While types are very useful, it’s still a good idea to do this in untyped (“dynamically typed”) languages like Elixir and Erlang. Transforming external data to a known structure that your application understands and refusing to operate on anything but that eliminates lots of possible issues on its own, and is frankly less work in the long run than validating and operating on more-or-less raw data. Don’t let the absence of types stop you from implementing a good idea. 
You don’t have to go so far as to declare a whole new struct every time, either, as you get many of the benefits just from consistently combining functions like leginfo_url? and leginfo_url_to_cite into a function that returns {:error, reason} | {:ok, {:ad_hoc_tag, value}}. For small things that can be good enough.
