lang.org/overviews/core/futures.html.

    Common Imports

    When working with Futures, you will often find that importing the whole concurrent package is convenient:

    import scala.concurrent._

    When using things like Futures, it is often required to have an implicit ExecutionContext in scope. The general advice for these implicits are as follows.

    If the code in question is a class or method definition, and no ExecutionContext is available, request one from the caller by adding an implicit parameter list:

    def myMethod(myParam: MyType)(implicit ec: ExecutionContext) = …
    //Or
    class MyClass(myParam: MyType)(implicit ec: ExecutionContext) { … }

    This allows the caller of the method, or creator of the instance of the class, to decide which ExecutionContext should be used.

    For typical REPL usage and experimentation, importing the global ExecutionContext is often desired.

    import scala.concurrent.ExcutionContext.Implicits.global

    Specifying Durations

    Operations often require a duration to be specified. A duration DSL is available to make defining these easier:

    import scala.concurrent.duration._
    val d: Duration = 10.seconds

    Using Futures For Non-blocking Computation

    Basic use of futures is easy with the factory method on Future, which executes a provided function asynchronously, handing you back a future result of that function without blocking the current thread. In order to create the Future you will need either an implicit or explicit ExecutionContext to be provided:

    import scala.concurrent._
    import ExecutionContext.Implicits.global  // implicit execution context
    
    val firstZebra: Future[Int] = Future {
      val words = Files.readAllLines("/etc/dictionaries-common/words").asScala
      words.indexOfSlice("zebra")
    }

    Avoid Blocking

    Although blocking is possible in order to await results (with a mandatory timeout duration):

    import scala.concurrent.duration._
    Await.result(firstZebra, 10.seconds)

    and although this is sometimes necessary to do, in particular for testing purposes, blocking in general is discouraged when working with Futures and concurrency in order to avoid potential deadlocks and improve performance. Instead, use callbacks or combinators to remain in the future domain:

    val animalRange: Future[Int] = for {
      aardvark <- firstAardvark
      zebra <- firstZebra
    } yield zebra - aardvark
    
    animalRange.onSuccess {
      case x if x > 500000 => println("It's a long way from Aardvark to Zebra")
    }
    Definition Classes
    scala
  • package duration
    Definition Classes
    concurrent
  • Await
  • Awaitable
  • Batchable
  • BlockContext
  • CanAwait
  • Channel
  • DelayedLazyVal
  • ExecutionContext
  • ExecutionContextExecutor
  • ExecutionContextExecutorService
  • Future
  • JavaConversions
  • OnCompleteRunnable
  • Promise
  • SyncChannel
  • SyncVar

object Future

Future companion object.

Source
Future.scala
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Value Members

  1. final def apply[T](body: => T)(implicit executor: ExecutionContext): Future[T]

    Starts an asynchronous computation and returns a Future instance with the result of that computation.

    Starts an asynchronous computation and returns a Future instance with the result of that computation.

    The following expressions are equivalent:

    val f1 = Future(expr)
    val f2 = Future.unit.map(_ => expr)
    val f3 = Future.unit.transform(_ => Success(expr))

    The result becomes available once the asynchronous computation is completed.

    T

    the type of the result

    body

    the asynchronous computation

    executor

    the execution context on which the future is run

    returns

    the Future holding the result of the computation

  2. final def delegate[T](body: => Future[T])(implicit executor: ExecutionContext): Future[T]

    Starts an asynchronous computation and returns a Future instance with the result of that computation once it completes.

    Starts an asynchronous computation and returns a Future instance with the result of that computation once it completes.

    The following expressions are semantically equivalent:

    val f1 = Future(expr).flatten
    val f2 = Future.delegate(expr)
    val f3 = Future.unit.flatMap(_ => expr)

    The result becomes available once the resulting Future of the asynchronous computation is completed.

    T

    the type of the result

    body

    the asynchronous computation, returning a Future

    executor

    the execution context on which the body is evaluated in

    returns

    the Future holding the result of the computation

  3. final def failed[T](exception: Throwable): Future[T]

    Creates an already completed Future with the specified exception.

    Creates an already completed Future with the specified exception.

    T

    the type of the value in the future

    exception

    the non-null instance of Throwable

    returns

    the newly created Future instance

  4. final def find[T](futures: collection.immutable.Iterable[Future[T]])(p: (T) => Boolean)(implicit executor: ExecutionContext): Future[Option[T]]

    Asynchronously and non-blockingly returns a Future that will hold the optional result of the first Future with a result that matches the predicate, failed Futures will be ignored.

    Asynchronously and non-blockingly returns a Future that will hold the optional result of the first Future with a result that matches the predicate, failed Futures will be ignored.

    T

    the type of the value in the future

    futures

    the scala.collection.immutable.Iterable of Futures to search

    p

    the predicate which indicates if it's a match

    returns

    the Future holding the optional result of the search

  5. final def firstCompletedOf[T](futures: IterableOnce[Future[T]])(implicit executor: ExecutionContext): Future[T]

    Asynchronously and non-blockingly returns a new Future to the result of the first future in the list that is completed.

    Asynchronously and non-blockingly returns a new Future to the result of the first future in the list that is completed. This means no matter if it is completed as a success or as a failure.

    T

    the type of the value in the future

    futures

    the IterableOnce of Futures in which to find the first completed

    returns

    the Future holding the result of the future that is first to be completed

  6. final def foldLeft[T, R](futures: collection.immutable.Iterable[Future[T]])(zero: R)(op: (R, T) => R)(implicit executor: ExecutionContext): Future[R]

    A non-blocking, asynchronous left fold over the specified futures, with the start value of the given zero.

    A non-blocking, asynchronous left fold over the specified futures, with the start value of the given zero. The fold is performed asynchronously in left-to-right order as the futures become completed. The result will be the first failure of any of the futures, or any failure in the actual fold, or the result of the fold.

    Example:

    val futureSum = Future.foldLeft(futures)(0)(_ + _)
    T

    the type of the value of the input Futures

    R

    the type of the value of the returned Future

    futures

    the scala.collection.immutable.Iterable of Futures to be folded

    zero

    the start value of the fold

    op

    the fold operation to be applied to the zero and futures

    returns

    the Future holding the result of the fold

  7. final def fromTry[T](result: Try[T]): Future[T]

    Creates an already completed Future with the specified result or exception.

    Creates an already completed Future with the specified result or exception.

    T

    the type of the value in the Future

    result

    the result of the returned Future instance

    returns

    the newly created Future instance

  8. final def reduceLeft[T, R >: T](futures: collection.immutable.Iterable[Future[T]])(op: (R, T) => R)(implicit executor: ExecutionContext): Future[R]

    Initiates a non-blocking, asynchronous, left reduction over the supplied futures where the zero is the result value of the first Future.

    Initiates a non-blocking, asynchronous, left reduction over the supplied futures where the zero is the result value of the first Future.

    Example:

    val futureSum = Future.reduceLeft(futures)(_ + _)
    T

    the type of the value of the input Futures

    R

    the type of the value of the returned Future

    futures

    the scala.collection.immutable.Iterable of Futures to be reduced

    op

    the reduce operation which is applied to the results of the futures

    returns

    the Future holding the result of the reduce

  9. final def sequence[A, CC[X] <: IterableOnce[X], To](in: CC[Future[A]])(implicit bf: BuildFrom[CC[Future[A]], A, To], executor: ExecutionContext): Future[To]

    Simple version of Future.traverse.

    Simple version of Future.traverse. Asynchronously and non-blockingly transforms, in essence, a IterableOnce[Future[A]] into a Future[IterableOnce[A]]. Useful for reducing many Futures into a single Future.

    A

    the type of the value inside the Futures

    CC

    the type of the IterableOnce of Futures

    To

    the type of the resulting collection

    in

    the IterableOnce of Futures which will be sequenced

    returns

    the Future of the resulting collection

  10. final def successful[T](result: T): Future[T]

    Creates an already completed Future with the specified result.

    Creates an already completed Future with the specified result.

    T

    the type of the value in the future

    result

    the given successful value

    returns

    the newly created Future instance

  11. final def traverse[A, B, M[X] <: IterableOnce[X]](in: M[A])(fn: (A) => Future[B])(implicit bf: BuildFrom[M[A], B, M[B]], executor: ExecutionContext): Future[M[B]]

    Asynchronously and non-blockingly transforms a IterableOnce[A] into a Future[IterableOnce[B]] using the provided function A => Future[B].

    Asynchronously and non-blockingly transforms a IterableOnce[A] into a Future[IterableOnce[B]] using the provided function A => Future[B]. This is useful for performing a parallel map. For example, to apply a function to all items of a list in parallel:

    val myFutureList = Future.traverse(myList)(x => Future(myFunc(x)))
    A

    the type of the value inside the Futures in the collection

    B

    the type of the value of the returned Future

    M

    the type of the collection of Futures

    in

    the collection to be mapped over with the provided function to produce a collection of Futures that is then sequenced into a Future collection

    fn

    the function to be mapped over the collection to produce a collection of Futures

    returns

    the Future of the collection of results

  12. final val unit: Future[Unit]

    A Future which is completed with the Unit value.

  13. object never extends Future[Nothing]

    A Future which is never completed.

Deprecated Value Members

  1. def fold[T, R](futures: IterableOnce[Future[T]])(zero: R)(op: (R, T) => R)(implicit executor: ExecutionContext): Future[R]

    A non-blocking, asynchronous fold over the specified futures, with the start value of the given zero.

    A non-blocking, asynchronous fold over the specified futures, with the start value of the given zero. The fold is performed on the thread where the last future is completed, the result will be the first failure of any of the futures, or any failure in the actual fold, or the result of the fold.

    Example:

    val futureSum = Future.fold(futures)(0)(_ + _)
    T

    the type of the value of the input Futures

    R

    the type of the value of the returned Future

    futures

    the IterableOnce of Futures to be folded

    zero

    the start value of the fold

    op

    the fold operation to be applied to the zero and futures

    returns

    the Future holding the result of the fold

    Annotations
    @deprecated
    Deprecated

    (Since version 2.12.0) use Future.foldLeft instead

  2. final def reduce[T, R >: T](futures: IterableOnce[Future[T]])(op: (R, T) => R)(implicit executor: ExecutionContext): Future[R]

    Initiates a non-blocking, asynchronous, fold over the supplied futures where the fold-zero is the result value of the first Future in the collection.

    Initiates a non-blocking, asynchronous, fold over the supplied futures where the fold-zero is the result value of the first Future in the collection.

    Example:

    val futureSum = Future.reduce(futures)(_ + _)
    T

    the type of the value of the input Futures

    R

    the type of the value of the returned Future

    futures

    the IterableOnce of Futures to be reduced

    op

    the reduce operation which is applied to the results of the futures

    returns

    the Future holding the result of the reduce

    Annotations
    @deprecated
    Deprecated

    (Since version 2.12.0) use Future.reduceLeft instead