Our solution to the following challenge: Conway's Game of Life without saving state and without using iteration (or anything provided by Enumerable). Also included is a shorter solution which does use Enumerable.

functional_alt_gol.rb

class NilClass
  def [](index) end
end

class Array
  def [](index)
    (index < 0) ? nil : at(index)
  end
end

class World
  ADJACENT_OFFSETS = {
    :northwest => [-1, 1],  :north => [0, 1],  :northeast => [1,1],
    :west      => [-1, 0],                     :east      => [1,0],
    :southwest => [-1, -1], :south => [0, -1], :southeast => [1,-1]
  }

  def self.next_world(current, x = 0, y = 0)
    alive_neighbors = count_neighbors(current, x, y)

    if x < current.length - 1
      next_world(current, x + 1, y)
    elsif y < current[x].length - 1
      next_world(current, 0, y + 1)
    end

    current[x][y] = current[x][y] ? [2,3].include?(alive_neighbors) : alive_neighbors == 3
    current
  end

  def self.count_neighbors(world, x, y)
    ADJACENT_OFFSETS.values.inject(0) do |sum, offset|
      sum + ( world[x + offset[0]][y + offset[1]] ? 1 : 0)
    end
  end

end

functional_gol.rb

class NilClass
  def [](index) end
end

class Array
  def [](index)
    (index < 0) ? nil : at(index)
  end
end

class World
  def self.next_world(current, x = 0, y = 0)
    alive_neighbors = neighbors(current, x, y)

    if x < current.length - 1
      next_world(current, x + 1, y)
    elsif y < current[x].length - 1
      next_world(current, 0, y + 1)
    end

    current[x][y] = calculate_state(current[x][y], alive_neighbors)
    current
  end

  def self.neighbors(world, x, y)
    alive_neighbors = 0
    alive_neighbors += 1 if world[x-1][y-1]
    alive_neighbors += 1 if world[x-1][y]
    alive_neighbors += 1 if world[x-1][y+1]
    alive_neighbors += 1 if world[x][y-1]
    alive_neighbors += 1 if world[x][y+1]
    alive_neighbors += 1 if world[x+1][y-1]
    alive_neighbors += 1 if world[x+1][y]
    alive_neighbors += 1 if world[x+1][y+1]
    alive_neighbors
  end

  def self.calculate_state(alive, alive_neighbors)
    if alive
      alive_neighbors == 2 or alive_neighbors == 3
    else
      alive_neighbors == 3
    end
  end
end