Scala Reflection Library 2.13.16 - scala.reflect.api.Constants.ConstantExtractor
Keywords: Scala Reflection Library 2.13.16 scala.reflect.api.Constants.ConstantExtractor

Scala Reflection Library2.13.16 < Back
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# Packages

-  package root
  Definition Classes
    root
-  package scala
  Definition Classes
    root
-  package reflect
  Definition Classes
    scala
-  package api

  EXPERIMENTAL

  EXPERIMENTAL

  The Scala Reflection API (located in scala-reflect.jar).

  In Scala 2.10.0, the Scala Reflection API and its implementation have an "experimental" status. This means that the API and the docs are not complete and can be changed in binary- and source-incompatible manner in 2.10.1. This also means that the implementation has some known issues.

  The following types are the backbone of the Scala Reflection API, and serve as a good starting point for information about Scala Reflection:

  - scala.reflect.api.Symbols
  - scala.reflect.api.Types
  - scala.reflect.api.Mirrors
  - scala.reflect.api.Universe

  For more information about Scala Reflection, see the Reflection Guide

  Definition Classes
    reflect
-  trait Constants extends AnyRef

  EXPERIMENTAL

  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. Examples include "true", "0" and "classOf[List]".

  `Constant` instances 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 the following kinds of 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 `String`.
  3. References to classes, typically constructed with scala.Predef#classOf - represented as types.
  4. References to enumeration values - represented as symbols.

  Instances are used to represent literals in abstract syntax trees, inside scala.reflect.api.Trees#Literal nodes.

  ##### 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).

      // File "JavaSimpleEnumeration.java"
      enum JavaSimpleEnumeration { FOO, BAR }

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

      // File "JavaAnnottee.java"
      @JavaSimpleAnnotation(
        classRef = JavaAnnottee.class,
        enumRef = JavaSimpleEnumeration.BAR
      )
      public class JavaAnnottee {}

      val javaArgs = typeOf[JavaAnnottee].typeSymbol.annotations(0).tree.children.tail

      def jArg[A](lhs: String): Option[A] = javaArgs
        .map { case NamedArg(lhs, Literal(const)) => (lhs.toString, const) }
        .find(_._1 == lhs)
        .map(_._2.value.asInstanceOf[A])

      // class reference, cast to Type
      val classRef = jArg[Type]("classRef").get
      println(showRaw(classRef))             // TypeRef(ThisType(<empty>), JavaAnnottee, List())
      println(cm.runtimeClass(classRef))     // class JavaAnnottee
      // enum value reference, cast to Symbol
      val enumRef = jArg[Symbol]("enumRef").get
      println(enumRef)                       // value BAR

      val siblings = enumRef.owner.info.decls
      val enumValues = siblings.filter(_.isJavaEnum)
      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

  Definition Classes
    api
- Constant
- ConstantApi
- ConstantExtractor

c

scala.reflect.api.Constants

# ConstantExtractor

###

#### 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.

Source
  Constants.scala

Linear Supertypes
AnyRef, Any
Type Hierarchy
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Ordering

1. Alphabetic
2. By Inheritance

Inherited

1. ConstantExtractor
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. Protected

### Instance Constructors

1.  new ConstantExtractor()

### Abstract Value Members

1.  abstract def apply(value: Any): Universe.Constant

   A factory method that produces `Constant` instances.

   A factory method that produces `Constant` instances.

   Notice that not any value can be passed to a constant: it must be either a primitive, a `String`, a type or a symbol. See the `Constant` class for more information.

2.  abstract def unapply(arg: Universe.Constant): Option[Any]

   An extractor that enables writing pattern matches against the `Constant` class.

### 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 Universe.ConstantExtractor toany2stringadd[Universe.ConstantExtractor] performed by method any2stringadd in scala.Predef.
   Definition Classes
     any2stringadd
4.  def ->[B](y: B): (Universe.ConstantExtractor, B)
   Implicit
     This member is added by an implicit conversion from Universe.ConstantExtractor toArrowAssoc[Universe.ConstantExtractor] 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(classOf[java.lang.CloneNotSupportedException]) @native()
8.  def ensuring(cond: (Universe.ConstantExtractor) => Boolean, msg: => Any): Universe.ConstantExtractor
   Implicit
     This member is added by an implicit conversion from Universe.ConstantExtractor toEnsuring[Universe.ConstantExtractor] performed by method Ensuring in scala.Predef.
   Definition Classes
     Ensuring
9.  def ensuring(cond: (Universe.ConstantExtractor) => Boolean): Universe.ConstantExtractor
   Implicit
     This member is added by an implicit conversion from Universe.ConstantExtractor toEnsuring[Universe.ConstantExtractor] performed by method Ensuring in scala.Predef.
   Definition Classes
     Ensuring
10.  def ensuring(cond: Boolean, msg: => Any): Universe.ConstantExtractor
    Implicit
      This member is added by an implicit conversion from Universe.ConstantExtractor toEnsuring[Universe.ConstantExtractor] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
11.  def ensuring(cond: Boolean): Universe.ConstantExtractor
    Implicit
      This member is added by an implicit conversion from Universe.ConstantExtractor toEnsuring[Universe.ConstantExtractor] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
12.  final def eq(arg0: AnyRef): Boolean
    Definition Classes
      AnyRef
13.  def equals(arg0: AnyRef): 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[_ <: AnyRef]
    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(classOf[java.lang.InterruptedException])
24.  final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
25.  final def wait(arg0: Long): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException]) @native()

### Deprecated Value Members

1.  def formatted(fmtstr: String): String
   Implicit
     This member is added by an implicit conversion from Universe.ConstantExtractor toStringFormat[Universe.ConstantExtractor] 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.

2.  def →[B](y: B): (Universe.ConstantExtractor, B)
   Implicit
     This member is added by an implicit conversion from Universe.ConstantExtractor toArrowAssoc[Universe.ConstantExtractor] performed by method ArrowAssoc in scala.Predef.
   Definition Classes
     ArrowAssoc
   Annotations
     @deprecated
   Deprecated

     (Since version 2.13.0) Use `->` instead. If you still wish to display it as one character, consider using a font with programming ligatures such as Fira Code.

### Inherited from AnyRef

### Inherited from Any

### Inherited by implicit conversion any2stringadd fromUniverse.ConstantExtractor to any2stringadd[Universe.ConstantExtractor]

### Inherited by implicit conversion StringFormat fromUniverse.ConstantExtractor to StringFormat[Universe.ConstantExtractor]

### Inherited by implicit conversion Ensuring fromUniverse.ConstantExtractor to Ensuring[Universe.ConstantExtractor]

### Inherited by implicit conversion ArrowAssoc fromUniverse.ConstantExtractor to ArrowAssoc[Universe.ConstantExtractor]

### Ungrouped