pi.rb

def pi k = nil
  if k
    a = Rational(4, 8 * k + 1)
    b = Rational(2, 8 * k + 4)
    c = Rational(1, 8 * k + 5)
    d = Rational(1, 8 * k + 6)

    return (a - b - c - d) * (16 ** -k)
  end

  return to_enum(:pi) unless block_given?

  k = 0
  while true
    yield pi(k)
    k += 1
  end
end

pi.reduce(0) do |acc, n|
  sum = acc + n.to_f
  break acc if sum == acc
  sum
end