lazy.rb

class Lazy < BasicObject
  def initialize(&blk)
    @blk = blk
  end

  def method_missing(name, *args, &blk)
    _resolve.send(name, *args, &blk)
  end

  def respond_to?(name)
    _resolve.respond_to?(name)
  end

  def _resolve
    @resolved ||= @blk.()
  end
end

def lazy(&blk)
  Lazy.new(&blk)
end

Cons = Struct.new(:car, :cdr) do
  include Enumerable

  def each(&blk)
    return to_enum unless blk
    blk.(car)
    cdr.each(&blk)
  end
end

# Infinite list of factorials
def fact_list(start = 1, acc = 1)
  Cons[start * acc, lazy { fact_list(start + 1, start * acc) }]
end

fact_list.take(5) # => [1, 2, 6, 24, 120]

# Circular lists
fizz = Cons['', Cons['', Cons['Fizz', ]]]
fizz.cdr.cdr.cdr = fizz

buzz = Cons['', Cons['', Cons['', Cons['', Cons['Buzz', ]]]]]
buzz.cdr.cdr.cdr.cdr.cdr = buzz

fizz_enum, buzz_enum = [fizz,buzz].map(&:each)

15.times do |i|
  fizzbuzz = [fizz_enum, buzz_enum].map(&:next).join
  puts fizzbuzz.empty? ? i+1 : fizzbuzz
end

# >> 1
# >> 2
# >> Fizz
# >> 4
# >> Buzz
# >> Fizz
# >> 7
# >> 8
# >> Fizz
# >> Buzz
# >> 11
# >> Fizz
# >> 13
# >> 14
# >> FizzBuzz