eval.ex

  def run(), do:  eval('5 * 5 + 2-2      + 10 *20*0') |> IO.inspect

  def eval(input), do: _tokenize(input, []) |> _eval1([]) |> _eval2()
  defp _tokenize([32 | input], result),                                                   do: _tokenize input, result
  defp _tokenize([char | input], [{:num, num} | result]) when char >= ?0 and char <= ?9,  do: _tokenize input, [{:num, num * 10 + (char - ?0)} | result]
  defp _tokenize([char | input], result)                 when char >= ?0 and char <= ?9,  do: _tokenize input, [{:num, (char - ?0)} | result]
  defp _tokenize([?* | input], result),                                                   do: _tokenize input, [:mul | result]
  defp _tokenize([?/ | input], result),                                                   do: _tokenize input, [:div | result]
  defp _tokenize([?+ | input], result),                                                   do: _tokenize input, [:add | result]
  defp _tokenize([?- | input], result),                                                   do: _tokenize input, [:sub | result]
  defp _tokenize([], result),                                                             do: result |> Enum.reverse()
  defp _eval1([{:num,left},:mul,{:num,right} | tail], result),                            do: _eval1 [{:num, left * right} | tail], result
  defp _eval1([{:num,left},:div,{:num,right} | tail], result),                            do: _eval1 [{:num, left / right} | tail], result
  defp _eval1([any | tail], result),                                                      do: _eval1 tail, [any | result]
  defp _eval1([], result),                                                                do: result |> Enum.reverse()
  defp _eval2([{:num,left},:add,{:num,right} | tail]),                                    do: _eval2 [{:num, left + right} | tail]
  defp _eval2([{:num,left},:sub,{:num,right} | tail]),                                    do: _eval2 [{:num, left - right} | tail]
  defp _eval2([{:num,value}]),                                                            do: value