Kotlin dog riddle solution

Animal.kt

// We are modelling different types of animals.
sealed class Animal {

    // Cats are simple. A cat is a cat.
    object Cat : Animal()

    sealed class Dog: Animal() {

        companion object : Dog()

        data class Breed(val breed: String) : Dog()
    }

    fun Dog(breed: String) = Dog.Breed(breed)

}


// Example usage
fun main() {

    // All of these lines should work:
    var animal: Animal
    animal = Cat
    animal = Dog
    animal = Dog("husky")

    // Implementation needs to work with an exhaustive when, such as this one:
    val opinion = when (animal) {
        is Cat -> {
            "Cat"
        }
        is Dog -> {
            if (animal.breed == null) "Generic dog" else "Specific dog: ${animal.breed}"
        }
    }

    // This should NOT compile, as for a dog without a breed, we want just the `Dog` syntax.
    Dog()

    // This should not compile either, for the same reason.
    Dog(null)

    // Finally, different Dogs created are expected to have different breeds when checked later:
    val animal1 = Dog("labrador")
    val animal2 = Dog("husky")
    println(animal1.breed) // labrador
    println(animal2.breed) // husky

}