Package scala.annotation

A base class for annotations. Annotations extending this class directly are not preserved for the Scala type checker and are also not stored as Java annotations in classfiles. To enable either or both of these, one needs to inherit from scala.annotation.StaticAnnotation or/and scala.annotation.ClassfileAnnotation.

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

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

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.

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).

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

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]

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. *

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

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

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 an error 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.

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.

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

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.