part6.rb

class CartesianProduct
  include Enumerable

  def initialize a, b
    @a = a
    @b = b
  end

  # Every method has an implied block that is sent into it, it does NOT need to be
  # specified as a parameter.
  def each
    # Just your average doubly-nested loop
    @a.each do |a_el|
      @b.each do |b_el|
        # yield will call whatever block gets passed into this method, passing
        # an array of the two elements as a parameter. block_given? returns false
        # if no block was passed to this method. Trying to call yield when no block
        # was passed to the method results in an exception being thrown.
        yield [a_el, b_el] if block_given?
      end
    end
  end
end

c = CartesianProduct.new([:a,:b], [4,5])
c.each { |el| puts el.inspect }