Constants

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(), 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
   }

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.