Pattern matching on Class[_] type? - scala

Question

I'm trying to use Scala pattern matching on Java Class[_] (in context of using Java reflection from Scala) but I'm getting some unexpected error. The following gives "unreachable code" on the line with case jLong
def foo[T](paramType: Class[_]): Unit = {
val jInteger = classOf[java.lang.Integer]
val jLong = classOf[java.lang.Long]
paramType match {
case jInteger => println("int")
case jLong => println("long")
}
}
Any ideas why this is happening ?

Answer 1

The code works as expected if you change the variable names to upper case (or surround them with backticks in the pattern):
scala> def foo[T](paramType: Class[_]): Unit = {
| val jInteger = classOf[java.lang.Integer]
| val jLong = classOf[java.lang.Long]
| paramType match {
| case `jInteger` => println("int")
| case `jLong` => println("long")
| }
| }
foo: [T](paramType: Class[_])Unit
scala> foo(classOf[java.lang.Integer])
int
In your code the jInteger in the first pattern is a new variable—it's not the jInteger from the surrounding scope. From the specification:
8.1.1 Variable Patterns
... A variable pattern x is a simple identifier which starts with a lower case letter. It
matches any value, and binds the variable name to that value.
...
8.1.5 Stable Identifier Patterns
... To resolve the syntactic overlap with a variable pattern, a stable
identifier pattern may not be a simple name starting with a lower-case
letter. However, it is possible to enclose a such a variable name in
backquotes; then it is treated as a stable identifier pattern.
See this question for more information.

Answer 2

On your pattern matching, each of these 2 cases try to create place holder names instead of matching the class type as expected.
If you use upper case in the starting character, you'll be fine
def foo[T](paramType: Class[_]): Unit = {
val JInteger = classOf[Int]
val JLong = classOf[Long]
paramType match {
case JInteger => println("int")
case JLong => println("long")
}
}
scala> foo(1.getClass)
int

Answer 3

JMPL is simple java library, which could emulate some of the features pattern matching, using Java 8 features.
matches(data).as(
Integer.class, i -> { System.out.println(i * i); },
Byte.class, b -> { System.out.println(b * b); },
Long.class, l -> { System.out.println(l * l); },
String.class, s -> { System.out.println(s * s); },
Null.class, () -> { System.out.println("Null value "); },
Else.class, () -> { System.out.println("Default value: " + data); }
);