在Scala中，Universe.TypeTag，TypeRef和Type有什么区别？

Question

There are 3 different classes all bounded to a universe: TypeTag , TypeRef , and Type . Why do we need all 3? If I only have a Type , how do I convert it to a TypeTag or TypeRef ?

Eg I have obtain a common super type through a Scala reflection API function:

val at = ...some TypeTag...
val bt = ...some TypeTag...

val tpe = ScalaReflection.universe.lub(List(at.tpe, bt.tpe))
ScalaReflection.universe.(tpe)

How do I convert it into TypeTag or TypeRef ?

Answer 1

TypeRef is a specific case of Type :

abstract type TypeRef >: Null <: Universe.TypeRefApi with Universe.Type 

You can check if a Type is actually a TypeRef by pattern-matching:

tpe match {
  case TypeRef(prefix, sym, args) =>
  // or alternately
  case typeRef: TypeRef =>
}

TypeTag[A] is a way to get access to Type which 1) works for different mirrors; 2) can be generated by the compiler automatically from the type parameter (note that Type is not generic).

To convert Type to TypeTag , from How to create a TypeTag manually? :

import scala.reflect.runtime.universe._

def typeToTypeTag[T](
  tpe: Type,
  mirror: reflect.api.Mirror[reflect.runtime.universe.type]
): TypeTag[T] = {
  TypeTag(mirror, new reflect.api.TypeCreator {
    def apply[U <: reflect.api.Universe with Singleton](m: reflect.api.Mirror[U]) = {
      assert(m eq mirror, s"TypeTag[$tpe] defined in $mirror cannot be migrated to $m.")
      tpe.asInstanceOf[U#Type]
    }
  })
}