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">>)).

calc1.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>>, CurrentNumber, ParsedNumbers) 
  when N >= $0, N =< $9 ->
    calc(Rest, [N|CurrentNumber], ParsedNumbers);

calc(<<_Sep, Rest/binary>>, CurrentNumber, ParsedNumbers) ->
    N = list_to_integer(lists:reverse(CurrentNumber)),
    calc(Rest, [], [N|ParsedNumbers]);

calc(<<"">>, [], ParsedNumbers) ->
    %% lists:sum(ParsedNumber) will give us the same result
    lists:foldl(fun(N, Acc) -> Acc + N end, 0, ParsedNumbers);

calc(<<"">>, CurrentNumber, ParsedNumbers) ->
    N = list_to_integer(lists:reverse(CurrentNumber)),
    calc(<<"">>, [], [N|ParsedNumbers]).

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

%%%
%%% Test suite
%%%

random_number() ->
    random:uniform(1000000).

%%% [2, 4, 6, 4] => <<"1 2 3 4>>"

random_number_string() ->
    Times = lists:seq(1, random:uniform(10)),
    F = fun(_) -> random_number() end,
    Numbers = lists:map(F, Times),
    BinaryList = lists:map(fun integer_to_binary/1, Numbers),
    F1 = fun(Bin, Acc) -> [Bin, <<" ">>|Acc] end,
    ListWithSpaces = lists:foldr(F1, [], BinaryList),
    {iolist_to_binary(ListWithSpaces), lists:sum(Numbers)}.

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)).

many_numbers_test() ->
    {Input, Sum} = random_number_string(),
    ?assertEqual(Sum, calc(Input)).