gistfile1.rb

class CartesianProduct
  include Enumerable
  def initalize(x, y)
    @x = x
    @y = y
  end

  def each
    return to_enum unless block_given?
    @x.each do |x|
       @y.each do |y|
         yield [x, y]
       end
    end
  end
end


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

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

error message
cartesian.rb:10:in `each': undefined method `each' for nil:NilClass (NoMethodError)
	from cartesian.rb:20:in `<main>'