gistfile1.rb

module Cart
  def add(buyer, catalog, product_id, quantity)
    raise ArgumentError unless user.kind_of?(Buyer)
    raise ArgumentError unless catalog.kind_of?(Catalog)
    # или через #respond_to?
    product = catalog.find_product(product_id)
    add_cart_items(quantity, CartItem.new(buyer,product))
  end
end

class User
  include Buyer
end

class Shop
  include Catalog
end