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scala.reflect.api

TypeTags

trait TypeTags extends AnyRef

A TypeTag[T] encapsulates the runtime type representation of some type T. Like scala.reflect.Manifest, the prime use case of TypeTags is to give access to erased types. However, TypeTags should be considered to be a richer replacement of the pre-2.10 notion of a Manifest, that are, in addition, fully integrated with Scala reflection.

There exist three different types of TypeTags:

  • scala.reflect.api.TypeTags#TypeTag.
    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].
  • scala.reflect.ClassTag.
    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. ClassTags provide access only to the runtime class of a type. Analogous to scala.reflect.ClassManifest
  • scala.reflect.api.TypeTags#WeakTypeTag.
    A type descriptor for abstract types (see description below).

Like Manifests, TypeTags are always generated by the compiler, and can be obtained in three ways:

#1 Via the methods typeTag, classTag, or 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)
WeakTypeTags

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

TypeTags correspond loosely to the pre-2.10 notion of scala.reflect.Manifests. 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.

  • scala.reflect.OptManifest is not supported.
    This is because Tags can reify arbitrary types, so they are always available. -
  • There is no equivalent for scala.reflect.AnyValManifest.
    Instead, one can compare their Tag with one of the base Tags (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.
  • There are no replacement for factory methods defined in the Manifest companion objects.
    Instead, one could generate corresponding types using the reflection APIs provided by Java (for classes) and Scala (for types).
  • Certain manifest operations(i.e., <:<, >:> and typeArguments) are not supported.
    Instead, one could use the reflection APIs provided by Java (for classes) and Scala (for types).

In Scala 2.10, scala.reflect.ClassManifests are deprecated, and it is planned to deprecate scala.reflect.Manifest in favor of TypeTags and ClassTags in an upcoming point release. Thus, it is advisable to migrate any Manifest-based APIs to use Tags.

For more information about TypeTags, see the Reflection Guide: TypeTags

Self Type
Universe
Source
TypeTags.scala
See also

scala.reflect.ClassTag, scala.reflect.api.TypeTags#TypeTag, scala.reflect.api.TypeTags#WeakTypeTag

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Type Members

  1. trait TypeTag[T] extends Universe.WeakTypeTag[T] with Equals with Serializable

    A TypeTag is a scala.reflect.api.TypeTags#WeakTypeTag with the additional static guarantee that all type references are concrete, i.e.

    A TypeTag is a scala.reflect.api.TypeTags#WeakTypeTag with the additional static guarantee that all type references are concrete, i.e. it does not contain any references to unresolved type parameters or abstract types.

    Annotations
    @implicitNotFound( "No TypeTag available for ${T}" )
    See also

    scala.reflect.api.TypeTags

  2. trait WeakTypeTag[T] extends Equals with Serializable

    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.

    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 TypeTags 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 TypeTags, see the Reflection Guide: TypeTags

    Annotations
    @implicitNotFound( "No WeakTypeTag available for ${T}" )
    See also

    scala.reflect.api.TypeTags

Abstract Value Members

  1. abstract def symbolOf[T](implicit arg0: Universe.WeakTypeTag[T]): Universe.TypeSymbol

    Type symbol of x as derived from a type tag.

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 TypeTags to any2stringadd[TypeTags] performed by method any2stringadd in scala.Predef.
    Definition Classes
    any2stringadd
  4. def ->[B](y: B): (TypeTags, B)
    Implicit
    This member is added by an implicit conversion from TypeTags to ArrowAssoc[TypeTags] 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[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @native() @throws( ... )
  8. def ensuring(cond: (TypeTags) ⇒ Boolean, msg: ⇒ Any): TypeTags
    Implicit
    This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  9. def ensuring(cond: (TypeTags) ⇒ Boolean): TypeTags
    Implicit
    This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  10. def ensuring(cond: Boolean, msg: ⇒ Any): TypeTags
    Implicit
    This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  11. def ensuring(cond: Boolean): TypeTags
    Implicit
    This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  12. final def eq(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  13. def equals(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  14. def finalize(): Unit
    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( classOf[java.lang.Throwable] )
  15. def formatted(fmtstr: String): String
    Implicit
    This member is added by an implicit conversion from TypeTags to StringFormat[TypeTags] performed by method StringFormat in scala.Predef.
    Definition Classes
    StringFormat
    Annotations
    @inline()
  16. final def getClass(): Class[_]
    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  17. def hashCode(): Int
    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  18. final def isInstanceOf[T0]: Boolean
    Definition Classes
    Any
  19. final def ne(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  20. final def notify(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native()
  21. final def notifyAll(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native()
  22. final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
    AnyRef
  23. def toString(): String
    Definition Classes
    AnyRef → Any
  24. def typeOf[T](implicit ttag: Universe.TypeTag[T]): Universe.Type

    Shortcut for implicitly[TypeTag[T]].tpe

  25. def typeTag[T](implicit ttag: Universe.TypeTag[T]): Universe.TypeTag[T]

    Shortcut for implicitly[TypeTag[T]]

  26. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  27. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  28. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @native() @throws( ... )
  29. def weakTypeOf[T](implicit attag: Universe.WeakTypeTag[T]): Universe.Type

    Shortcut for implicitly[WeakTypeTag[T]].tpe

  30. def weakTypeTag[T](implicit attag: Universe.WeakTypeTag[T]): Universe.WeakTypeTag[T]

    Shortcut for implicitly[WeakTypeTag[T]]

  31. def [B](y: B): (TypeTags, B)
    Implicit
    This member is added by an implicit conversion from TypeTags to ArrowAssoc[TypeTags] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc
  32. object TypeTag extends Serializable

    Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.

  33. object WeakTypeTag extends Serializable

    Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion any2stringadd from TypeTags to any2stringadd[TypeTags]

Inherited by implicit conversion StringFormat from TypeTags to StringFormat[TypeTags]

Inherited by implicit conversion Ensuring from TypeTags to Ensuring[TypeTags]

Inherited by implicit conversion ArrowAssoc from TypeTags to ArrowAssoc[TypeTags]

TypeTags

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