Scala asInstanceOf with parameterized types
scala
Solution
Unfortunately, this in an inherent limitation of `asInstanceOf`. I'm actually surprised to see the scaladoc mention it in details:
Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression `1.asInstanceOf[String]` will throw a `ClassCastException` at runtime, while the expression `List(1).asInstanceOf[List[String]]` will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.
If you're mainly concerned about failing fast on wrong cast for traversable which would likely be the main issue when getting stuff back from your DB/memcached interface, I was playing around forcing a cast of the head for traversable objects:
def failFastCast[A: Manifest, T[A] <: Traversable[A]](as: T[A], any: Any) = {
val res = any.asInstanceOf[T[A]]
if (res.isEmpty) res
else {
manifest[A].newArray(1).update(0, res.head) // force exception on wrong type
res
}
}
On a simple example it works:
scala> val x = List(1, 2, 3): Any
x: Any = List(1, 2, 3)
scala> failFastCast(List[String](), x)
java.lang.ArrayStoreException: java.lang.Integer
[...]
scala> failFastCast(List[Int](), x)
res22: List[Int] = List(1, 2, 3)
But not on a more complex one:
val x = Map(1 -> ("s" -> 1L)): Any
failFastCast(Map[Int, (String, String)](), x) // no throw
I wonder if there is a way to recursively drill down into A to keep casting until there is no more type parameters...
Problem
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]]. ``` def castToType[A](x: Any): A = { // throws if A is not the right type x.asInstanceOf[A] } ``` Right now, due to type erasure (I believe), the code merrily works even when the type is not correct. The error only manifests on access, witha ClassCastException. ``` val x = List(1, 2, 3) val y = castToType[List[String]](x) y(0) --> throws java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String ``` Is there a way I can use manifests to make this work properly? Thanks!