Scala example source code file (Constants.scala)

This example Scala source code file (Constants.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

any, anyref, constant, constantapi, constantextractor, constants, null, option, type, universe

The Constants.scala Scala example source code

/* NSC -- new Scala compiler
 * Copyright 2005-2013 LAMP/EPFL
 * @author  Martin Odersky
 */

package scala
package reflect
package api

/**
 * <span class="badge badge-red" style="float: right;">EXPERIMENTAL</span>
 *
 *  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).
 *    1. String literals.
 *    1. References to classes (typically constructed with [[scala.Predef#classOf]]).
 *    1. 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(newTermName(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://issues.scala-lang.org/browse/SI-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
 *  }
 *  }}}
 *
 *  @contentDiagram hideNodes "*Api"
 *  @group ReflectionAPI
 */
trait Constants {
  self: Universe =>

  /**
   *  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
   *  [[http://www.scala-lang.org/docu/files/ScalaReference.pdf 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 ([[scala.Byte `Byte`]], [[scala.Short `Short`]], [[scala.Int `Int`]], [[scala.Long `Long`]], [[scala.Float `Float`]], [[scala.Double `Double`]], [[scala.Char `Char`]], [[scala.Boolean `Boolean`]] and [[scala.Unit `Unit`]]) - represented directly as the corresponding type
   *    1. String literals - represented as instances of the `String`.
   *    1. References to classes, typically constructed with [[scala.Predef#classOf]] - represented as [[scala.reflect.api.Types#Type types]].
   *    1. References to enumeration values - represented as [[scala.reflect.api.Symbols#Symbol 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 [[scala.reflect.api.Mirrors#RuntimeMirror#runtimeClass `runtimeClass`]] method of a
   *  mirror such as [[scala.reflect.api.Mirrors#RuntimeMirror `RuntimeMirror`]] (the simplest way to get such a mirror is using
   *  [[scala.reflect.runtime#currentMirror `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(newTermName(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://issues.scala-lang.org/browse/SI-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
   *  }
   *  }}}
   *  @template
   *  @group Constants
   */
  type Constant >: Null <: AnyRef with ConstantApi

  /** The constructor/extractor for `Constant` instances.
   *  @group Extractors
   */
  val Constant: ConstantExtractor

  /** An extractor class to create and pattern match with syntax `Constant(value)`
   *  where `value` is the Scala value of the constant.
   *  @group Extractors
   */
  abstract class ConstantExtractor {
    /** A factory method that produces [[Constant `Constant`]] instances.
     *
     *  Notice that not any value can be passed to a constant: it must be either a primitive, a `String`, a
     *  [[scala.reflect.api.Types#Type type]] or a [[scala.reflect.api.Symbols#Symbol symbol]].
     *  See [[Constant the `Constant` class]] for more information.
     */
    def apply(value: Any): Constant
    /** An extractor that enables writing pattern matches against the [[Constant `Constant`]] class. */
    def unapply(arg: Constant): Option[Any]
  }

  /** The API of [[Constant]] instances.
   *  @group API
   */
  abstract class ConstantApi {
    /** Payload of the constant, that can be accessed directly or pattern matched against. */
    val value: Any

    /** Scala type that describes the constant. It is generated automatically based on the type of the value. */
    def tpe: Type
  }
}