Scala example source code file (TypeTags.scala)

This example Scala source code file (TypeTags.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, serializedtypetag, singleton, type, typecreator, typetag, u, universe, weaktypetag

The TypeTags.scala Scala example source code

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

package scala
package reflect
package api

import java.lang.{ Class => jClass }
import scala.language.implicitConversions

/**
 * A `TypeTag[T]` encapsulates the runtime type representation of some type `T`.
 * Like [[scala.reflect.Manifest]], the prime use case of `TypeTag`s is to give access
 * to erased types. However, `TypeTag`s should be considered to be a richer
 * replacement of the pre-2.10 notion of a [[scala.reflect.Manifest Manifest]], that
 * are, in addition, fully integrated with Scala reflection.
 *
 * There exist three different types of `TypeTags`:
 *
 *  <ul>
 *  <li>[[scala.reflect.api.TypeTags#TypeTag]]. <br/>A full type descriptor of a Scala type.
 *  For example, a `TypeTag[List[String]]` contains all type information,
 *  in this case, of type `scala.List[String]`.</li>
 *
 *  <li>[[scala.reflect.ClassTag]]. <br/>A partial type descriptor of a Scala type. For
 *  example, a `ClassTag[List[String]]` contains only the erased class
 *  type information, in this case, of type `scala.collection.immutable.List`.
 *  `ClassTag`s provide access only to the runtime class of a type.
 *  Analogous to [[scala.reflect.ClassManifest]]</li>
 *
 *  <li>[[scala.reflect.api.TypeTags#WeakTypeTag]]. <br/>A type descriptor for abstract
 *  types (see description below).</li>
 *  </ul>
 *
 * Like [[scala.reflect.Manifest Manifest]]s, `TypeTag`s are always generated by the
 * compiler, and can be obtained in three ways:
 *
 * === #1 Via the methods [[scala.reflect.api.TypeTags#typeTag typeTag]],
 * [[scala.reflect#classTag classTag]], or [[scala.reflect.api.TypeTags#weakTypeTag weakTypeTag]] ===
 *
 * For example:
 * {{{
 *   import scala.reflect.runtime.universe._
 *   val tt = typeTag[Int]
 *
 *   import scala.reflect._
 *   val ct = classTag[String]
 * }}}
 *
 * Each of these methods constructs a `TypeTag[T]` or `ClassTag[T]` for the given
 * type argument `T`.
 *
 * === #2 Using an implicit parameter of type `TypeTag[T]`, `ClassTag[T]`, or `WeakTypeTag[T]
 *
 * For example:
 * {{{
 *   import scala.reflect.runtime.universe._
 *
 *   def paramInfo[T](x: T)(implicit tag: TypeTag[T]): Unit = {
 *     val targs = tag.tpe match { case TypeRef(_, _, args) => args }
 *     println(s"type of $x has type arguments $targs")
 *   }
 *
 *   scala> paramInfo(42)
 *   type of 42 has type arguments List()
 *
 *   scala> paramInfo(List(1, 2))
 *   type of List(1, 2) has type arguments List(Int)
 * }}}
 *
 * === #3 Context bound of a type parameter ===
 *
 * ...on methods or classes. The above example can be implemented as follows:
 *
 * {{{
 *   import scala.reflect.runtime.universe._
 *
 *   def paramInfo[T: TypeTag](x: T): Unit = {
 *     val targs = typeOf[T] match { case TypeRef(_, _, args) => args }
 *     println(s"type of $x has type arguments $targs")
 *   }
 *
 *   scala> paramInfo(42)
 *   type of 42 has type arguments List()
 *
 *   scala> paramInfo(List(1, 2))
 *   type of List(1, 2) has type arguments List(Int)
 * }}}
 *
 * === `WeakTypeTag`s ===
 *
 *`WeakTypeTag[T]` generalizes `TypeTag[T]`. Unlike a regular `TypeTag`, components of
 * its type representation can be references to type parameters or abstract types.
 * However, `WeakTypeTag[T]` tries to be as concrete as possible, i.e. if type tags
 * are available for the referenced type arguments or abstract types, they are used to
 * embed the concrete types into the `WeakTypeTag[T]`.
 *
 * Continuing the example above:
 * {{{
 * def weakParamInfo[T](x: T)(implicit tag: WeakTypeTag[T]): Unit = {
 *   val targs = tag.tpe match { case TypeRef(_, _, args) => args }
 *   println(s"type of $x has type arguments $targs")
 * }
 *
 * scala> def foo[T] = weakParamInfo(List[T]())
 * foo: [T]=> Unit
 *
 * scala> foo[Int]
 * type of List() has type arguments List(T)
 * }}}
 *
 * === TypeTags and Manifests ===
 *
 * `TypeTag`s correspond loosely to the pre-2.10 notion of
 * [[scala.reflect.Manifest]]s. While [[scala.reflect.ClassTag]] corresponds to
 * [[scala.reflect.ClassManifest]] and [[scala.reflect.api.TypeTags#TypeTag]] mostly
 * corresponds to [[scala.reflect.Manifest]], other pre-2.10 `Manifest` types do not
 * have a direct correspondence with a 2.10 "`Tag`" type.
 *
 * <ul>
 * <li>'''[[scala.reflect.OptManifest]] is not supported.''' <br/>This is because `Tag`s
 * can reify arbitrary types, so they are always available.<li>
 *
 * <li>'''There is no equivalent for [[scala.reflect.AnyValManifest]].''' <br/>Instead, one
 * can compare their `Tag` with one of the base `Tag`s (defined in the corresponding
 * companion objects) in order to find out whether or not it represents a primitive
 * value class. Additionally, it's possible to simply use
 * `<tag>.tpe.typeSymbol.isPrimitiveValueClass`.</li>
 *
 * <li>'''There are no replacement for factory methods defined in the `Manifest`
 * companion objects'''. <br/>Instead, one could generate corresponding types using the
 * reflection APIs provided by Java (for classes) and Scala (for types).</li>
 *
 * <li>'''Certain manifest operations(i.e., <:<, >:> and typeArguments) are not
 * supported.''' <br/>Instead, one could use the reflection APIs provided by Java (for
 * classes) and Scala (for types).</li>
 *</ul>
 *
 * In Scala 2.10, [[scala.reflect.ClassManifest]]s are deprecated, and it is planned
 * to deprecate [[scala.reflect.Manifest]] in favor of `TypeTag`s and `ClassTag`s in
 * an upcoming point release. Thus, it is advisable to migrate any `Manifest`-based
 * APIs to use `Tag`s.
 *
 * For more information about `TypeTag`s, see the
 * [[http://docs.scala-lang.org/overviews/reflection/typetags-manifests.html Reflection Guide: TypeTags]]
 *
 * @see [[scala.reflect.ClassTag]], [[scala.reflect.api.TypeTags#TypeTag]], [[scala.reflect.api.TypeTags#WeakTypeTag]]
 * @group ReflectionAPI
 */
trait TypeTags { self: Universe =>

  import definitions._

  /**
   * If an implicit value of type `WeakTypeTag[T]` is required, the compiler will create one,
   * and the reflective representation of `T` can be accessed via the `tpe` field.
   * Components of `T` can be references to type parameters or abstract types. Note that `WeakTypeTag`
   * makes an effort to be as concrete as possible, i.e. if `TypeTag`s are available for the referenced type arguments
   * or abstract types, they are used to embed the concrete types into the WeakTypeTag. Otherwise the WeakTypeTag will
   * contain a reference to an abstract type. This behavior can be useful, when one expects `T` to be perhaps be partially
   * abstract, but requires special care to handle this case. However, if `T` is expected to be fully known, use
   * [[scala.reflect.api.TypeTags#TypeTag]] instead, which statically guarantees this property.
   *
   * For more information about `TypeTag`s, see the
   * [[http://docs.scala-lang.org/overviews/reflection/typetags-manifests.html Reflection Guide: TypeTags]]
   *
   * @see [[scala.reflect.api.TypeTags]]
   * @group TypeTags
   */
  @annotation.implicitNotFound(msg = "No WeakTypeTag available for ${T}")
  trait WeakTypeTag[T] extends Equals with Serializable {
    /**
     * The underlying `Mirror` of this type tag.
     */
    val mirror: Mirror

    /**
     * Migrates the expression into another mirror, jumping into a different universe if necessary.
     *
     * Migration means that all symbolic references to classes/objects/packages in the expression
     * will be re-resolved within the new mirror (typically using that mirror's classloader).
     */
    def in[U <: Universe with Singleton](otherMirror: scala.reflect.api.Mirror[U]): U # WeakTypeTag[T]

    /**
     * Reflective representation of type T.
     */
    def tpe: Type

    override def canEqual(x: Any) = x.isInstanceOf[WeakTypeTag[_]]

    override def equals(x: Any) = x.isInstanceOf[WeakTypeTag[_]] && this.mirror == x.asInstanceOf[WeakTypeTag[_]].mirror && this.tpe == x.asInstanceOf[WeakTypeTag[_]].tpe

    override def hashCode = mirror.hashCode * 31 + tpe.hashCode

    override def toString = "WeakTypeTag[" + tpe + "]"
  }

  /**
   * Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.
   * @group TypeTags
   */
  object WeakTypeTag {
    val Byte    : WeakTypeTag[scala.Byte]       = TypeTag.Byte
    val Short   : WeakTypeTag[scala.Short]      = TypeTag.Short
    val Char    : WeakTypeTag[scala.Char]       = TypeTag.Char
    val Int     : WeakTypeTag[scala.Int]        = TypeTag.Int
    val Long    : WeakTypeTag[scala.Long]       = TypeTag.Long
    val Float   : WeakTypeTag[scala.Float]      = TypeTag.Float
    val Double  : WeakTypeTag[scala.Double]     = TypeTag.Double
    val Boolean : WeakTypeTag[scala.Boolean]    = TypeTag.Boolean
    val Unit    : WeakTypeTag[scala.Unit]       = TypeTag.Unit
    val Any     : WeakTypeTag[scala.Any]        = TypeTag.Any
    val AnyVal  : WeakTypeTag[scala.AnyVal]     = TypeTag.AnyVal
    val AnyRef  : WeakTypeTag[scala.AnyRef]     = TypeTag.AnyRef
    val Object  : WeakTypeTag[java.lang.Object] = TypeTag.Object
    val Nothing : WeakTypeTag[scala.Nothing]    = TypeTag.Nothing
    val Null    : WeakTypeTag[scala.Null]       = TypeTag.Null


    def apply[T](mirror1: scala.reflect.api.Mirror[self.type], tpec1: TypeCreator): WeakTypeTag[T] =
      new WeakTypeTagImpl[T](mirror1.asInstanceOf[Mirror], tpec1)

    def unapply[T](ttag: WeakTypeTag[T]): Option[Type] = Some(ttag.tpe)
  }

  /* @group TypeTags */
  private class WeakTypeTagImpl[T](val mirror: Mirror, val tpec: TypeCreator) extends WeakTypeTag[T] {
    lazy val tpe: Type = tpec(mirror)
    def in[U <: Universe with Singleton](otherMirror: scala.reflect.api.Mirror[U]): U # WeakTypeTag[T] = {
      val otherMirror1 = otherMirror.asInstanceOf[scala.reflect.api.Mirror[otherMirror.universe.type]]
      otherMirror.universe.WeakTypeTag[T](otherMirror1, tpec)
    }
    private def writeReplace(): AnyRef = new SerializedTypeTag(tpec, concrete = false)
  }

  /**
   * A `TypeTag` is a [[scala.reflect.api.TypeTags#WeakTypeTag]] with the additional
   * static guarantee that all type references are concrete, i.e. it does <b>not</b> contain any references to
   * unresolved type parameters or abstract types.
   *
   * @see [[scala.reflect.api.TypeTags]]
   * @group TypeTags
   */
  @annotation.implicitNotFound(msg = "No TypeTag available for ${T}")
  trait TypeTag[T] extends WeakTypeTag[T] with Equals with Serializable {
    /**
     * @inheritdoc
     */
    override def in[U <: Universe with Singleton](otherMirror: scala.reflect.api.Mirror[U]): U # TypeTag[T]

    override def canEqual(x: Any) = x.isInstanceOf[TypeTag[_]]

    override def equals(x: Any) = x.isInstanceOf[TypeTag[_]] && this.mirror == x.asInstanceOf[TypeTag[_]].mirror && this.tpe == x.asInstanceOf[TypeTag[_]].tpe

    override def hashCode = mirror.hashCode * 31 + tpe.hashCode

    override def toString = "TypeTag[" + tpe + "]"
  }

  /**
   * Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.
   * @group TypeTags
   */
  object TypeTag {
    val Byte:    TypeTag[scala.Byte]       = new PredefTypeTag[scala.Byte]       (ByteTpe,    _.TypeTag.Byte)
    val Short:   TypeTag[scala.Short]      = new PredefTypeTag[scala.Short]      (ShortTpe,   _.TypeTag.Short)
    val Char:    TypeTag[scala.Char]       = new PredefTypeTag[scala.Char]       (CharTpe,    _.TypeTag.Char)
    val Int:     TypeTag[scala.Int]        = new PredefTypeTag[scala.Int]        (IntTpe,     _.TypeTag.Int)
    val Long:    TypeTag[scala.Long]       = new PredefTypeTag[scala.Long]       (LongTpe,    _.TypeTag.Long)
    val Float:   TypeTag[scala.Float]      = new PredefTypeTag[scala.Float]      (FloatTpe,   _.TypeTag.Float)
    val Double:  TypeTag[scala.Double]     = new PredefTypeTag[scala.Double]     (DoubleTpe,  _.TypeTag.Double)
    val Boolean: TypeTag[scala.Boolean]    = new PredefTypeTag[scala.Boolean]    (BooleanTpe, _.TypeTag.Boolean)
    val Unit:    TypeTag[scala.Unit]       = new PredefTypeTag[scala.Unit]       (UnitTpe,    _.TypeTag.Unit)
    val Any:     TypeTag[scala.Any]        = new PredefTypeTag[scala.Any]        (AnyTpe,     _.TypeTag.Any)
    val AnyVal:  TypeTag[scala.AnyVal]     = new PredefTypeTag[scala.AnyVal]     (AnyValTpe,  _.TypeTag.AnyVal)
    val AnyRef:  TypeTag[scala.AnyRef]     = new PredefTypeTag[scala.AnyRef]     (AnyRefTpe,  _.TypeTag.AnyRef)
    val Object:  TypeTag[java.lang.Object] = new PredefTypeTag[java.lang.Object] (ObjectTpe,  _.TypeTag.Object)
    val Nothing: TypeTag[scala.Nothing]    = new PredefTypeTag[scala.Nothing]    (NothingTpe, _.TypeTag.Nothing)
    val Null:    TypeTag[scala.Null]       = new PredefTypeTag[scala.Null]       (NullTpe,    _.TypeTag.Null)

    def apply[T](mirror1: scala.reflect.api.Mirror[self.type], tpec1: TypeCreator): TypeTag[T] =
      new TypeTagImpl[T](mirror1.asInstanceOf[Mirror], tpec1)

    def unapply[T](ttag: TypeTag[T]): Option[Type] = Some(ttag.tpe)
  }

  /* @group TypeTags */
  private class TypeTagImpl[T](mirror: Mirror, tpec: TypeCreator) extends WeakTypeTagImpl[T](mirror, tpec) with TypeTag[T] {
    override def in[U <: Universe with Singleton](otherMirror: scala.reflect.api.Mirror[U]): U # TypeTag[T] = {
      val otherMirror1 = otherMirror.asInstanceOf[scala.reflect.api.Mirror[otherMirror.universe.type]]
      otherMirror.universe.TypeTag[T](otherMirror1, tpec)
    }
    private def writeReplace(): AnyRef = new SerializedTypeTag(tpec, concrete = true)
  }

  /* @group TypeTags */
  private class PredefTypeCreator[T](copyIn: Universe => Universe#TypeTag[T]) extends TypeCreator {
    def apply[U <: Universe with Singleton](m: scala.reflect.api.Mirror[U]): U # Type = {
      copyIn(m.universe).asInstanceOf[U # TypeTag[T]].tpe
    }
  }

  /* @group TypeTags */
  private class PredefTypeTag[T](_tpe: Type, copyIn: Universe => Universe#TypeTag[T]) extends TypeTagImpl[T](rootMirror, new PredefTypeCreator(copyIn)) {
    override lazy val tpe: Type = _tpe
    private def writeReplace(): AnyRef = new SerializedTypeTag(tpec, concrete = true)
  }

  /**
   * Shortcut for `implicitly[WeakTypeTag[T]]`
   * @group TypeTags
   */
  def weakTypeTag[T](implicit attag: WeakTypeTag[T]) = attag

  /**
   * Shortcut for `implicitly[TypeTag[T]]`
   * @group TypeTags
   */
  def typeTag[T](implicit ttag: TypeTag[T]) = ttag

  // big thanks to Viktor Klang for this brilliant idea!
  /**
   * Shortcut for `implicitly[WeakTypeTag[T]].tpe`
   * @group TypeTags
   */
  def weakTypeOf[T](implicit attag: WeakTypeTag[T]): Type = attag.tpe

  /**
   * Shortcut for `implicitly[TypeTag[T]].tpe`
   * @group TypeTags
   */
  def typeOf[T](implicit ttag: TypeTag[T]): Type = ttag.tpe

  /**
   * Type symbol of `x` as derived from a type tag.
   * @group TypeTags
   */
  def symbolOf[T: WeakTypeTag]: TypeSymbol
}

private[scala] class SerializedTypeTag(var tpec: TypeCreator, var concrete: Boolean) extends Serializable {
  private def writeObject(out: java.io.ObjectOutputStream): Unit = {
    out.writeObject(tpec)
    out.writeBoolean(concrete)
  }

  private def readObject(in: java.io.ObjectInputStream): Unit = {
    tpec = in.readObject().asInstanceOf[TypeCreator]
    concrete = in.readBoolean()
  }

  private def readResolve(): AnyRef = {
    import scala.reflect.runtime.universe._
    if (concrete) TypeTag(rootMirror, tpec)
    else WeakTypeTag(rootMirror, tpec)
  }
}