Scala double definition (2 methods have the same type erasure)

I like Michael Krämer's idea to use implicits, but I think it can be applied more directly:

case class IntList(list: List[Int])
case class StringList(list: List[String])

implicit def il(list: List[Int]) = IntList(list)
implicit def sl(list: List[String]) = StringList(list)

def foo(i: IntList) { println("Int: " + i.list)}
def foo(s: StringList) { println("String: " + s.list)}

I think this is quite readable and straightforward.

[Update]

There is another easy way which seems to work:

def foo(p: List[String]) { println("Strings") }
def foo[X: ClassTag](p: List[Int]) { println("Ints") }
def foo[X: ClassTag, Y: ClassTag](p: List[Double]) { println("Doubles") }

For every version you need an additional type parameter, so this doesn't scale, but I think for three or four versions it's fine.

[Update 2]

For exactly two methods I found another nice trick:

def foo(list: => List[Int]) = { println("Int-List " + list)}
def foo(list: List[String]) = { println("String-List " + list)}

Instead of inventing dummy implicit values, you can use the DummyImplicit defined in Predef which seems to be made exactly for that:

class TestMultipleDef {
  def foo(p:List[String]) = ()
  def foo(p:List[Int])(implicit d: DummyImplicit) = ()
  def foo(p:List[java.util.Date])(implicit d1: DummyImplicit, d2: DummyImplicit) = ()
}

To understand Michael Krämer's solution, it's necessary to recognize that the types of the implicit parameters are unimportant. What is important is that their types are distinct.

The following code works in the same way:

class TestDoubleDef {
   object dummy1 { implicit val dummy: dummy1.type = this }
   object dummy2 { implicit val dummy: dummy2.type = this }

   def foo(p:List[String])(implicit d: dummy1.type) = {}
   def foo(p:List[Int])(implicit d: dummy2.type) = {}
}

object App extends Application {
   val a = new TestDoubleDef()
   a.foo(1::2::Nil)
   a.foo("a"::"b"::Nil)
}

At the bytecode level, both foo methods become two-argument methods since JVM bytecode knows nothing of implicit parameters or multiple parameter lists. At the callsite, the Scala compiler selects the appropriate foo method to call (and therefore the appropriate dummy object to pass in) by looking at the type of the list being passed in (which isn't erased until later).

While it's more verbose, this approach relieves the caller of the burden of supplying the implicit arguments. In fact, it even works if the dummyN objects are private to the TestDoubleDef class.

Due to the wonders of type erasure, the type parameters of your methods' List get erased during compilation, thus reducing both methods to the same signature, which is a compiler error.

Scala double definition (2 methods have the same type erasure)

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