defmodule Parse do
  def number([ ?- | tail]), do: _number_digits(tail, 0) * -1
  def number([ ?+ | tail]), do: _number_digits(tail, 0)
  def number(str), do: _number_digits(str, 0)

  defp _number_digits([], value), do: value
  defp _number_digits([digit | tail], value) when digit in '0123456789' do
    _number_digits(tail, value * 10 + digit - ?0)
  end
  defp _number_digits([non_digit | _ ], _) do
    raise "Invalid digit '#{[non_digit]}'"
  end
end