Scala asInstanceOf with parameterized types

Question

I would like to write a function that casts to type A, where A can be e.g. List[Int], or a more complicated parameterized type like Map[Int, List[Int]].

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Answer 1

You are indeed correct - type erasure means that you cannot "cast" in such a way as to distinguish between List[Int] and List[String], for example. However, you can improve on the cast which you are performing, whereby A is erased in such a way as to mean that you cannot distinguish between an Int and a String:

def cast[A](a : Any) = a.asInstanceOf[A]
//... is erased to
def erasedCast(a : Any) = a.asInstanceOf[Any]

What you need are reified generics, using manifests

def cast[A <: AnyRef : Manifest](a : Any) : A 
  = manifest[A].erasure.cast(a).asInstanceOf[A]

Whilst the final cast is erased to AnyRef, at least you should have the correct Class[_] instance (manifest.erasure) to get the top level type correct. In action:

scala> cast[String]("Hey")
res0: String = Hey

scala> cast[java.lang.Integer]("Hey")
  java.lang.ClassCastException
    at java.lang.Class.cast(Class.java:2990)
    at .cast(:7)
    at .(:9)

scala> cast[List[String]](List("Hey"))
res2: List[String] = List(Hey)

scala> cast[List[Int]](List("Hey"))
res3: List[Int] = List(Hey)

My advice is not to use nested reflection to decide whether the target was really a List[Int]: this is not generally feasible. For what should the following return?

cast[List[Int]](List[AnyVal](1, 2))