How to write "asInstanceOfOption" in Scala

Question

Is it possible to write an "asInstanceOfOption" method that would do what is intended by the following (bogus) code?

def asInstanceOfOption[T](o: Any): Option[T] =
   if (o.isInstanceOf[T]) Some(o.asInstanceOf[T]) else None

Answer 1

You could use manifests to get around the fact that the type T is erased at compile time:

scala> import scala.reflect._
import scala.reflect._

scala> def asInstanceOfOption[B](x : Any)(implicit m: Manifest[B]) : Option[B] = {
   | if (Manifest.singleType(x) <:< m)
   |   Some(x.asInstanceOf[B])
   | else
   |   None
   | }
asInstanceOfOption: [B](x: Any)(implicit m: scala.reflect.Manifest[B])Option[B]

Then this could be used:

scala> asInstanceOfOption[Int]("Hello")
res1: Option[Int] = None

scala> asInstanceOfOption[String]("World")
res2: Option[String] = Some(World)

You could even use implicit conversions to get this to be a method available on Any. I think I prefer the method name matchInstance:

implicit def any2optionable(x : Any) = new { //structural type
  def matchInstance[B](implicit m: Manifest[B]) : Option[B] = {
    if (Manifest.singleType(x) <:< m)
      Some(x.asInstanceOf[B])
    else
      None
    } 
  }   
}

Now you can write code like:

"Hello".matchInstance[String] == Some("Hello") //true
"World".matchInstance[Int] == None             //true