> (people writing dynamically typed languages eventually resort to type comments)

This has never been an issue in Elixir, because instead of a comment, you'd just improve the pattern matching in the function definition.

    def blah(%{students: [%{firstname: firstname, lastname: lastname}|[]], count: cnt}) when is_int(cnt):
      fullname = firstname <> lastname

Is a valid function declaration, which specifies that blah takes a dictionary that contains at least 2 keys, :count, who's value is an integer, and :students, who's value is a length-1 list who's first element is a dictionary that contains the keys :firstname and :lastname