Scala Reflection Library 2.13.16 - scala.reflect.api.TypeTags.WeakTypeTag
Keywords: Scala Reflection Library 2.13.16 scala.reflect.api.TypeTags.WeakTypeTag

Scala Reflection Library2.13.16 < Back
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# Packages

-  package root
  Definition Classes
    root
-  package scala
  Definition Classes
    root
-  package reflect
  Definition Classes
    scala
-  package api

  EXPERIMENTAL

  EXPERIMENTAL

  The Scala Reflection API (located in scala-reflect.jar).

  In Scala 2.10.0, the Scala Reflection API and its implementation have an "experimental" status. This means that the API and the docs are not complete and can be changed in binary- and source-incompatible manner in 2.10.1. This also means that the implementation has some known issues.

  The following types are the backbone of the Scala Reflection API, and serve as a good starting point for information about Scala Reflection:

  - scala.reflect.api.Symbols
  - scala.reflect.api.Types
  - scala.reflect.api.Mirrors
  - scala.reflect.api.Universe

  For more information about Scala Reflection, see the Reflection Guide

  Definition Classes
    reflect
-  trait TypeTags extends AnyRef

  A `TypeTag[T]` encapsulates the runtime type representation of some type `T`.

  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 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`. `ClassTag`s 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, `TypeTag`s 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

  `TypeTag`s 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 `Tag`s 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 `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`.
  - 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 `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 Reflection Guide: TypeTags

  Definition Classes
    api
  See also

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

- TypeTag
- WeakTypeTag

t

scala.reflect.api.TypeTags

# WeakTypeTag

###
Companion object WeakTypeTag

#### 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. 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 Reflection Guide: TypeTags

Annotations
  @implicitNotFound()
Source
  TypeTags.scala
See also

  scala.reflect.api.TypeTags

Linear Supertypes
java.io.Serializable, Equals, AnyRef, Any
Known Subclasses
TypeTag
Type Hierarchy

Ordering

1. Alphabetic
2. By Inheritance

Inherited

1. WeakTypeTag
2. Serializable
3. Equals
4. AnyRef
5. Any

Implicitly

1. by any2stringadd
2. by StringFormat
3. by Ensuring
4. by ArrowAssoc

1. Hide All
2. Show All

Visibility

1. Public
2. Protected

### Abstract Value Members

1.  abstract def in[U <: Universe with Singleton](otherMirror: Mirror[U]): in.U.WeakTypeTag[T]

   Migrates the expression into another mirror, jumping into a different universe if necessary.

   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).

2.  abstract val mirror: Universe.Mirror

   The underlying `Mirror` of this type tag.

3.  abstract def tpe: Universe.Type

   Reflective representation of type T.

### 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 Universe.WeakTypeTag[T] toany2stringadd[Universe.WeakTypeTag[T]] performed by method any2stringadd in scala.Predef.
   Definition Classes
     any2stringadd
4.  def ->[B](y: B): (Universe.WeakTypeTag[T], B)
   Implicit
     This member is added by an implicit conversion from Universe.WeakTypeTag[T] toArrowAssoc[Universe.WeakTypeTag[T]] 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 canEqual(x: Any): Boolean
   Definition Classes
     WeakTypeTag → Equals
8.  def clone(): AnyRef
   Attributes
     protected[lang]
   Definition Classes
     AnyRef
   Annotations
     @throws(classOf[java.lang.CloneNotSupportedException]) @native()
9.  def ensuring(cond: (Universe.WeakTypeTag[T]) => Boolean, msg: => Any): Universe.WeakTypeTag[T]
   Implicit
     This member is added by an implicit conversion from Universe.WeakTypeTag[T] toEnsuring[Universe.WeakTypeTag[T]] performed by method Ensuring in scala.Predef.
   Definition Classes
     Ensuring
10.  def ensuring(cond: (Universe.WeakTypeTag[T]) => Boolean): Universe.WeakTypeTag[T]
    Implicit
      This member is added by an implicit conversion from Universe.WeakTypeTag[T] toEnsuring[Universe.WeakTypeTag[T]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
11.  def ensuring(cond: Boolean, msg: => Any): Universe.WeakTypeTag[T]
    Implicit
      This member is added by an implicit conversion from Universe.WeakTypeTag[T] toEnsuring[Universe.WeakTypeTag[T]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
12.  def ensuring(cond: Boolean): Universe.WeakTypeTag[T]
    Implicit
      This member is added by an implicit conversion from Universe.WeakTypeTag[T] toEnsuring[Universe.WeakTypeTag[T]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
13.  final def eq(arg0: AnyRef): Boolean
    Definition Classes
      AnyRef
14.  def equals(x: Any): Boolean
    Definition Classes
      WeakTypeTag → Equals → AnyRef → Any
15.  def finalize(): Unit
    Attributes
      protected[lang]
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.Throwable])
16.  final def getClass(): Class[_ <: AnyRef]
    Definition Classes
      AnyRef → Any
    Annotations
      @native()
17.  def hashCode(): Int
    Definition Classes
      WeakTypeTag → AnyRef → Any
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
      WeakTypeTag → AnyRef → Any
24.  final def wait(): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
25.  final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
26.  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 Universe.WeakTypeTag[T] toStringFormat[Universe.WeakTypeTag[T]] 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): (Universe.WeakTypeTag[T], B)
   Implicit
     This member is added by an implicit conversion from Universe.WeakTypeTag[T] toArrowAssoc[Universe.WeakTypeTag[T]] 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 java.io.Serializable

### Inherited from Equals

### Inherited from AnyRef

### Inherited from Any

### Inherited by implicit conversion any2stringadd fromUniverse.WeakTypeTag[T] to any2stringadd[Universe.WeakTypeTag[T]]

### Inherited by implicit conversion StringFormat fromUniverse.WeakTypeTag[T] to StringFormat[Universe.WeakTypeTag[T]]

### Inherited by implicit conversion Ensuring fromUniverse.WeakTypeTag[T] to Ensuring[Universe.WeakTypeTag[T]]

### Inherited by implicit conversion ArrowAssoc fromUniverse.WeakTypeTag[T] to ArrowAssoc[Universe.WeakTypeTag[T]]

### Ungrouped