My erlang practice of the day.

string_calculator.erl

-module(kata).
-import(lists, [map/2, foldl/3]).
-include_lib("eunit/include/eunit.hrl").

add("")        -> 0;
add(Operators) ->
  lists:foldl(fun(Number, Acumulator) -> Number + Acumulator end, 0, extractNumbers(Operators, ",\n")).

extractNumbers(String, Separator) ->
  map(fun erlang:list_to_integer/1, string:tokens(String, Separator)).

it_returns_0_for_an_empty_string_test() ->
  ?assertEqual(0, add("")).

it_returns_the_number_for_a_single_number_test() ->
  ?assertEqual(1, add("1")).

it_returns_the_sum_of_two_numbers_for_two_numbers_test() ->
  ?assertEqual(3, add("1,2")).

it_can_sum_more_than_two_numbers_test() ->
  ?assertEqual(6, add("1,2,3")).

it_can_handle_new_line_as_separator_test() ->
  ?assertEqual(6, add("1,2\n3")).

Second take on the problem. I think it reads better:

-module(kata).
-import(string, [tokens/2]).
-import(lists, [map/2, sum/1]).
-include_lib("eunit/include/eunit.hrl").

add("")     -> 0;
add(String) -> sum(extract_numbers(String)).

extract_numbers(String) ->
  Separators = ",\n",
  map(fun list_to_integer/1 , tokens(String, Separators)).

it_returns_0_for_an_empty_string_test() ->
  ?assertEqual(0, add("")).

it_returns_the_number_for_a_single_number_test() ->
  ?assertEqual(1, add("1")).

it_returns_the_sum_for_two_numbers_test() ->
  ?assertEqual(3, add("1,2")).

it_returns_the_sum_for_n_numbers_test() ->
  ?assertEqual(110, add("1,2,3,4,15,35,50")).

it_can_handle_both_comma_and_new_line_as_separators_test() ->
  ?assertEqual(6, add("1,2\n3")).