Roman to decimal number converter (tail-recursive)

roman_to_num.erl

-module(roman_to_num).
% https://github.com/ruby-fatecsp/dojos/blob/8cb15023eec6d4a5c9cdcf1723adff1ed5864a03/roman_to_numerals/lib/roman_to_num.rb
-export([convert/1, test/0]).

digit_to_num(D) -> 
  % io:format("~p~n", [D]),
  case D of
  
    $I -> 1;
    $V -> 5;
    $X -> 10;
    $L -> 50;
    $C -> 100;
    $D -> 500;
    $M -> 1000;
    _  -> erlang:error(badarg)
end.


convert(Roman) -> convert(Roman, 0).

convert([Head|[]], Acc) -> Acc + digit_to_num(Head);

convert([Head1, Head2 | Tail], Acc) -> 
  Head1AsNum = digit_to_num(Head1),
  Head2AsNum = digit_to_num(Head2),
  NewAcc = if
    Head1AsNum < Head2AsNum ->
      Acc - Head1AsNum;
    true -> 
      Acc + Head1AsNum
  end,
  convert([Head2|Tail], NewAcc) .


test() ->
  20    = convert("XX"),
  22    = convert("XXII"),
  4     = convert("IV"),
  5     = convert("V"),
  6     = convert("VI"),
  7     = convert("VII"),
  9     = convert("IX"),
  11    = convert("XI"),
  2011  = convert("MDDXI"),
  test_passed.