Adds Haskell style list comprehensions to the Ruby Array

array_comprehension.rb

class Array

  #
  # Iterates over each permutation of the enumerable values within the
  # {Array}.
  #
  # @yield [list]
  #   The given block will be passed each permutation of the enumerable
  #   values in the {Array}.
  #
  # @yieldparam [Array] list
  #   A permutation of the enumerable values within the {Array}.
  #
  # @return [Enumerator]
  #   If no block is given, an enumerator object will be returned.
  #
  # @example
  #   [(1..5),(1..4),(1..3)].comprehension.to_a
  #   # => [
  #     [1, 1, 1], [1, 1, 2], [1, 1, 3],
  #     [1, 2, 1], [1, 2, 2], [1, 2, 3],
  #     [1, 3, 1], [1, 3, 2], [1, 3, 3],
  #     [1, 4, 1], [1, 4, 2], [1, 4, 3],
  #     [2, 1, 1], [2, 1, 2], [2, 1, 3],
  #     [2, 2, 1], [2, 2, 2], [2, 2, 3],
  #     [2, 3, 1], [2, 3, 2], [2, 3, 3],
  #     [2, 4, 1], [2, 4, 2], [2, 4, 3],
  #     [3, 1, 1], [3, 1, 2], [3, 1, 3],
  #     [3, 2, 1], [3, 2, 2], [3, 2, 3],
  #     [3, 3, 1], [3, 3, 2], [3, 3, 3],
  #     [3, 4, 1], [3, 4, 2], [3, 4, 3],
  #     [4, 1, 1], [4, 1, 2], [4, 1, 3],
  #     [4, 2, 1], [4, 2, 2], [4, 2, 3],
  #     [4, 3, 1], [4, 3, 2], [4, 3, 3],
  #     [4, 4, 1], [4, 4, 2], [4, 4, 3],
  #     [5, 1, 1], [5, 1, 2], [5, 1, 3],
  #     [5, 2, 1], [5, 2, 2], [5, 2, 3],
  #     [5, 3, 1], [5, 3, 2], [5, 3, 3],
  #     [5, 4, 1], [5, 4, 2], [5, 4, 3]
  #   ]
  #
  # @note
  #   This currently will not run under Ruby 1.8, due to the fact that the
  #   Ruby 1.8 `Enumerable` class not provide the `#peek` method. Additionally,
  #   the `Enumerator` implementation in Ruby 1.8 is much slower than that of
  #   Ruby 1.9. Please upgrade to 1.9.2 for the best Ruby experience.
  #
  def comprehension
    return enum_for(:comprehension) unless block_given?

    ranges = self.map do |range|
      if range.kind_of?(Enumerable)
        range
      elsif range.kind_of?(Proc)
        Enumerator.new(&range)
      else
        (range..range)
      end
    end

    cycles = ranges.map { |range| range.cycle }

    loop do
      yield cycles.map { |cycle| cycle.peek }

      (cycles.length - 1).downto(0) do |index|
        cycles[index].next
        break unless cycles[index].peek == ranges[index].first

        return nil if index == 0
      end
    end
  end

end

I wonder if there are nice ways to include predicates in this, or is it already there and my eyes glossed over it?

Current version of this is in combinatorics.

I suppose I could add support for lambdas by passing them the half-built Array.

[(0..255), ->(a) { a[0].chr }]