parse2.exs

defmodule Parse do
  def calculate(expression) do
    { left, rest } = Enum.split_while(expression, &(!(&1 in '+-*/')))
    [ op | right ] = rest
    { result, _ } = Code.eval_quoted { :erlang.list_to_atom([op]), [],
      [value(left), value(right)] }
    result
  end

  defp value(digits) do
    digits |> to_string |> String.strip |> :erlang.binary_to_integer
  end
end

IO.inspect Parse.calculate('123+123')
IO.inspect Parse.calculate(' 123 + 123 ')
IO.inspect Parse.calculate('12 * 12')