Simple calculator kata in Erlang

calc.erl

-module(calc).

-export([calc/1]).

-include_lib("eunit/include/eunit.hrl").

calc(<<"">>) ->
    0;

calc(Input) 
  when is_binary(Input) ->
    calc(Input, []).

calc(<<N, Rest/binary>>, ParsedNumbers) 
  when N >= $0, N =< $9 ->
    calc(Rest, [N - $0|ParsedNumbers]);

calc(<<_Sep, Rest/binary>>, ParsedNumbers) ->
    calc(Rest, ParsedNumbers);

calc(<<"">>, ParsedNumbers) ->
    lists:foldl(fun(N, Acc) -> Acc + N end, 0, ParsedNumbers).

integer_to_binary(N) 
  when is_integer(N) ->
    list_to_binary(integer_to_list(N)).

%%%
%%% Test suite
%%%

empty_string_test() ->
    ?assertEqual(0, calc(<<"">>)).

single_number_test() ->
    ?assertEqual(9, calc(<<"9">>)).

random_number_test() ->
    N = random:uniform(9),
    Bin = integer_to_binary(N),
    ?assertEqual(N, calc(Bin)).

two_number_test() ->
    ?assertEqual(3, calc(<<"1 2">>)).