class Cat(let name: String, let lives: Int64) class Dog(let name: String, let years: Int64) union Pet of Cat, Dog fun describe(pet: Pet): String = if pet ... is Cat(let name, let lives) { "cat $name has $lives lives" } ... is Dog(let name, let years) { "dog $name is $years of age" } describe(Cat("Molly", 9)) // → cat Molly has 9 lives