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scala.annotation

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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.

Attributes

Type members

Classlikes

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.

Attributes

Source: Annotation.scala
Supertypes: class Object

trait Matchable

class Any
Known subtypes: trait StaticAnnotation

class beanGetter

class beanSetter

class companionClass

class companionMethod

class companionObject

class field

class getter

class languageFeature

class param

class setter

class uncheckedCaptures

class uncheckedStable

class uncheckedVariance

trait ConstantAnnotation

trait ClassfileAnnotation

class elidable

class implicitAmbiguous

class implicitNotFound

class nowarn

class SerialVersionUID

class deprecated

class deprecatedInheritance

class deprecatedOverriding

trait MacroAnnotation

trait RefiningAnnotation

class alpha

class capability

class compileTimeOnly

class constructorOnly

class experimental

class widen

class publicInBinary

class retains[Elems]

class retainsByName[Elems]

class retainsCap

class showAsInfix

class static

class strictfp

class switch

class tailrec

class targetName

class threadUnsafe

class transparentTrait

class unroll

class unspecialized

class unused

class varargs

class BeanProperty

class BooleanBeanProperty

class consume

class inferredDepFun

class refineOverride

class rootCapability

class untrackedCaptures

class use

class deprecatedName

class inline

class native

class noinline

class specialized

class throws[T]

class transient

class volatile

trait TypeConstraint

class fromAbove

class patternType

class SplicedType

class main

class unchecked
Show all

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
}

Attributes

Source: ConstantAnnotation.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any
Known subtypes: trait ClassfileAnnotation

class elidable

class implicitAmbiguous

class implicitNotFound

class nowarn

class SerialVersionUID

class deprecated

class deprecatedInheritance

class deprecatedOverriding
Show all

A base trait for annotations that yield proper subtypes of the types they annotate. Refining annotations are more "sticky" than normal ones. They are conceptually kept around when normal refinements would also not be stripped away.

Attributes

Source: RefiningAnnotation.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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.

Attributes

Source: StaticAnnotation.scala
Supertypes: class Annotation

class Object

trait Matchable

class Any
Known subtypes: class beanGetter

class beanSetter

class companionClass

class companionMethod

class companionObject

class field

class getter

class languageFeature

class param

class setter

class uncheckedCaptures

class uncheckedStable

class uncheckedVariance

trait ConstantAnnotation

trait ClassfileAnnotation

class elidable

class implicitAmbiguous

class implicitNotFound

class nowarn

class SerialVersionUID

class deprecated

class deprecatedInheritance

class deprecatedOverriding

trait MacroAnnotation

trait RefiningAnnotation

class alpha

class capability

class compileTimeOnly

class constructorOnly

class experimental

class widen

class publicInBinary

class retains[Elems]

class retainsByName[Elems]

class retainsCap

class showAsInfix

class static

class strictfp

class switch

class tailrec

class targetName

class threadUnsafe

class transparentTrait

class unroll

class unspecialized

class unused

class varargs

class BeanProperty

class BooleanBeanProperty

class consume

class inferredDepFun

class refineOverride

class rootCapability

class untrackedCaptures

class use

class deprecatedName

class inline

class native

class noinline

class specialized

class throws[T]

class transient

class volatile
Show all

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.

Attributes

Source: TypeConstraint.scala
Supertypes: class Annotation

class Object

trait Matchable

class Any

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).

Value parameters

message: the error message to print during compilation if a reference remains after type checking

Attributes

Source: compileTimeOnly.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

An annotation that goes on parameters of classes or traits. It asserts that the parameter is used only for initialization and is not kept in the class as a field. Violations of this assertion are flagged as compile errors. The annotation is particularly useful for implicit parameters since for these a textual scan is not sufficient to know where they are used. Note: the annotation is copied from constructor parameters to corresponding class fields. But it is checked that the field is eliminated before code is generated.

Attributes

Source: constructorOnly.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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`
}

Note for Scala 3 users: If you're using Scala 3, the annotation exists since Scala 3 uses the Scala 2 standard library, but it's unsupported by the Scala 3 compiler. Instead, to achieve the same result you'd want to utilize the inline if feature to introduce behavior that makes a method de facto elided at compile-time.

type LogLevel = Int

object LogLevel:
  inline val Info = 0
  inline val Warn = 1
  inline val Debug = 2

inline val appLogLevel = LogLevel.Warn

inline def log(msg: String, inline level: LogLevel): Unit =
  inline if (level <= appLogLevel) then println(msg)

log("Warn log", LogLevel.Warn)

log("Debug log", LogLevel. Debug)

Attributes

Companion: object
Source: elidable.scala
Supertypes: trait ConstantAnnotation

trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any
Show all

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.

Attributes

Companion: class
Source: elidable.scala
Supertypes: class Object

trait Matchable

class Any
Self type: elidable.type

An annotation that can be used to mark a definition as experimental.

Attributes

See also: https://dotty.epfl.ch/docs/reference/other-new-features/experimental-defs
Source: experimental.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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]

Attributes

Source: implicitAmbiguous.scala
Supertypes: trait ConstantAnnotation

trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any
Show all

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]
    ^

Attributes

Source: implicitNotFound.scala
Supertypes: trait ConstantAnnotation

trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any
Show all

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. The unused annotation warning is emitted in category unused-nowarn and can be selectively managed using -Wconf:cat=unused-nowarn:s.

Attributes

Source: nowarn.scala
Supertypes: trait ConstantAnnotation

trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any
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A binary API is a definition that is annotated with @publicInBinary. This annotation can be placed on def, val, lazy val, var, class constructors, object, and given definitions. A binary API will be publicly available in the bytecode. Tools like TASTy MiMa will take this into account to check compatibility.

This annotation cannot be used on private/private[this] definitions.

@publicInBinary can be used to guarantee access to private[T]/protected definitions:

within inline definitions,
against previous binary where this definitions was public or less private,
or through JVM reflection.

Removing this annotation from a non-public definition is a binary incompatible change. Adding this annotation to a non-public definition can also cause binary incompatibilities if the definition is accessed in an inline definition (these can be checked using -WunstableInlineAccessors).

Attributes

Source: publicInBinary.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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

Value parameters

enabled: whether to show this type as an infix type operator.

Attributes

Source: showAsInfix.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

https://github.com/scala/scala.github.com/pull/491

Attributes

Source: static.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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

Attributes

Source: strictfp.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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.

Attributes

Source: switch.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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.

Attributes

Source: tailrec.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

An annotation that defines an external name for a definition. If an targetName(extname) annotation is given for a method or some other definition, its implementation will use the name extname instead of the regular name.

Attributes

Source: targetName.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

This annotation can only be used on a field which defines a lazy val. When this annotation is used, the initialization of the lazy val will use a faster mechanism which is not thread-safe.

Attributes

Source: threadUnsafe.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

An annotation that can be used from Scala 2 to mark a trait as transparent. Scala 3 code would use the modifier transparent instead. Transparent traits are not inferred when combined with other types in an intersection. See reference/other-new-features/transparent-traits.html for details.

Attributes

Source: transparentTrait.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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

Attributes

Source: unspecialized.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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.

Attributes

Source: unused.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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.

Attributes

Source: varargs.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

Deprecated classlikes

A base class for classfile annotations.

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

Attributes

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
Source: ClassfileAnnotation.scala
Supertypes: trait ConstantAnnotation

trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any
Show all

An annotation that defines an external name for a definition. If an alpha(extname) annotation is given for a method or some other definition, its implementation will use the name extname instead of the regular name. An alpha annotation is mandatory for definitions with symbolic names.

Attributes

Deprecated: use @targetName instead
Source: alpha.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

Experimental classlikes

Base trait for macro annotation implementation. Macro annotations can transform definitions and add new definitions.

See: MacroAnnotation.transform

Attributes

Experimental: true
Source: MacroAnnotation.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

Marks an annotated class as a capability. If the annotation is present and -Ycc is set, any (possibly aliased or refined) instance of the class type is implicitly augmented with the universal capture set. Example

Attributes

Deprecated: To make a class a capability, let it derive from the `Capability` trait instead
Experimental: true
Source: capability.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

Annotations to control the behavior of the compiler check for safe initialization of static obects.

Programmers usually do not need to use any annotations. They are intended for complex initialization code in static objects.

Attributes

Experimental: true
Source: init.scala
Supertypes: class Object

trait Matchable

class Any
Self type: init.type

An annotation that indicates capture of a set of references under capture checking.

T @retains[x.type | y.type | z.type]

is the internal representation used for the capturing type

T ^ {x, y, z}

The annotation can also be written explicitly if one wants to avoid the non-standard capturing type syntax.

Attributes

Experimental: true
Source: retains.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

An annotation that indicates capture of an enclosing by-name type

Attributes

Experimental: true
Source: retainsByName.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

Equivalent in meaning to @retains[cap.type], but consumes less bytecode.

Attributes

Experimental: true
Source: retains.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

Attributes

Experimental: true
Source: unroll.scala
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any

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