Nit: Kotlin doesn't exactly have optionals, it has essentially a type modifier (?) that represents a "nullable" type, i.e. the sum of a type T and the value 'nu

Nit: Kotlin doesn't exactly have optionals, it has essentially a type modifier (?) that represents a "nullable" type, i.e. the sum of a type T and the value 'null'. This is indeed safer and terser, therefore strictly more powerful than Java's Optional type, but it begs a question: Why not allow users to create similar higher-kinded types instead of baking this one very specific type kind into the compiler? Their recent decision to axe typeclasses in favor of multiple function receivers, which solves a small subset of problems that HKTs do in Scala and Haskell, make me look for greener pastures whenever I need them.