Question

Is it possible to do something like this in Scala:

class MyTest {
   def foo[A <: String _or_ A <: Int](p:List[A]) =  {} 
}

That is, the type A could be a String or Int. Is this possible?

(Similar question here)

Answer 1

Not really possible as you put it, but you can do it using the type class pattern. For example, from here:

sealed abstract class Acceptable[T]
object Acceptable {
  implicit object IntOk extends Acceptable[Int]
  implicit object LongOk extends Acceptable[Long]
}

def f[T: Acceptable](t: T) = t

scala> f(1)
res0: Int = 1

scala> f(1L)
res1: Long = 1

scala> f(1.0)
<console>:8: error: could not find implicit value for parameter ev: Acceptable[Double]
f(1.0)
^

EDIT

This works if class and object are companions. On REPL, if you type each on a different line (ie, a “result” appears between them), they are not companions. You can type it like below, though:

scala> sealed abstract class Acceptable[T]; object Acceptable {
     |   implicit object IntOk extends Acceptable[Int]
     |   implicit object LongOk extends Acceptable[Long]
     | }
defined class Acceptable
defined module Acceptable