A recursive approach to the Cartesian product

cartesian.rb

# irb(main):001:0> cartprod [[:a, :b], (1..3), [true, false]]
# => [[:a, 1, true], [:a, 1, false], [:a, 2, true], [:a, 2, false],
#     [:a, 3, true], [:a, 3, false], [:b, 1, true], [:b, 1, false],
#     [:b, 2, true], [:b, 2, false], [:b, 3, true], [:b, 3, false]]
def cartprod(colls)
  if colls.empty?
    [[]]
  else
    rest_cartprod = cartprod colls[1..-1]
    colls[0].reduce [] do |all, x|
      all + rest_cartprod.map { |tail| [x] + tail }
    end
  end
end

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