字符串与二进制

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

    def number([ ?- | tail ]) do
        _number_digits(tail, 0)
    end

    def number(str) do
        _number_digits(str, 0)
    end

    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