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.

  def find_citations(html) do
    html
    |> Floki.parse_document!(html)
    |> Floki.attribute("a", "href")
    |> List.flatten()
    |> Enum.map(&URI.parse/1)
    |> Enum.reduce([], &parse_valid/2)
    |> Enum.sort()
    |> Enum.uniq()
  end

  def parse_valid(uri, valids) do
    with %{host: "leginfo.legislature.ca.gov", query: query} <- uri,
         %{"lawCode" => code, "sectionNum" => section} <- URI.decode_query(query) do
      [String.replace_suffix("CA #{@code_abbrevs[@cal_codes[code]]} Section #{section}", ".", "") | valids]
    else
      _ -> valids
    end
  end

Is this something you had in mind (not tested)? In the above code nothing happens in else clause of with, the accumulator is just passed, but you could add here different pattern matches and maybe also parse the data and add it to the accumulator.

Also, since there’s a lot of enum calls consider using stream.

Maybe I don’t understand what is parse don’t validate from haskell.

There are lots of data validation / casting libraries in Elixir that help with this sort of thing in different use cases. I wrote one myself GitHub - Adzz/data_schema: Declarative schemas for data transformations. · GitHub

but as mentioned it doesn’t mean everything has to be a non primitive type, it just means you should do type casting at the edge of the system and be confident that if you get further in that that type casting happened.