Gleam doesn’t support interfaces. Not generics. You are completely right.
Haskell allows both sorts of generics. In Haskell parlance they call this higher-kinded polymorphism and the generic version of map they call fmap (as a method of the class Functor).
Gauche has a generic sequence interface which is great, and it's one of the reasons as a Python user I like Gauche as my "daily driver" Scheme.