parameterized type checking in Scala

Question

I want to be able to assert the type of a parameterized List in Scala. For e.g. I have the following types:

class Animal
class Cat extends Animal
class Dog extends Animal

And I instantiate lists as follows:

val l1 = List(new Cat())
val l2 = List(new Dog())
val m = l1 ::: l2

When I try to assert the type of list m using the scalatest Matchers, I am only able to assert the generic type, not the parameterized type.

import org.scalatest.Matchers._

m shouldBe a [List[_]]

What I actually want to do is to assert that m is of type List[Animal] and NOT of type List[Cat] or List[Dog]

Is there any way I can do that?

Does not the compiler already proves that for you? Or do you have a `List[Any]`? — Luis Miguel Mejía Suárez, Mar 18 '19 at 21:51
`scalatest.Matchers` appears to be runtime so it can't verify types erased by type erasure. — jwvh, Mar 18 '19 at 22:22

score 4 · Answer 1 · answered Mar 18 '19 at 22:20

You can't assert the actual type parameter in run time because of type erasure. And even if you circumvented the type erasure with somethinglike new MyList[Cat]{/** saves the actual type parameter in anonymous class definition */} it would make little sense since List[Cat] IS actually a List[Animal] in Scala.

score 4 · Answer 2 · answered Mar 19 '19 at 09:04

Type erasure will prevent you from checking the type parameter at runtime. This is because any List[T] becomes a List[Any] at runtime (the actual value of T is "erased").

However, as pointed out by others, the unit test you are writing is not needed because the compiler will automatically infer the type of m to be List[Animal]. This is because List is declared as covariant (the + in the List[+T] definition).

See this answer for more details about covariance.

parameterized type checking in Scala

2 Answers2