Packages

t

scala.reflect.api

Constants

trait Constants extends AnyRef

EXPERIMENTAL

According to the section 6.24 "Constant Expressions" of the Scala language specification, certain expressions (dubbed constant expressions) can be evaluated by the Scala compiler at compile-time.

scala.reflect.api.Constants#Constant instances represent certain kinds of these expressions (with values stored in the value field and its strongly-typed views named booleanValue, intValue etc.), namely:

  1. Literals of primitive value classes (bytes, shorts, ints, longs, floats, doubles, chars, booleans and voids).
  2. String literals.
  3. References to classes (typically constructed with scala.Predef#classOf).
  4. References to enumeration values.

Such constants are used to represent literals in abstract syntax trees (the scala.reflect.api.Trees#Literal node) and literal arguments for Java class file annotations (the scala.reflect.api.Annotations#LiteralArgument class).

Example

The value field deserves some explanation. Primitive and string values are represented as themselves, whereas references to classes and enums are a bit roundabout.

Class references are represented as instances of scala.reflect.api.Types#Type (because when the Scala compiler processes a class reference, the underlying runtime class might not yet have been compiled). To convert such a reference to a runtime class, one should use the runtimeClass method of a mirror such as scala.reflect.api.Mirrors#RuntimeMirror (the simplest way to get such a mirror is using scala.reflect.runtime.package#currentMirror).

Enumeration value references are represented as instances of scala.reflect.api.Symbols#Symbol, which on JVM point to methods that return underlying enum values. To inspect an underlying enumeration or to get runtime value of a reference to an enum, one should use a scala.reflect.api.Mirrors#RuntimeMirror (the simplest way to get such a mirror is again scala.reflect.runtime.package#currentMirror).

enum JavaSimpleEnumeration { FOO, BAR }

import java.lang.annotation.*;
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface JavaSimpleAnnotation {
  Class<?> classRef();
  JavaSimpleEnumeration enumRef();
}

@JavaSimpleAnnotation(
  classRef = JavaAnnottee.class,
  enumRef = JavaSimpleEnumeration.BAR
)
public class JavaAnnottee {}
import scala.reflect.runtime.universe._
import scala.reflect.runtime.{currentMirror => cm}

object Test extends App {
  val jann = typeOf[JavaAnnottee].typeSymbol.annotations(0).javaArgs
  def jarg(name: String) = jann(TermName(name)).asInstanceOf[LiteralArgument].value

  val classRef = jarg("classRef").typeValue
  println(showRaw(classRef))             // TypeRef(ThisType(<empty>), JavaAnnottee, List())
  println(cm.runtimeClass(classRef))     // class JavaAnnottee

  val enumRef = jarg("enumRef").symbolValue
  println(enumRef)                       // value BAR

  val siblings = enumRef.owner.info.decls
  val enumValues = siblings.filter(sym => sym.isVal && sym.isPublic)
  println(enumValues)                    // Scope{
                                         //   final val FOO: JavaSimpleEnumeration;
                                         //   final val BAR: JavaSimpleEnumeration
                                         // }

  // doesn't work because of https://github.com/scala/bug/issues/6459
  // val enumValue = mirror.reflectField(enumRef.asTerm).get
  val enumClass = cm.runtimeClass(enumRef.owner.asClass)
  val enumValue = enumClass.getDeclaredField(enumRef.name.toString).get(null)
  println(enumValue)                     // BAR
}
Self Type
Universe
Source
Constants.scala
Linear Supertypes
AnyRef, Any
Known Subclasses
Ordering
  1. Grouped
  2. Alphabetic
  3. By Inheritance
Inherited
  1. Constants
  2. AnyRef
  3. Any
Implicitly
  1. by any2stringadd
  2. by StringFormat
  3. by Ensuring
  4. by ArrowAssoc
  1. Hide All
  2. Show All
Visibility
  1. Public
  2. All

Type Members

  1. abstract type Constant >: Null <: Universe.ConstantApi

    This "virtual" case class represents the reflection interface for literal expressions which can not be further broken down or evaluated, such as "true", "0", "classOf[List]".

    This "virtual" case class represents the reflection interface for literal expressions which can not be further broken down or evaluated, such as "true", "0", "classOf[List]". Such values become parts of the Scala abstract syntax tree representing the program. The constants correspond to section 6.24 "Constant Expressions" of the Scala Language Specification.

    Such constants are used to represent literals in abstract syntax trees (the scala.reflect.api.Trees#Literal node) and literal arguments for Java class file annotations (the scala.reflect.api.Annotations#LiteralArgument class).

    Constants can be matched against and can be constructed directly, as if they were case classes:

    assert(Constant(true).value == true)
    Constant(true) match {
      case Constant(s: String) =>  println("A string: " + s)
      case Constant(b: Boolean) => println("A boolean value: " + b)
      case Constant(x) =>          println("Something else: " + x)
    }

    Constant instances can wrap certain kinds of these expressions:

    1. Literals of primitive value classes (Byte, Short, Int, Long, Float, Double, Char, Boolean and Unit) - represented directly as the corresponding type
    2. String literals - represented as instances of the String.
    3. References to classes, typically constructed with scala.Predef#classOf - represented as types.
    4. References to enumeration values - represented as symbols.

    Class references are represented as instances of scala.reflect.api.Types#Type (because when the Scala compiler processes a class reference, the underlying runtime class might not yet have been compiled). To convert such a reference to a runtime class, one should use the runtimeClass method of a mirror such as RuntimeMirror (the simplest way to get such a mirror is using scala.reflect.runtime.currentMirror).

    Enumeration value references are represented as instances of scala.reflect.api.Symbols#Symbol, which on JVM point to methods that return underlying enum values. To inspect an underlying enumeration or to get runtime value of a reference to an enum, one should use a scala.reflect.api.Mirrors#RuntimeMirror (the simplest way to get such a mirror is again scala.reflect.runtime.package#currentMirror).

    Usage example:

    enum JavaSimpleEnumeration { FOO, BAR }
    
    import java.lang.annotation.*;
    @Retention(RetentionPolicy.RUNTIME)
    @Target({ElementType.TYPE})
    public @interface JavaSimpleAnnotation {
      Class<?> classRef();
      JavaSimpleEnumeration enumRef();
    }
    
    @JavaSimpleAnnotation(
      classRef = JavaAnnottee.class,
      enumRef = JavaSimpleEnumeration.BAR
    )
    public class JavaAnnottee {}
    import scala.reflect.runtime.universe._
    import scala.reflect.runtime.{currentMirror => cm}
    
    object Test extends App {
      val jann = typeOf[JavaAnnottee].typeSymbol.annotations(0).javaArgs
      def jarg(name: String) = jann(TermName(name)) match {
        // Constant is always wrapped into a Literal or LiteralArgument tree node
        case LiteralArgument(ct: Constant) => value
        case _ => sys.error("Not a constant")
      }
    
      val classRef = jarg("classRef").value.asInstanceOf[Type]
                                             // ideally one should match instead of casting
      println(showRaw(classRef))             // TypeRef(ThisType(<empty>), JavaAnnottee, List())
      println(cm.runtimeClass(classRef))     // class JavaAnnottee
    
      val enumRef = jarg("enumRef").value.asInstanceOf[Symbol]
                                             // ideally one should match instead of casting
      println(enumRef)                       // value BAR
    
      val siblings = enumRef.owner.info.decls
      val enumValues = siblings.filter(sym => sym.isVal && sym.isPublic)
      println(enumValues)                    // Scope{
                                             //   final val FOO: JavaSimpleEnumeration;
                                             //   final val BAR: JavaSimpleEnumeration
                                             // }
    
      // doesn't work because of https://github.com/scala/bug/issues/6459
      // val enumValue = mirror.reflectField(enumRef.asTerm).get
      val enumClass = cm.runtimeClass(enumRef.owner.asClass)
      val enumValue = enumClass.getDeclaredField(enumRef.name.toString).get(null)
      println(enumValue)                     // BAR
    }
  2. abstract class ConstantApi extends AnyRef

    The API of Constant instances.

  3. abstract class ConstantExtractor extends AnyRef

    An extractor class to create and pattern match with syntax Constant(value) where value is the Scala value of the constant.

Abstract Value Members

  1. abstract val Constant: Universe.ConstantExtractor

    The constructor/extractor for Constant instances.

Concrete Value Members

  1. final def !=(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  2. final def ##(): Int
    Definition Classes
    AnyRef → Any
  3. def +(other: String): String
    Implicit
    This member is added by an implicit conversion from Constants to any2stringadd[Constants] performed by method any2stringadd in scala.Predef.
    Definition Classes
    any2stringadd
  4. def ->[B](y: B): (Constants, B)
    Implicit
    This member is added by an implicit conversion from Constants to ArrowAssoc[Constants] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()
  5. final def ==(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  6. final def asInstanceOf[T0]: T0
    Definition Classes
    Any
  7. def clone(): AnyRef
    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( ... ) @native()
  8. def ensuring(cond: (Constants) ⇒ Boolean, msg: ⇒ Any): Constants
    Implicit
    This member is added by an implicit conversion from Constants to Ensuring[Constants] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  9. def ensuring(cond: (Constants) ⇒ Boolean): Constants
    Implicit
    This member is added by an implicit conversion from Constants to Ensuring[Constants] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  10. def ensuring(cond: Boolean, msg: ⇒ Any): Constants
    Implicit
    This member is added by an implicit conversion from Constants to Ensuring[Constants] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  11. def ensuring(cond: Boolean): Constants
    Implicit
    This member is added by an implicit conversion from Constants to Ensuring[Constants] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  12. final def eq(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  13. def equals(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  14. def finalize(): Unit
    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( classOf[java.lang.Throwable] )
  15. final def getClass(): Class[_]
    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  16. def hashCode(): Int
    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  17. final def isInstanceOf[T0]: Boolean
    Definition Classes
    Any
  18. final def ne(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  19. final def notify(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native()
  20. final def notifyAll(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native()
  21. final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
    AnyRef
  22. def toString(): String
    Definition Classes
    AnyRef → Any
  23. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  24. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  25. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... ) @native()
  26. def [B](y: B): (Constants, B)
    Implicit
    This member is added by an implicit conversion from Constants to ArrowAssoc[Constants] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc

Deprecated Value Members

  1. def formatted(fmtstr: String): String
    Implicit
    This member is added by an implicit conversion from Constants to StringFormat[Constants] performed by method StringFormat in scala.Predef.
    Definition Classes
    StringFormat
    Annotations
    @deprecated @inline()
    Deprecated

    (Since version 2.12.16) Use formatString.format(value) instead of value.formatted(formatString), or use the f"" string interpolator. In Java 15 and later, formatted resolves to the new method in String which has reversed parameters.

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion any2stringadd from Constants to any2stringadd[Constants]

Inherited by implicit conversion StringFormat from Constants to StringFormat[Constants]

Inherited by implicit conversion Ensuring from Constants to Ensuring[Constants]

Inherited by implicit conversion ArrowAssoc from Constants to ArrowAssoc[Constants]

Constants

API

The methods available for each reflection entity, without the implementation. Since the reflection entities are later overridden by runtime reflection and macros, their API counterparts guarantee a minimum set of methods that are implemented.

Extractors

Extractors provide the machinery necessary to allow pattern matching and construction of reflection entities that is similar to case classes, although the entities are only abstract types that are later overridden.

Ungrouped