A modern ruby Money class, for ActiveShipping?

money.rb

require 'bigdecimal'

module ActiveShipping #:nodoc:
  class Money
    SCALE = 2
    OPERAND_CLASSES = [self, Rational, BigDecimal, Integer].freeze

    attr_reader :amount
    protected :amount

    def initialize numeric
      @amount = coerce_numeric numeric
    end

    def amount_in_cents
      (amount * 100).to_i
    end

    %i[+@ -@ abs].each do |method|
      define_method(method) { self.class.new(amount.public_send(method)) }
    end
    
    %i[+ - * /].each do |method|
      define_method(method) do |other|
        self.class.new(amount.public_send(method, coerce_numeric(other)))
      end
    end

    def to_d
      (amount.numerator.to_d / amount.denominator).round(
        SCALE,
        BigDecimal::ROUND_HALF_EVEN
      )
    end

    def to_s
      '$%.*f' % [SCALE, to_d]
    end

    protected
    
    def to_r
      amount
    end

    private

    def coerce_numeric numeric
      assert_valid_numeric numeric
      numeric.to_r
    end

    def assert_valid_numeric numeric
      unless OPERAND_CLASSES.any? { |klass| numeric.kind_of?(klass) }
        raise TypeError,
          "Operand must be #{OPERAND_CLASSES.join(', ')}, not #{numeric.class}"
      end
    end

  end
end