Get TypeTag[A] from Class[A]

Question

I have createOld method that I need to override and I cannot change it. I would like to use TypeTag to pattern match provided type in createN

Answer 1

So it is not very safe and correct (cause you don't use the power of scala type system), but you can make (using your logic) to do the following:

def createNew[A](implicit t: TypeTag[A]): A = {
  val result: Any = t.tpe.toString match {
    case "C1" => new C1
    case "C2" => new C2
  }
  result.asInstanceOf[A]
}

createNew[C1] //> its all ok
createNew[C2] //> its all ok
createNew[C3] //> crashes here; lets pretend we got C3 class

To use it with createOld, just pass implicit argument:

def createOld[A](c: Class[A])(implicit t: TypeTag[A]): A = createNew[A]

createOld[C1] //> its all ok
createOld[C2] //> its all ok
createOld[C3] //> crashes here; lets pretend we got C3 class

I think I should not tell you twice that it is not very good. We can improve this code by using shapeless:

Lets create a poly function, which has a TypeTag as an argument:

 import shapeless._; import scala.reflect.runtime.universe._;

 def getTypeTag[T](implicit t: TypeTag[T]) = t //> to get TypeTag of a class

 // here is low prority implicit
 trait createPolyNewErr extends Poly1 {
   implicit def newErr[T] = at[T](_ => "Error can not create object of this class")
 } 

 object createPolyBew extends createPolyNewError {
   implicit def newC1 = at[TypeTag[C1]](_ => new C1)
   implicit def newC2 = at[TypeTag[C2]](_ => new C2)
 }

 createPolyNew(getTypeTag[C1]) //> success
 createPolyNew(getTypeTag[C2]) //> success
 createPolyNew(getTypeTag[C3]) //> String: Error can not create object of this class no crash!

We also can write a function, in order not to use function getTypeTag[T] every time:

 def createPoly[T]
 (implicit t: TypeTag[T],
         cse: poly.Case[createPolyNew.type, TypeTag[T] :: HNil]) = cse(t)

 createPoly[C1] //> its all ok
 createPoly[C2] //> its all ok
 createPoly[C3] //> String: Error can not create object of this class no crash!