A simple currency class

currency.rb

class Currency
  def initialize(value)
    @value = (value || 0).to_d.round(2)
  end

  attr_reader :value

  alias_method :to_d, :value

  def to_currency; self; end
  alias_method :dollars, :to_currency

  def method_missing(method, *args, &blk)
    ret = @value.send(method, *args, &blk)
    ret.is_a?(Numeric) ? Currency.new(ret) : ret
  end

  def eql?(other)
    self.class.equal?(other.class) && @value == other.to_d
  end

  def ==(other)
    @value == other.to_d
  end

  def <=>(other)
    @value <=> other.to_d
  end

  def coerce(thing)
    [thing.to_currency, @value]
  end

  def is_a?(klass)
    super || @value.is_a?(klass)
  end
  alias_method :kind_of?, :is_a?

  def to_str
    sprintf("%0.2f", @value)
  end
  alias_method :to_s, :to_str

  def inspect
    "#<Currency:#{to_s}>"
  end
end

object_to_currency.rb

class Object
  def to_currency
    Currency.new(self)
  end
  alias_method :dollars, :to_currency
end