Scala generic method - No ClassTag available for T - when using Collection

Question

I want to leverage Scala reflection to implicitly pass a ClassTag.

There are plenty solutions on StackOverflow on how to accomplish this.

import scala.reflect.ClassTag

// animal input
trait AnimalInput {}
case class DogInput() extends AnimalInput
case class CatInput() extends AnimalInput

// animals
trait Animal[T <: AnimalInput] {
  def getInput(implicit ct: ClassTag[T]): Class[T] = {
    ct.runtimeClass.asInstanceOf[Class[T]]
  }
}
object Dog extends Animal[DogInput] {}
object Cat extends Animal[CatInput] {}

I can test that this is working well:

println(Dog.getInput) // Success: "DogInput"
println(Cat.getInput) // Success: "CatInput"

The problem is, the second I reference these objects in any collection, I run into trouble:

// Failure: "No ClassTag available for animal.T"
List(Dog, Cat).foreach(animal => println(animal.getInput)) 

I think I understand why this is happening but I'm not sure how to work around it.

Thank you in advance for your help!

Liberati

You could store the classtag. But what do you really want to achieve with all this? Sounds like XY problem. – Ballerina

This code compiles fine – Schrader

@AdamBerry Please see updates with magnet and with abstract class – Woorali

@Schrader No ClassTag available for animal.T would be for type member rather then type parameter scastie.scala-lang.org/v79Y37eDRXuoauWA9b8uhQ OP's code compiled but worked not as he wanted. – Woorali

Answer 1

Actually, I can't reproduce No ClassTag available for animal.T. Your code compiles and runs in Scala 2.13.9: https://scastie.scala-lang.org/DmytroMitin/lonnNg0fR1qb4Lg7ChDBqg

Maybe by failure you meant that for collection you don't receive classes DogInput, CatInput.

Actually, I managed to reproduce No ClassTag available for animal.T for type member T <: AnimalInput rather than type parameter: https://scastie.scala-lang.org/v79Y37eDRXuoauWA9b8uhQ

This question is very close to recent question Trying to extract the TypeTag of a Sequence of classes that extend a trait with different generic type parameters

See the reasons there.

Either use a heterogeneous collection

object classPoly extends Poly1 {
  implicit def cse[T <: AnimalInput : ClassTag, A](implicit
    ev: A <:< Animal[T]
  ): Case.Aux[A, Class[T]] =
    at(_ => classTag[T].runtimeClass.asInstanceOf[Class[T]])
}

(Dog :: Cat :: HNil).map(classPoly).toList.foreach(println)
// class DogInput
// class CatInput

or use runtime reflection

trait Animal[T <: AnimalInput] {
  def getInput: Class[T] = {
    val classSymbol = runtimeMirror.classSymbol(this.getClass)
    val animalSymbol = typeOf[Animal[_]].typeSymbol 
    val extendeeType = classSymbol.typeSignature.baseType(animalSymbol)
    val extenderSymbol = extendeeType.typeArgs.head.typeSymbol.asClass
    runtimeMirror.runtimeClass(extenderSymbol).asInstanceOf[Class[T]]
  }
}

List(Dog, Cat).foreach(animal => println(animal.getInput))
// class DogInput
// class CatInput

The easiest is

trait Animal[T <: AnimalInput] {
  def getInput: Class[T]
}
object Dog extends Animal[DogInput] {
  val getInput = classOf[DogInput]
}
object Cat extends Animal[CatInput] {
  val getInput = classOf[CatInput]
}

---

One more option is magnet pattern (1 2 3 4 5 6)

trait AnimalMagnet[T <: AnimalInput] {
  def getInput: Class[T]
}

import scala.language.implicitConversions

implicit def animalToMagnet[A, T <: AnimalInput : ClassTag](a: A)(implicit
  ev: A <:< Animal[T]
): AnimalMagnet[T] = new AnimalMagnet[T] {
  override def getInput: Class[T] = classTag[T].runtimeClass.asInstanceOf[Class[T]]
}

List[AnimalMagnet[_]](Dog, Cat).foreach(animal => println(animal.getInput))
//class DogInput
//class CatInput

---

Also you can move ClassTag implicit from the method to the trait (and make the trait an abstract class)

abstract class Animal[T <: AnimalInput](implicit ct: ClassTag[T]) {
  def getInput: Class[T] = ct.runtimeClass.asInstanceOf[Class[T]]
}

List(Dog, Cat).foreach(animal => println(animal.getInput))
// class DogInput
// class CatInput

Woorali