How to downcast with reflection in Scala?

Question

Suppose I have a non-sealed trait A and need a function convert[T <: A]: A => Option[T]:

def convert[T <: A](a: A): Option[T] = // don't compile
  if (a is instance of T) then Some(a) else None

How would you suggest implement that convert ?

P.S I don't like reflection but need to deal with legacy, which uses it.

You could use `ClassTag`, see [this answer](http://stackoverflow.com/a/18136667/406435). `def convert[T <: A : ClassTag](a: A) = {val clazz = implicitly[ClassTag[T]].runtimeClass; if (clazz.isInstance(a)) Some(a.asInstanceOf[T]) else None}` — senia, Mar 30 '15 at 12:00
@senia Why `Some(a.asInstanceOf[T])` but not just `Some(a)` ? — Michael, Mar 30 '15 at 12:32
type of `a` is `A`, so type of `Some(a)` is `Some[A]`, not `Some[T]` — senia, Mar 30 '15 at 12:43
@senia What if I replace `if (clazz.isInstance(a)) ` with `clazz == a.getClass` ? Does it work the same ? — Michael, Mar 30 '15 at 12:49
No. `a` could be an instance of subtype of `T`. In this case `a` is instance of `T`, but class of `a` is not `T`. — senia, Mar 30 '15 at 12:53

score 2 · Accepted Answer · answered Mar 30 '15 at 13:14

2

Rather than isInstance, you can use the ClassTag in a match expression, which is slightly clearer:

def convert[T <: A](a: A)(implicit ct: ClassTag[T]) = a match {
  case ct(t) => Some(t)
  case _ => None
}

answered Mar 30 '15 at 13:14

lmm

Very nice ! Did not know about `ClassTag.unapply`. Thank you. – Michael Mar 30 '15 at 13:36
That said, you can even write `case t: T => Some(t)`, Scala is smart enough to figure it out. – Vladimir Matveev Mar 30 '15 at 14:00
@VladimirMatveev Did not get it, sorry. Could you give a full example, please ? – Michael Mar 30 '15 at 14:34
1

@Michael, sure, see [here](https://gist.github.com/netvl/de3e4388d2e8f2fbc143). In other words, adding `ClassTag` or `TypeTag` context bound allows matching with the bounded generic parameter type safely. – Vladimir Matveev Mar 30 '15 at 14:40
@VladimirMatveev Thank you. Neat and very simple. Probably the best possible solution. – Michael Mar 30 '15 at 17:46

1 Answers1