Scala cast to generic type (for generic numerical

Question

I'm trying to implement a generic function that wraps a mathematical Java function. For simplicity, we can assume that the Java function (Java 7) takes one parameter and returns a result, both of type java.lang.Double. Of course, the wrapper function should take a parameter and a result, both of generic but numeric type A. The problem is that I'm not able to cast the result back to type A in the wrapper function. Where/what is the problem?

Note: (I'm a newbie on Scala and used the following reference to solve the problem.)

• I'm using implicit type trait Numeric to enforce that template types are numeric. How do I implement a generic mathematical function in Scala
• I found 'import scala.collection.JavaConversions._' which I do not fully understand, but it seems to me that it's rather for conversion of Java collections and doesn't solve my problem, does it? Iterating over Java collections in Scala
• As described here, I tried to use Manifest to enforce correct casting (Variant B) which, however, ends up in an exception java.lang.ClassCastException. Scala asInstanceOf with parameterized types

Variant A

package test

object mytest {
  def f[A](x: A)(implicit num: Numeric[A]): A = {
    val result = new java.lang.Double(num.toDouble(x))
    result.asInstanceOf[A]
  }

  def main(args: Array[String]) {
    // 'Some code'
  }
}

'Some code' A1

result val result = f(3)

Output:

Exception in thread "main" java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer
    at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:105)
    at test.mytest$.main(test.scala:10)
    at test.mytest.main(test.scala)

'Some code' A2

println(f(3))

Output:

3.0

'Some code' A3

println(f(3).getClass)

Output:

int

Variant B

package test

object mytest {
  def f[A : Manifest](x: A)(implicit num: Numeric[A]): A = {
    val result = new java.lang.Double(num.toDouble(x))
    manifest[A].erasure.cast(result).asInstanceOf[A]
  }

  def main(args: Array[String]) {
    val result = f(3)
  }
}

Output:

(This is the same also for equivalents of variants A1, A2, and A3, because the exception is now thrown in line 6 of function f.)

Exception in thread "main" java.lang.ClassCastException: Cannot cast java.lang.Double to int
    at java.lang.Class.cast(Class.java:3176)
    at test.mytest$.f(test.scala:6)
    at test.mytest$.main(test.scala:10)
    at test.mytest.main(test.scala)

Answer 1

You can't cast Double to Integer anyway (because it's boxed). Numeric just allows you to have a set of mathematical operations (by import num._) on your T - nothing else (you don't need some mediate type for that). A2 works only because println expecting Any as a result of f(3), so T is automatically inferred to Any; println(f[Int](3)) will never work, f[Any](3) will always work.

If you want to implement generic function which operates Doubles (you may need that only if you have operations specific to Double) - you should better return Double. If you can't - you will have to construct the type manually by analizing T:

def f[A](x: A)(implicit num: Numeric[A]): A = {
   val result = new java.lang.Double(num.toDouble(x))
   (x match {
       case x: Double => result
       case x: Int => result.toInt
       case x: Float => result.toFloat
       case x: Long => result.toLong
   }).asInstanceOf[A]
}

The reason why you can't just do result.doubleValue.asInstanceOf[A] here is that A is already boxed. @specialized annotation doesn't work for asInstanceOf (type A is still boxed)

UPDATE: actually @specialized works:

def f[@specialized(Int, Double, Long, Float) A](x: A)(implicit num: Numeric[A]): A = {
   val result = new java.lang.Double(num.toDouble(x)) 

   result.doubleValue.asInstanceOf[A]
}

But it doesn't work in Scala REPL - so be careful!

P.S. Manifests are deprecated: use classTag/typeTag instead