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, aTypeTag[List[String]]contains all type information, in this case, of typescala.List[String]. - scala.reflect.ClassTag.
A partial type descriptor of a Scala type. For example, aClassTag[List[String]]contains only the erased class type information, in this case, of typescala.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 becauseTags can reify arbitrary types, so they are always available. - - There is no equivalent for scala.reflect.AnyValManifest.
Instead, one can compare theirTagwith one of the baseTags (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
Manifestcompanion 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
-
trait
TypeTag[T] extends Universe.WeakTypeTag[T] with Equals with Serializable
A
TypeTagis a scala.reflect.api.TypeTags#WeakTypeTag with the additional static guarantee that all type references are concrete, i.e.A
TypeTagis 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
-
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 ofTcan be accessed via thetpefield.If an implicit value of type
WeakTypeTag[T]is required, the compiler will create one, and the reflective representation ofTcan be accessed via thetpefield. Components ofTcan be references to type parameters or abstract types. Note thatWeakTypeTagmakes an effort to be as concrete as possible, i.e. ifTypeTags 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 expectsTto be perhaps be partially abstract, but requires special care to handle this case. However, ifTis 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
Abstract Value Members
-
abstract
def
symbolOf[T](implicit arg0: Universe.WeakTypeTag[T]): Universe.TypeSymbol
Type symbol of
xas derived from a type tag.
Concrete Value Members
-
final
def
!=(arg0: Any): Boolean
- Definition Classes
- AnyRef → Any
-
final
def
##(): Int
- Definition Classes
- AnyRef → Any
- def +(other: String): String
- def ->[B](y: B): (TypeTags, B)
-
final
def
==(arg0: Any): Boolean
- Definition Classes
- AnyRef → Any
-
final
def
asInstanceOf[T0]: T0
- Definition Classes
- Any
-
def
clone(): AnyRef
- Attributes
- protected[java.lang]
- Definition Classes
- AnyRef
- Annotations
- @native() @throws( ... )
- 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
- 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
-
final
def
getClass(): Class[_]
- Definition Classes
- AnyRef → Any
- Annotations
- @native()
-
def
hashCode(): Int
- Definition Classes
- AnyRef → Any
- Annotations
- @native()
-
final
def
isInstanceOf[T0]: Boolean
- Definition Classes
- Any
-
final
def
ne(arg0: AnyRef): Boolean
- Definition Classes
- AnyRef
-
final
def
notify(): Unit
- Definition Classes
- AnyRef
- Annotations
- @native()
-
final
def
notifyAll(): Unit
- Definition Classes
- AnyRef
- Annotations
- @native()
-
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
- @native() @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)
-
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.