scala.annotation

Type members

Classlikes

abstract class Annotation
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.
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@deprecated("Annotation classes need to be written in Java in order to be stored in classfiles in a Java-compatible manner", "2.13.0")
A base class for classfile annotations. These are stored as Java annotations] in classfiles.
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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
}
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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.
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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.
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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 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.
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@deprecated("use @targetName instead")
final class alpha(externalName: String) extends StaticAnnotation
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.
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final class compileTimeOnly(message: String) extends StaticAnnotation
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 Params
message
the error message to print during compilation if a reference remains after type checking
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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.
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final class elidable(val level: Int) extends StaticAnnotation
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`
}
Companion
object
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object elidable
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.
Companion
class
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final class implicitAmbiguous(msg: String) extends 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. 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]
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final class implicitNotFound(msg: String) extends 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. 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]
    ^
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class nowarn(value: String) extends ConstantAnnotation
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.
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@deprecatedInheritance("Scheduled for being final in the future", "2.13.0")
class showAsInfix(enabled: Boolean) extends StaticAnnotation
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 Params
enabled
whether to show this type as an infix type operator.
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final class static extends StaticAnnotation
https://github.com/scala/scala.github.com/pull/491
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@deprecatedInheritance("Scheduled for being final in the future", "2.13.0")
class strictfp extends StaticAnnotation
If this annotation is present on a method or its enclosing class, the strictfp flag will be emitted.
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final class switch extends StaticAnnotation
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.
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final class tailrec extends StaticAnnotation
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.
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final class targetName(name: String) extends StaticAnnotation
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.
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final class threadUnsafe extends StaticAnnotation
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.
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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.
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@deprecatedInheritance("Scheduled for being final in the future", "2.13.0")
A method annotation which suppresses the creation of additional specialized forms based on enclosing specialized type parameters.
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class unused(message: String) extends StaticAnnotation
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.
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final class varargs extends StaticAnnotation
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.
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