Why do I need an extra asInstanceOf - scala

Question

I wrote this code
listOfClassNames.map{ className =>
Try {
GuiceInjector.getInstance(Class.forName(className)).asInstanceOf[BaseClass]
} recover {
case _ => Option.empty[(String, BaseClass)]
} match {
case Success(bc) => Some((className, bc))
case _ => Option.empty[(String, BaseClass)]
}
}
The above code throws an error
type mismatch;
[error] found : List[Option[(String, Object)]]
[error] required: List[Option[(String, BaseClass)]]
Now if I change the code to
listOfClassNames.map{ className =>
Try {
GuiceInjector.getInstance(Class.forName(className)).asInstanceOf[BaseClass]
} recover {
case _ => Option.empty[(String, BaseClass)]
} match {
case Success(bc) => Some((className, bc.asInstanceOf[BaseClass]))
case _ => Option.empty[(String, BaseClass)]
}
}
Now it works. but according to me the second asInstanceOf is unnecessary because the first object itself was typecast. no?

Answer 1

You have to remove the call to recover. It's not needed anyway, because you process the error case inside match, but it messes up the types and correctness of your code.
Consider the types: Try { /* ... */.asInstanceOf[BaseClass] }
has type Try[BaseClass].
Then Try { /* ... */ } recover { case _ => Option.empty[(String, BaseClass)] } is a Try of the common supertype of BaseClass and Option[(String, BaseClass)], so it's Try[AnyRef].
Thus inside the match in case Success(bc) => this bc has type AnyRef, and at runtime it can be either an instance of BaseClass or None. If the original fetch of BaseClass from Guice fails, this bc is None, and you get a ClassCastException with the additional isInstanceOf.