Question

Say I have something like this:

obj match {
    case objTypeOne : TypeOne => Some(objTypeOne)
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case _ => None
}

Now I want to generalise, to pass in one of the types to match:

obj match {
    case objTypeOne : clazz => Some(objTypeOne)
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case _ => None
}

But this isn’t allowed, I think for syntactic rather than semantic reasons (although I guess also that even though the clazz is a Class[C] the type is erased and so the type of the Option will be lost).

I ended up with:

if(clazzOne.isAssignableFrom(obj.getClass)) Some(clazz.cast(obj))
if(obj.isInstanceOf[TypeTwo]) Some(obj.asInstanceOf[TypeTwo])
None

I just wondered if there was a nicer way.

Answer 1

You could use pattern guards to achieve that. Try something like this:

obj match {
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case objTypeOne if clazz.isAssignableFrom(objTypeOne.getClass) => Some(clazz.cast(objTypeOne))
    case _ => None
}