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 i

Answer 1

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 2

To consider inheritance:

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

Answer 3

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 4

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!