fibonacci.rb

class Fibonacci

  class << self
    include Enumerable

    def each
      (1..Float::INFINITY).lazy.each do |i|
        yield fib(i)
      end
    end

    private

    def fib(i)
      return if i < 1
      @fib ||= {1 => 0, 2 => 1}
      return @fib[i] if @fib[i]
      @fib[i] = @fib[i-1] + @fib[i-2]
      @fib[i]
    end

  end

end

# Fibonacci.take(10)
#  => [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]