yield

Example: drawolver.rb You can write some incredibly sophisticated code in ruby if you embrace blocks, take for example drawolver.rb where we extend an instance of Array to produce a differently shaped iterator using one_of_each, kudos to Florian Jenett who created the original version:-

module ExtendedArray
  # send one item from each array, expects array to be 2D:
  # array [[1,2,3], [a,b,c]] sends
  # [1,a] , [2,b] , [3,c]
  def one_of_each(&block)
    i = 0
    one = self[0]
    two = self[1]
    mi = one.length > two.length ? two.length : one.length
    while i < mi do
      yield(one[i], two[i])
      i += 1
    end
  end
end

Here is creation of the ExtendedArray instance where r1 and r2 are instances of Vec3D

ext_array = [r1, r2].extend ExtendedArray # extend an instance of Array
ext_array.one_of_each do |v1, v2|          
  v1.to_vertex(renderer)
  v2.to_vertex(renderer)
end

Example: LSystems Here is a simpler example that is to produce LSystems grammar

class Grammar
  attr_reader :axiom, :rules
  def initialize(axiom, rules)
    @axiom = axiom
    @rules = rules
  end

  def expand(production, iterations, &block)
    production.each_char do |token|
      if rules.key?(token) && iterations > 0
        expand(rules[token], iterations - 1, &block)
      else
        yield token
      end
    end
  end

  def each(gen)
    expand(axiom, gen) { |token| yield token }
  end

  def generate(gen)
    [].tap do |output|
      each(gen) { |token| output << token }
    end
  end
end