Lazy enumerators aren't magic

0_enumerator.rb

class Benerator
  def initialize(&block)
    @block = block
  end

  def next
    as_array.shift
  end

  def as_array
    # Load the output of the generator into memory, #next simply reads from the array
    @as_array ||= [].tap { |a| @block.call(->(x){a << x })  }
  end
end

enum = Benerator.new do |y|
  y.yield :hello
  y.yield :world
end

puts "First: #{enum.next}"
puts "Second: #{enum.next}"

1_lazy_enumerator.rb

require 'fiber'

class LazyBenerator
  def initialize(&block)
    @block = block
  end

  def next
    fiber.resume
  end

  def fiber
    # Create a fiber that returns each successive yield when #next is called
    @fiber ||= Fiber.new do
      @block.call(->(x) {
        Fiber.yield x
      })
    end
  end
end

enum = LazyBenerator.new do |y|
  y.yield :hello
  y.yield :world
end

puts "First: #{enum.next}"
puts "Second: #{enum.next}"

2_infinit_fib.rb

fib = LazyBenerator.new do |yielder|
  x, y = 1, 1
  loop do
    yielder.yield x
    x, y = y, x + y
  end
end

10.times do
  puts fib.next
end