fizzbuzz.rb

def fizzbuzz
  Enumerator.new do |y|
    i = 1
    loop do
      y << case
           when i % 15 == 0 then "FizzBuzz"
           when i %  5 == 0 then "Buzz"
           when i %  3 == 0 then "Fizz"
           else                  i
           end
      i += 1
    end
  end
end

f = fizzbuzz
f.next                   # => 1
f.next                   # => 2
f.next                   # => "Fizz"
f.next                   # => 4
f.next                   # => "Buzz"
f.next                   # => "Fizz"
f.next                   # => 7
f.next                   # => 8
f.next                   # => "Fizz"
f.next                   # => "Buzz"
f.next                   # => 11
f.next                   # => "Fizz"
f.next                   # => 13
f.next                   # => 14
f.next                   # => "FizzBuzz"
f.next                   # => 16
f.next                   # => 17
f.next                   # => "Fizz"
f.next                   # => 19
f.next                   # => "Buzz"
f.next                   # => "Fizz"
f.next                   # => 22

fizzbuzz.lazy.first(30)         # => [1, 2, "Fizz", 4, "Buzz", "Fizz", 7, 8, "Fizz", "Buzz", 11, "Fizz", 13, 14, "FizzBuzz", 16, 17, "Fizz", 19, "Buzz", "Fizz", 22, 23, "Fizz", "Buzz", 26, "Fizz", 28, 29, "FizzBuzz"]