[leshow]

Sum and product types are essential for programming. After you have used a language with them you will miss them dearly in any other language you use.

Typeclasses and traits (if you're writing Rust) coupled with ad hoc polymorphism are a succinct and beautiful method of abstraction. You will miss them also in almost any other language.

Those features in a language (functional or not) will let you confidently refactor code, and (IMO) will lead to less mistakes and better abstractions.