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

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)

Answer 4

To consider inheritance:
val what = c match {
case q if classOf[Int].isAssignableFrom(q) => "int!"
case q if classOf[Float].isAssignableFrom(q) => "float!"
}