Is it possible to convert a TypeTag to a Manifest?

Question

Our library uses TypeTags, but now we need to interact with another library which requires Manifests. Is there any simple way to create a Manifest from a TypeTag?

Answer 1

gourlaysama's anwer uses Class[_], thus type arguments are being erased. I've come up with an implementation that preserves the type arguments here: How to maintain type parameter during TypeTag to Manifest conversion?

Here's the code:

  def toManifest[T:TypeTag]: Manifest[T] = {
    val t = typeTag[T]
    val mirror = t.mirror
    def toManifestRec(t: Type): Manifest[_] = {
      val clazz = ClassTag[T](mirror.runtimeClass(t)).runtimeClass
      if (t.typeArgs.length == 1) {
        val arg = toManifestRec(t.typeArgs.head)
        ManifestFactory.classType(clazz, arg)
      } else if (t.typeArgs.length > 1) {
        val args = t.typeArgs.map(x => toManifestRec(x))
        ManifestFactory.classType(clazz, args.head, args.tail: _*)
      } else {
        ManifestFactory.classType(clazz)
      }
    }
    toManifestRec(t.tpe).asInstanceOf[Manifest[T]]
  }

Answer 2

If you try naively to summon a Manifest when a TypeTag is present, the compiler will give you a hint as to the solution:

import reflect.runtime.universe._
import reflect.ClassTag

def test[A : TypeTag] = manifest[A]

---

error: to create a manifest here, it is necessary to interoperate with the type
tag `evidence$1` in scope.
however typetag -> manifest conversion requires a class tag for the corresponding
type to be present.
to proceed add a class tag to the type `A` (e.g. by introducing a context bound)
and recompile.
   def test[A : TypeTag] = manifest[A]
                                   ^

So if you have a ClassTag in scope, the compiler will be able to create the necessary Manifest. You have two options:

• Add a second context bound everywhere TypeTag is, as in:

  def test[A : TypeTag : ClassTag] = manifest[A] // this compiles
  

• Or first convert the TypeTag to a ClassTag, then ask for a Manifest:

  def test[A](implicit ev: TypeTag[A]) = {
    // typeTag to classTag
    implicit val cl = ClassTag[A]( ev.mirror.runtimeClass( ev.tpe ) )
  
    // with an implicit classTag in scope, you can get a manifest
    manifest[A]
  }