How can I match classes in a Scala "match" statement?

Question

How can I use a "match" statement to identify the value of a class variable? The following is invalid, and I can't find an acceptable variant -- other than if ... else if ... else ...

val c: Class[_] = classOf[Int]
val what = c match { case classOf[Int] => "int!"; case classOf[Float] => "float!" }

The compiler complains: error: not found: type classOf

And of course, I can't use Class[Int] because that type information is erased:

c match { case Class[Int] => "int!"; case Class[Float] => "float!" }
error: type Class of type Class does not take type parameters.

I've also tried variants like Int.class, all to no avail. (And I don't really want to convert to strings: I feel it's important to have the compiler catch renamed/moved classes.)

Am I being dense, or have I stumbled into a Scala blind spot?

Answer 1

The verbose case comparison works:

val what = c match {   case q if q == classOf[Int] => "int!"   case q if q == classOf[Float] => "float!" } 

Of course, being a lower-case identifier, classOf should not work directly in a case statement anyway. However, neither does an escaped

case `classOf`[Int] 

work in this case, so you’ll have to go with the if-guard.

Answer 2

You can match on class values if you create a stable identifier (ie. a val) for them,

scala> val c: Class[_] = classOf[Int]
c: Class[_] = int

scala> val ClassOfInt = classOf[Int]
ClassOfInt: java.lang.Class[Int] = int

scala> val ClassOfFloat = classOf[Float]
ClassOfFloat: java.lang.Class[Float] = float

scala> val what = c match {
     |     case ClassOfInt => "int!"
     |     case ClassOfFloat => "float!"
     | }
what: String = int!

Note that you can't match on type (ie. Class[Int]) because erasure means that the different type instantiations of Class[T] are indistinguishable at runtime ... hence the warning below

scala> val what = c match {
     |     case _: Class[Int] => "int!"
     |     case _: Class[Float] => "float!"
     | }
warning: there were 2 unchecked warnings; re-run with -unchecked for details
what: java.lang.String = int!

Answer 3

I encountered the same problem and placing the class in a 'stable identifier' wasn't that practical. I found the next best thing was to have tidy 'else if' statements.

Using this method:

private def is[T <: AnyRef : Manifest](implicit cls: Class[_]) = 
    cls == manifest[T].runtimeClass

I can write:

  implicit val arg = cls   
  if (is[ClassA]) ...
  else if (is[ClassB]) ...
  ...
  else throw new IllegalArgumentException("Unknown class: " + cls)