An alternative Factory pattern in Scala to compare with the one presented in [Design Patterns in Scala](http://pavelfatin.com/design-patterns-in-scala/). This one involves a bit more boilerplate, but will not compile if you ask for an unknown type and is easier to extend.

gistfile1.scala

trait Animal
private class Dog extends Animal
private class Cat extends Animal

trait Wanna
case object WannaCat extends Wanna
case object WannaDog extends Wanna

trait AnimalFactory[Wanna] {
  def create(): Animal
}

object Animal {
  def apply[T <: Wanna](kind: T)(implicit factory: AnimalFactory[T]) =
   factory.create()
}

implicit val canMakeCat = new AnimalFactory[WannaCat.type] {
  def create(): Animal = new Cat()
}

implicit val canMakeDog = new AnimalFactory[WannaDog.type] {
  def create(): Animal = new Dog()
}

You can then create animals with:

scala> val cat = Animal(WannaCat)
cat: Animal = Cat@46a1d830

scala> val dog = Animal(WannaDog)
dog: Animal = Dog@1afa463f