cartesian product

cartesian_product.rb

def ghetto_cartesian_product colls
  if colls.empty?
    [[]]
  else
    colls.first.reduce([]) do |memo, e|
      rest = colls.drop 1
      tails = ghetto_cartesian_product(rest).map do |tail|
        [e].concat tail
      end
      memo.concat tails
    end
  end
end

# so good
def lazy_cartesian_product colls
  colls.first.enum_for :product, *colls.drop(1)
end