How to create a TypeTag manually?

Question

I\'m interested in creating a TypeTag manually (since 2.10M5):

object X {
  import reflect.runtime.universe._
  def tt[A : TypeTag](a: A) = typeTag[A] // how         


        

Answer 1

Based on Get TypeTag[A] from Class[A]:

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]
    }
  })
}

For example this can be used to get the TypeTag for a part of another TypeTag:

def inside[A, B](tag: TypeTag[(A, B)]): (TypeTag[A], TypeTag[B]) = {
  val tpes = tag.tpe.asInstanceOf[TypeRefApi].args
  val tagA = typeToTypeTag[A](tpes(0), tag.mirror)
  val tagB = typeToTypeTag[B](tpes(1), tag.mirror)
  return (tagA, tagB)
}

This works in Scala 2.10.2:

scala> inside(typeTag[(Int, Double)])
res0: (reflect.runtime.universe.TypeTag[Int], reflect.runtime.universe.TypeTag[Double]) = (TypeTag[Int],TypeTag[Double])

The limitation of being tied to a particular Mirror is likely not a problem as long as you don't have multiple ClassLoaders.