Change Return type of a function

changereturntype.scala

{
      trait Aux[T] {
        type R
        def apply(t: T): R
      }

      implicit val sa = new Aux[String] {
         type R = Boolean
         def apply(s: String): Boolean = s.isEmpty
      }

      implicit val ia = new Aux[Int] {
         type R = String
         def apply(i: Int): String = i.toString
      }

      def foo[T](t: T)(implicit aux: Aux[T]): aux.R = aux(t)
  }

/*
@ val b: Boolean = foo("")
b: Boolean = true
@ val c: String = foo(2)
c: String = "2"
@ val c: Boolean = foo(4)
cmd4.sc:1: type mismatch;
 found   : ammonite.$sess.cmd0.ia.R
    (which expands to)  String
 required: Boolean
val c: Boolean = foo(4)
                    ^
Compilation Failed
*/