foo-example.scala

object FooExample {
  sealed trait Foo {
    def i: Int
  }

  private case class SecretFoo(i: Int) extends Foo

  /* Without the explicit return type, we'd get this compiler error:
   *
   *     <console>:14: error: private class SecretFoo escapes its defining scope...
   *
   */
  def buildFoo(i: Int): Either[String, Foo] = if (i < 0)
    Left("Need non-negative integer!")
  else
    Right(SecretFoo(i))
}