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. 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
Self Type
Universe
Source
Constants.scala
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Inherited
  1. Constants
  2. AnyRef
  3. Any
Implicitly
  1. by any2stringadd
  2. by StringFormat
  3. by Ensuring
  4. by ArrowAssoc
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Visibility
  1. Public
  2. Protected

Type Members

  1. abstract type Constant >: Null <: Universe.ConstantApi
  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 toany2stringadd[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 toArrowAssoc[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(classOf[java.lang.CloneNotSupportedException]) @native()
  8. def ensuring(cond: (Constants) => Boolean, msg: => Any): Constants
    Implicit
    This member is added by an implicit conversion from Constants toEnsuring[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 toEnsuring[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 toEnsuring[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 toEnsuring[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: 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 Constants toStringFormat[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.

  2. def [B](y: B): (Constants, B)
    Implicit
    This member is added by an implicit conversion from Constants toArrowAssoc[Constants] 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 fromConstants to any2stringadd[Constants]

Inherited by implicit conversion StringFormat fromConstants to StringFormat[Constants]

Inherited by implicit conversion Ensuring fromConstants to Ensuring[Constants]

Inherited by implicit conversion ArrowAssoc fromConstants to ArrowAssoc[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