Pattern matching on a list of generics - scala

Question

I have a class which contains a sequence of a generic type like:
sealed trait Interface {}
case class Imp1() extends Interface {}
case class Imp2() extends Interface {}
case class Wrapper[+I <: Interface](interface: I) {}
case class WrapperList(list: Seq[Wrapper[Interface]]) {
...
}
In the WrapperList I want to be able to iterate through the Sequence of wrappers and pattern match on each ones type, eg.
def getImp1s(remaining: Seq[Wrapper[Interface]] = list): Seq[Wrapper[Imp1]] = {
if (remaining.length == 0) Seq()
else remaining.head match {
case wrapper: Wrapper[Imp1] => get(remaining.tail) :+ wrapper
case _ => get(remaining.tail)
}
}
As you can probably guess I'm running into
non-variable type argument Playground.Imp1 in type pattern Playground.Wrapper[Playground.Imp1] is unchecked since it is eliminated by erasure
To overcome this I was under the impression I could use TypeTags or ClassTags to preserve the type, something like:
case class Wrapper[+I <: Interface](interface: I)(implicit tag: TypeTag[I]) {}
However this doesn't seem to work, I still get the same warning. Could someone explain how I can match using the TypeTag? I'd rather avoid creating concrete versions of my generic class which extend a generic abstract class, but do understand that this is maybe the easiest solution.
Thanks for your help :)

Answer 1

Try
import shapeless.TypeCase
val `Wrapper[Imp1]` = TypeCase[Wrapper[Imp1]]
def getImp1s(remaining: Seq[Wrapper[Interface]]): Seq[Wrapper[Imp1]] = {
if (remaining.isEmpty) Seq()
else remaining.head match {
case `Wrapper[Imp1]`(wrapper) => getImp1s(remaining.tail) :+ wrapper
case _ => getImp1s(remaining.tail)
}
}
getImp1s(Seq(Wrapper(Imp1()), Wrapper(Imp2()), Wrapper(new Interface {})))
// List(Wrapper(Imp1()))
getImp1s(Seq(Wrapper(Imp2()), Wrapper(Imp1()), Wrapper(new Interface {})))
// List(Wrapper(Imp1()))
The same can be achieved without Shapeless with custom extractor
object `Wrapper[Imp1]` {
def unapply(arg: Any): Option[Wrapper[Imp1]] = arg match {
case Wrapper(Imp1()) => Some(Wrapper(Imp1()))
case _ => None
}
}
or directly
def getImp1s(remaining: Seq[Wrapper[Interface]]): Seq[Wrapper[Imp1]] = {
if (remaining.isEmpty) Seq()
else remaining.head match {
case Wrapper(Imp1()) => getImp1s(remaining.tail) :+ Wrapper(Imp1())
case _ => getImp1s(remaining.tail)
}
}
or
def getImp1s(remaining: Seq[Wrapper[Interface]]): Seq[Wrapper[Imp1]] = {
if (remaining.isEmpty) Seq()
else remaining.head match {
case Wrapper(_: Imp1) => getImp1s(remaining.tail) :+ Wrapper(Imp1())
case _ => getImp1s(remaining.tail)
}
}