Tell me about the Unit and Nothing classes in Kotlin.

Unit is a singleton and is used as a return type for functions that do not return any meaningful value. It is analogous to void in Java, but unlike void, Unit is a real type.

Nothing is a special type indicating that a function will never complete normally (for example, it throws an exception or runs an infinite loop). It can be used as a return type or as the type of a property that will never have a value. It is a subtype of all other types in Kotlin.