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 annotation
    Definition Classes
    scala
  • package meta

    When defining a field, the Scala compiler creates up to four accessors for it: a getter, a setter, and if the field is annotated with @BeanProperty, a bean getter and a bean setter.

    When defining a field, the Scala compiler creates up to four accessors for it: a getter, a setter, and if the field is annotated with @BeanProperty, a bean getter and a bean setter.

    For instance in the following class definition

    class C(@myAnnot @BeanProperty var c: Int)

    there are six entities which can carry the annotation @myAnnot: the constructor parameter, the generated field and the four accessors.

    By default, annotations on (val-, var- or plain) constructor parameters end up on the parameter, not on any other entity. Annotations on fields by default only end up on the field.

    The meta-annotations in package scala.annotation.meta are used to control where annotations on fields and class parameters are copied. This is done by annotating either the annotation type or the annotation class with one or several of the meta-annotations in this package.

    Annotating the annotation type

    The target meta-annotations can be put on the annotation type when instantiating the annotation. In the following example, the annotation @Id will be added only to the bean getter getX.

    import javax.persistence.Id
class A {
  @(Id @beanGetter) @BeanProperty val x = 0
}

    In order to annotate the field as well, the meta-annotation @field would need to be added.

    The syntax can be improved using a type alias:

    object ScalaJPA {
  type Id = javax.persistence.Id @beanGetter
}
import ScalaJPA.Id
class A {
  @Id @BeanProperty val x = 0
}

    Annotating the annotation class

    For annotations defined in Scala, a default target can be specified in the annotation class itself, for example

    @getter
class myAnnotation extends Annotation

    This only changes the default target for the annotation myAnnotation. When instantiating the annotation, the target can still be specified as described in the last section.

  • package unchecked
  • Annotation
  • ClassfileAnnotation
  • ConstantAnnotation
  • StaticAnnotation
  • TypeConstraint
  • compileTimeOnly
  • elidable
  • implicitAmbiguous
  • implicitNotFound
  • nowarn
  • showAsInfix
  • strictfp
  • switch
  • tailrec
  • unspecialized
  • unused
  • varargs
  • package beans
    Definition Classes
    scala
  • package collection
    Definition Classes
    scala
  • package compat
    Definition Classes
    scala
  • 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 http://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 io
    Definition Classes
    scala
  • package jdk
    Definition Classes
    scala
  • package math

    The package object scala.math contains methods for performing basic numeric operations such as elementary exponential, logarithmic, root and trigonometric functions.

    The package object scala.math contains methods for performing basic numeric operations such as elementary exponential, logarithmic, root and trigonometric functions.

    All methods forward to java.lang.Math unless otherwise noted.

    Definition Classes
    scala
    See also

    java.lang.Math

  • package ref
    Definition Classes
    scala
  • package reflect
    Definition Classes
    scala
  • package sys

    The package object scala.sys contains methods for reading and altering core aspects of the virtual machine as well as the world outside of it.

    The package object scala.sys contains methods for reading and altering core aspects of the virtual machine as well as the world outside of it.

    Definition Classes
    scala
  • package util
    Definition Classes
    scala
p

scala

annotation

package annotation

Content Hierarchy
Ordering
  1. Alphabetic
Visibility
  1. Public
  2. Protected

Package Members

  1. package meta

    When defining a field, the Scala compiler creates up to four accessors for it: a getter, a setter, and if the field is annotated with @BeanProperty, a bean getter and a bean setter.

    When defining a field, the Scala compiler creates up to four accessors for it: a getter, a setter, and if the field is annotated with @BeanProperty, a bean getter and a bean setter.

    For instance in the following class definition

    class C(@myAnnot @BeanProperty var c: Int)

    there are six entities which can carry the annotation @myAnnot: the constructor parameter, the generated field and the four accessors.

    By default, annotations on (val-, var- or plain) constructor parameters end up on the parameter, not on any other entity. Annotations on fields by default only end up on the field.

    The meta-annotations in package scala.annotation.meta are used to control where annotations on fields and class parameters are copied. This is done by annotating either the annotation type or the annotation class with one or several of the meta-annotations in this package.

    Annotating the annotation type

    The target meta-annotations can be put on the annotation type when instantiating the annotation. In the following example, the annotation @Id will be added only to the bean getter getX.

    import javax.persistence.Id
class A {
  @(Id @beanGetter) @BeanProperty val x = 0
}

    In order to annotate the field as well, the meta-annotation @field would need to be added.

    The syntax can be improved using a type alias:

    object ScalaJPA {
  type Id = javax.persistence.Id @beanGetter
}
import ScalaJPA.Id
class A {
  @Id @BeanProperty val x = 0
}

    Annotating the annotation class

    For annotations defined in Scala, a default target can be specified in the annotation class itself, for example

    @getter
class myAnnotation extends Annotation

    This only changes the default target for the annotation myAnnotation. When instantiating the annotation, the target can still be specified as described in the last section.

  2. package unchecked

Type Members

  1. abstract class Annotation extends AnyRef

    A base class for annotations.

    A base class for annotations.

    Annotations extending this class directly are not preserved in the classfile. To enable storing annotations in the classfile's Scala signature and make it available to Scala reflection and other tools, the annotation needs to inherit from scala.annotation.StaticAnnotation.

    Annotation classes defined in Scala are not stored in classfiles in a Java-compatible manner and therefore not visible in Java reflection. In order to achieve this, the annotation has to be written in Java.

  2. trait ConstantAnnotation extends Annotation with StaticAnnotation

    Annotation classes extending this trait only accept constant values as arguments.

    Annotation classes extending this trait only accept constant values as arguments.

    Note that this trait extends StaticAnnotation, so constant annotations are persisted in the classfile.

    The implementation requires arguments of constant annotations to be passed as named arguments, except if there is a single argument, which then defines the annotation's parameter named value.

    Constant annotations may use default arguments. Note that the internal representation of an annotation usage (which is visible for compiler plugins, for example) only contains arguments that are explicitly provided.

    Constant annotations are not allowed to define auxiliary constructors, and the primary constructor is required to have a single parameter list.

    Example:

    class Ann(value: Int, x: Int = 0) extends scala.annotation.ConstantAnnotation
class Test {
  def someInt = 0
  @Ann(value = 0, x = 1) def g = 0
  @Ann(0) def f = 0                 // Internal representation contains `@Ann(value = 0)`
  @Ann(someInt)                     // error: argument needs to be a compile-time constant
}
  3. trait StaticAnnotation extends Annotation

    A base class for static annotations.

    A base class for static annotations. These are available to the Scala type checker or Scala reflection, even across different compilation units.

    Annotation classes defined in Scala are not stored in classfiles in a Java-compatible manner and therefore not visible in Java reflection. In order to achieve this, the annotation has to be written in Java.

  4. trait TypeConstraint extends Annotation

    A marker for annotations that, when applied to a type, should be treated as a constraint on the annotated type.

    A marker for annotations that, when applied to a type, should be treated as a constraint on the annotated type.

    A proper constraint should restrict the type based only on information mentioned within the type. A Scala compiler can use this assumption to rewrite the contents of the constraint as necessary. To contrast, a type annotation whose meaning depends on the context where it is written down is not a proper constrained type, and this marker should not be applied. A Scala compiler will drop such annotations in cases where it would rewrite a type constraint.

  5. final class compileTimeOnly extends Annotation with StaticAnnotation

    An annotation that designates that an annottee should not be referred to after type checking (which includes macro expansion).

    An annotation that designates that an annottee should not be referred to after type checking (which includes macro expansion).

    Examples of potential use: 1) The annottee can only appear in the arguments of some other macro that will eliminate it from the AST during expansion. 2) The annottee is a macro and should have been expanded away, so if hasn't, something wrong has happened. (Comes in handy to provide better support for new macro flavors, e.g. macro annotations, that can't be expanded by the vanilla compiler).

    Annotations
    @getter() @setter() @beanGetter() @beanSetter() @companionClass() @companionMethod()
  6. final class elidable extends Annotation with StaticAnnotation

    An annotation for methods whose bodies may be excluded from compiler-generated bytecode.

    An annotation for methods whose bodies may be excluded from compiler-generated bytecode.

    Behavior is influenced by passing -Xelide-below <arg> to scalac. Calls to methods marked elidable (as well as the method body) will be omitted from generated code if the priority given the annotation is lower than that given on the command line.

    @elidable(123)           // annotation priority
scalac -Xelide-below 456 // command line priority

    The method call will be replaced with an expression which depends on the type of the elided expression. In decreasing order of precedence:

    Unit            ()
Boolean         false
T <: AnyVal     0
T >: Null       null
T >: Nothing    Predef.???

    Complete example:

    import scala.annotation._, elidable._
object Test extends App {
  def expensiveComputation(): Int = { Thread.sleep(1000) ; 172 }

  @elidable(WARNING) def warning(msg: String) = println(msg)
  @elidable(FINE) def debug(msg: String)      = println(msg)
  @elidable(FINE) def computedValue           = expensiveComputation()

  warning("Warning! Danger! Warning!")
  debug("Debug! Danger! Debug!")
  println("I computed a value: " + computedValue)
}
% scalac example.scala && scala Test
Warning! Danger! Warning!
Debug! Danger! Debug!
I computed a value: 172

// INFO lies between WARNING and FINE
% scalac -Xelide-below INFO example.scala && scala Test
Warning! Danger! Warning!
I computed a value: 0

    Note that only concrete methods can be marked @elidable. A non-annotated method is not elided, even if it overrides / implements a method that has the annotation.

    Also note that the static type determines which annotations are considered:

    import scala.annotation._, elidable._
class C { @elidable(0) def f(): Unit = ??? }
object O extends C { override def f(): Unit = println("O.f") }
object Test extends App {
  O.f()      // not elided
  (O: C).f() // elided if compiled with `-Xelide-below 1`
}
  7. final class implicitAmbiguous extends Annotation with StaticAnnotation

    To customize the error message that's emitted when an implicit search finds multiple ambiguous values, annotate at least one of the implicit values @implicitAmbiguous.

    To customize the error message that's emitted when an implicit search finds multiple ambiguous values, annotate at least one of the implicit values @implicitAmbiguous. Assuming the implicit value is a method with type parameters X1,..., XN, the error message will be the result of replacing all occurrences of ${Xi} in the string msg with the string representation of the corresponding type argument Ti.

    If more than one @implicitAmbiguous annotation is collected, the compiler is free to pick any of them to display.

    Nice errors can direct users to fix imports or even tell them why code intentionally doesn't compile.

    trait =!=[C, D]

implicit def neq[E, F] : E =!= F = null

@annotation.implicitAmbiguous("Could not prove ${J} =!= ${J}")
implicit def neqAmbig1[G, H, J] : J =!= J = null
implicit def neqAmbig2[I] : I =!= I = null

implicitly[Int =!= Int]
    Annotations
    @getter()
  8. final class implicitNotFound extends Annotation with StaticAnnotation

    To customize the error message that's emitted when an implicit of type C[T1,..., TN] cannot be found, annotate the class C with @implicitNotFound.

    To customize the error message that's emitted when an implicit of type C[T1,..., TN] cannot be found, annotate the class C with @implicitNotFound. Assuming C has type parameters X1, ..., XN, the error message will be the result of replacing all occurrences of ${Xi} in the string msg with the string representation of the corresponding type argument Ti. The annotation is effectively inherited by subtypes if they are not annotated.

    The annotation can also be attached to implicit parameters. In this case, ${Xi} can refer to type parameters in the current scope. The @implicitNotFound message on the parameter takes precedence over the one on the parameter's type.

    import scala.annotation.implicitNotFound

@implicitNotFound("Could not find an implicit C[${T}, ${U}]")
class C[T, U]

class K[A] {
  def m[B](implicit c: C[List[A], B]) = 0
  def n[B](implicit @implicitNotFound("Specific message for C of list of ${A} and ${B}") c: C[List[A], B]) = 1
}

object Test {
  val k = new K[Int]
  k.m[String]
  k.n[String]
}

    The compiler issues the following error messages:

    Test.scala:13: error: Could not find an implicit C[List[Int], String]
  k.m[String]
     ^
Test.scala:14: error: Specific message for C of list of Int and String
  k.n[String]
     ^

  9. class nowarn extends Annotation with ConstantAnnotation

    An annotation for local warning suppression.

    An annotation for local warning suppression.

    The optional value parameter allows selectively silencing messages, see scalac -Wconf:help for help. Examples:

    def f = {
  1: @nowarn // don't warn "a pure expression does nothing in statement position"
  2
}

@nowarn def f = { 1; deprecated() } // don't warn

@nowarn("msg=pure expression does nothing")
def f = { 1; deprecated() } // show deprecation warning

    To ensure that a @nowarn annotation actually suppresses a warning, enable -Xlint:unused or -Wunused:nowarn.

  10. class showAsInfix extends Annotation with StaticAnnotation

    This annotation configures how Scala prints two-parameter generic types.

    This annotation configures how Scala prints two-parameter generic types.

    By default, types with symbolic names are printed infix; while types without them are printed using the regular generic type syntax.

    Example of usage:

    scala> class Map[T, U]
defined class Map

scala> def foo: Int Map Int = ???
foo: Map[Int,Int]

scala> @showAsInfix class Map[T, U]
defined class Map

scala> def foo: Int Map Int = ???
foo: Int Map Int
    Annotations
    @deprecatedInheritance(message = "Scheduled for being final in the future", since = "2.13.0")
  11. class strictfp extends Annotation with StaticAnnotation

    If this annotation is present on a method or its enclosing class, the strictfp flag will be emitted.

    If this annotation is present on a method or its enclosing class, the strictfp flag will be emitted.

    Annotations
    @deprecatedInheritance(message = "Scheduled for being final in the future", since = "2.13.0")
  12. final class switch extends Annotation with StaticAnnotation

    An annotation to be applied to a match expression.

    An annotation to be applied to a match expression. If present, the compiler will verify that the match has been compiled to a tableswitch or lookupswitch and issue a warning if it instead compiles into a series of conditional expressions. Example usage:

    val Constant = 'Q'
def tokenMe(ch: Char) = (ch: @switch) match {
  case ' ' | '\t' | '\n'  => 1
  case 'A' | 'Z' | '$'    => 2
  case '5' | Constant     => 3  // a non-literal may prevent switch generation: this would not compile
  case _                  => 4
}

    Note: for pattern matches with one or two cases, the compiler generates jump instructions. Annotating such a match with @switch does not issue any warning.

  13. final class tailrec extends Annotation with StaticAnnotation

    A method annotation which verifies that the method will be compiled with tail call optimization.

    A method annotation which verifies that the method will be compiled with tail call optimization.

    If it is present, the compiler will issue an error if the method cannot be optimized into a loop.

  14. class unspecialized extends Annotation with StaticAnnotation

    A method annotation which suppresses the creation of additional specialized forms based on enclosing specialized type parameters.

    A method annotation which suppresses the creation of additional specialized forms based on enclosing specialized type parameters.

    Annotations
    @deprecatedInheritance(message = "Scheduled for being final in the future", since = "2.13.0")
  15. class unused extends Annotation with StaticAnnotation

    Mark an element unused for a given context.

    Mark an element unused for a given context.

    Unused warnings are suppressed for elements known to be unused.

    For example, a method parameter may be marked @unused because the method is designed to be overridden by an implementation that does use the parameter.

    Annotations
    @getter() @setter()
  16. final class varargs extends Annotation with StaticAnnotation

    A method annotation which instructs the compiler to generate a Java varargs-style forwarder method for interop.

    A method annotation which instructs the compiler to generate a Java varargs-style forwarder method for interop. This annotation can only be applied to methods with repeated parameters.

Deprecated Type Members

  1. trait ClassfileAnnotation extends Annotation with ConstantAnnotation

    A base class for classfile annotations.

    A base class for classfile annotations. These are stored as Java annotations] in classfiles.

    Annotations
    @deprecated
    Deprecated

    (Since version 2.13.0) Annotation classes need to be written in Java in order to be stored in classfiles in a Java-compatible manner

Value Members

  1. object elidable

    This useless appearing code was necessary to allow people to use named constants for the elidable annotation.

    This useless appearing code was necessary to allow people to use named constants for the elidable annotation. This is what it takes to convince the compiler to fold the constants: otherwise when it's time to check an elision level it's staring at a tree like

    (Select(Level, Select(FINEST, Apply(intValue, Nil))))

    instead of the number 300.

Ungrouped