Statically-types languages are a form of automatically-verified documentation, and an opportunity to name semantic properties different modules have in common. Both of those are great, but it is awkward that it is usually treated as an all-or-nothing matter.

Almost no language offers what I actually want: duck typing plus the ability to specify named interfaces for function inputs. Probably the closest I've found is Ruby with a linter to enforce RDoc comments on any public methods.