Selectively disable subsumption in Scala? (correctly type List.contains)

Question

List(\"a\").contains(5)

Because an Int can never be contained in a list of String

Answer 1

Why not use an equality typeclass?

scala> val l = List(1,2,3)
l: List[Int] = List(1, 2, 3)

scala> class EQ[A](a1:A) { def ===(a2:A) = a1 == a2 } 
defined class EQ

scala> implicit def toEQ[A](a1:A) = new EQ(a1)
toEQ: [A](a1: A)EQ[A]

scala> l exists (1===)
res7: Boolean = true

scala> l exists ("1"===)
:14: error: type mismatch;
 found   : java.lang.String => Boolean
 required: Int => Boolean
              l exists ("1"===)
                           ^

scala> List("1","2")
res9: List[java.lang.String] = List(1, 2)

scala> res9 exists (1===)
:14: error: type mismatch;
 found   : Int => Boolean
 required: java.lang.String => Boolean
              res9 exists (1===)