Combine multiple is in when clause in Kotlin

Question

Suppose I have the following:

fun makeSound(val animal: Animal) = when(animal) {
  is Lion -> animal.roar()
  is TRex -> animal.roar()
  is Cow -> animal.moo()
}

Usually I would simplify this by simply adding a RoaringAnimal interface and asking is RoaringAnimal. But is there another way of combining multiple is clauses into one?

Answer 1

Normally you can combine the clauses as shown in Yoni's answer.

But in the specific case that roar is defined on Lion and TRex, but not on Animal, you can't.

This is because the compiler inserts a smart cast:

is Lion -> animal.roar()

is really

is Lion -> (animal as Lion).roar()

but in is Lion, is TRex -> clause, it wouldn't know what cast to insert.

In principle the compiler could be extended to handle such cases by inserting another when:

is Lion, is TRex -> animal.roar()

would become

is Lion, is TRex -> when(animal) {
    is Lion -> animal.roar() // works as before
    is TRex -> animal.roar()
}

but I wouldn't expect this to happen

Answer 2

UPDATE: the answer below was written before the question specified that roar was a method on the animal parameter. As the question now stands, the answer below will no longer work, however it still shows how multiple conditions can be combined in one line in a when statement.

You can combine them:

fun makeSound(animal: Animal) = when(animal) {
  is Lion, is TRex -> roar()
  is Cow -> moo()
}