extension.scala

class Foo {
  def bar(i: Int): String = i.toString
}

object Foo {
  implicit class Syntax(val self: Foo) extends AnyVal {
    def baz(b: Boolean): String = b.toString
  }
}

val f = new Foo
f.bar(42)     // => "42"
f.baz(true)   // => "true"