http://confreaks.tv/videos/rubyconf2012-y-not-adventures-in-functional-programming

z_combinator.rb

class ZCombinator
  def initialize improver
    @improver = improver
  end

  def call gen
    eat_its_tail(gen).(
      ->(gen) { @improver.(applicatively_eat_its_tail gen) }
    ).(gen)
  end

  private

  def eat_its_tail gen
    ->(gen) { gen.(gen) }
  end

  def applicatively_eat_its_tail gen
    ->(_) { gen.(gen).(_) }
  end
end

triangle  = ZCombinator.new ->(partial) { ->(n) { n < 1 ? 0 : n + partial.(n.pred) } }
factorial = ZCombinator.new ->(partial) { ->(n) { n < 1 ? 1 : n * partial.(n.pred) } }
fibonacci = ZCombinator.new ->(partial) { ->(n) { n < 2 ? n : partial.(n.pred) + partial.(n.pred.pred) } }

Array.new(10) { |n| triangle.(n) }
#=> [0, 1, 3, 6, 10, 15, 21, 28, 36, 45]
Array.new(10) { |n| factorial.(n) }
#=> [1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880]
Array.new(10) { |n| fibonacci.(n) }
#=> [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]