Solution to The Dog Riddle

Animal.kt

sealed class Animal {
    object Cat: Animal()
}

class Dog(breedParam: String) : Animal() {
    private constructor() : this("")

    val breed: String? = if (breedParam == "") null else breedParam

    companion object {
        val Dog = Dog()
    }
}

Main.kt

import Animal.Cat
import Dog.Companion.Dog

fun main() {
    val cat: Animal = Cat
    val dog: Animal = Dog
    val husky: Animal = Dog("husky")
    val emptyDog: Animal = Dog() // error
    val nullDog: Animal = Dog(null) // error

    val opinion = when (val animal = getAnimal()) {
        is Cat -> {
            "Cat"
        }
        is Dog -> {
            if (animal.breed == null)
                "Generic Dog"
            else
                "Specific dog: ${animal.breed}"
        }
    }

    println(opinion)

    val labrador = Dog("labrador")
    val corgi = Dog("corgi")
    println(labrador.breed) // labrador
    println(corgi.breed) // corgi
}

fun getAnimal(): Animal {
    return Dog("golden retriever")
}