[Touche]

> This is a limitation of those languages, which should not force a language-agnostic protocol to adopt the lowest common denominator of expressiveness.

It's an intentional decision made by those languages in order to focus on other things. If your intent is to be language-agnostic, then yeah, going with lowest common denominator concepts is exactly what you need to do. If you just want to write a Haskell auth implementation using your favorite pet language features, then write a Haskell auth implementation.