TypeTags

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)

`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

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 culd 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
See also: scala.reflect.ClassTag, scala.reflect.api.TypeTags#TypeTag, scala.reflect.api.TypeTags#WeakTypeTag

Linear Supertypes

AnyRef, Any

Known Subclasses

JavaUniverse, Universe, Universe

Type Hierarchy Learn more about scaladoc diagrams

Type Members

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.
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.

Value Members

final def !=(arg0: AnyRef): Boolean

Definition Classes
AnyRef
final def !=(arg0: Any): Boolean

Definition Classes
Any
final def ##(): Int

Definition Classes
AnyRef → Any
def +(other: String): String

Implicit information
This member is added by an implicit conversion from TypeTags to StringAdd[TypeTags] performed by method StringAdd in scala.Predef.
Definition Classes
StringAdd
def ->[B](y: B): (TypeTags, B)

Implicit information
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()
final def ==(arg0: AnyRef): Boolean

Definition Classes
AnyRef
final def ==(arg0: Any): Boolean

Definition Classes
Any
object TypeTag extends Serializable

Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.
object WeakTypeTag extends Serializable

Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.
final def asInstanceOf[T0]: T0

Definition Classes
Any
def clone(): AnyRef

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( ... )
def ensuring(cond: (TypeTags) ⇒ Boolean, msg: ⇒ Any): TypeTags

Implicit information
This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: (TypeTags) ⇒ Boolean): TypeTags

Implicit information
This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: Boolean, msg: ⇒ Any): TypeTags

Implicit information
This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: Boolean): TypeTags

Implicit information
This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
final def eq(arg0: AnyRef): Boolean

Definition Classes
AnyRef
def equals(arg0: Any): Boolean

Definition Classes
AnyRef → Any
def finalize(): Unit

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( classOf[java.lang.Throwable] )
def formatted(fmtstr: String): String

Implicit information
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()
final def getClass(): Class[_]

Definition Classes
AnyRef → Any
def hashCode(): Int

Definition Classes
AnyRef → Any
final def isInstanceOf[T0]: Boolean

Definition Classes
Any
final def ne(arg0: AnyRef): Boolean

Definition Classes
AnyRef
final def notify(): Unit

Definition Classes
AnyRef
final def notifyAll(): Unit

Definition Classes
AnyRef
final def synchronized[T0](arg0: ⇒ T0): T0

Definition Classes
AnyRef
def toString(): String

Definition Classes
AnyRef → Any
def typeOf[T](implicit ttag: Universe.TypeTag[T]): Universe.Type

Shortcut for implicitly[TypeTag[T]].tpe
def typeTag[T](implicit ttag: Universe.TypeTag[T]): Universe.TypeTag[T]

Shortcut for implicitly[TypeTag[T]]
final def wait(): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
final def wait(arg0: Long, arg1: Int): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
final def wait(arg0: Long): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
def weakTypeOf[T](implicit attag: Universe.WeakTypeTag[T]): Universe.Type

Shortcut for implicitly[WeakTypeTag[T]].tpe
def weakTypeTag[T](implicit attag: Universe.WeakTypeTag[T]): Universe.WeakTypeTag[T]

Shortcut for implicitly[WeakTypeTag[T]]
def →[B](y: B): (TypeTags, B)

Implicit information
This member is added by an implicit conversion from TypeTags to ArrowAssoc[TypeTags] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc

trait TypeTags extends AnyRef

#1 Via the methods typeTag, classTag, or weakTypeTag

#2 Using an implicit parameter of type TypeTag[T], ClassTag[T], or WeakTypeTag[T]

#3 Context bound of a type parameter

WeakTypeTags

TypeTags and Manifests

Type Members

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

trait WeakTypeTag[T] extends Equals with Serializable

Value Members

final def !=(arg0: AnyRef): Boolean

final def !=(arg0: Any): Boolean

final def ##(): Int

def +(other: String): String

def ->[B](y: B): (TypeTags, B)

final def ==(arg0: AnyRef): Boolean

final def ==(arg0: Any): Boolean

object TypeTag extends Serializable

object WeakTypeTag extends Serializable

final def asInstanceOf[T0]: T0

def clone(): AnyRef

def ensuring(cond: (TypeTags) ⇒ Boolean, msg: ⇒ Any): TypeTags

def ensuring(cond: (TypeTags) ⇒ Boolean): TypeTags

def ensuring(cond: Boolean, msg: ⇒ Any): TypeTags

def ensuring(cond: Boolean): TypeTags

final def eq(arg0: AnyRef): Boolean

def equals(arg0: Any): Boolean

def finalize(): Unit

def formatted(fmtstr: String): String

final def getClass(): Class[_]

def hashCode(): Int

final def isInstanceOf[T0]: Boolean

final def ne(arg0: AnyRef): Boolean

final def notify(): Unit

final def notifyAll(): Unit

final def synchronized[T0](arg0: ⇒ T0): T0

def toString(): String

def typeOf[T](implicit ttag: Universe.TypeTag[T]): Universe.Type

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

final def wait(): Unit

final def wait(arg0: Long, arg1: Int): Unit

final def wait(arg0: Long): Unit

def weakTypeOf[T](implicit attag: Universe.WeakTypeTag[T]): Universe.Type

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

def →[B](y: B): (TypeTags, B)

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion StringAdd from TypeTags to StringAdd[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

Ungrouped

#2 Using an implicit parameter of type `TypeTag[T]`, `ClassTag[T]`, or `WeakTypeTag[T]`

`WeakTypeTag`s