scala.annotation (original) (raw)

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

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

Known subtypes

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

Known subtypes

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

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

Known subtypes

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

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

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

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

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

Self type

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

Attributes

Source

implicitAmbiguous.scala

Supertypes

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 <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>A</mi><mi>a</mi><mi>n</mi><mi>d</mi></mrow><annotation encoding="application/x-tex">{A} and </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord"><span class="mord mathnormal">A</span></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span></span></span></span>{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

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

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

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

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

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

Attributes

Source

strictfp.scala

Supertypes

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

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

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

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

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

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

Attributes

Source

unspecialized.scala

Supertypes

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

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

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

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

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

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

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

Self type

An annotation on (part of) a parameter type that allows implicit conversions for its arguments. The into modifier on parameter types in Scala 3 is mapped to this annotation. The annotation is intended to be used directly in Scala 2 sources only. For Scala 3, the into modifier should be preferred.

Attributes

Experimental

true

Source

into.scala

Supertypes

An annotation that indicates capture of a set of references under -Ycc.

T @retains(x, y, z)

is the internal representation used for the capturing type

{x, y, z}  T

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

Internal use, only for parameters of retains and retainsByName.

Attributes

Experimental

true

Source

retains.scala

Supertypes

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

Attributes

Experimental

true

Source

retainsByName.scala

Supertypes

Equivalent in meaning to @retains(cap), but consumes less bytecode.

Attributes

Experimental

true

Source

retains.scala

Supertypes