Lazy comprehension

gistfile1.rb

def comprehend(*enums)
  cycles = enums.map(&:cycle)

  Enumerator.new do |comprehended|
    loop do
      value = cycles.map(&:peek)
      value = yield value if block_given?
      comprehended << value
      (cycles.length - 1).downto(0) do |index|
        cycles[index].next
        break unless cycles[index].peek == enums[index].first
        raise StopIteration if index == 0
      end
    end
  end.lazy # requires Ruby > 2.0
end

comprehend(1..2, 4..6) { |a, b| puts "Generating #{a},#{b}"; [a, b] }.select { |a, b| (a + b).odd? }.take(2).each {|i| p i}

# Generating 1,4
# [1, 4]
# Generating 1,5
# Generating 1,6
# [1, 6]
#