Ruby fiber "dead fiber called"

Question

Playing round with fibers and I know I'm doing something stupid here. But trying to create a simple enumerable type thing based on fibers (it's obviously trivial using Enumerable). When I run the following I get a dead fiber error after it prints out the first couple of iterations. Does anyone know why?

class SumPow
  def initialize(a)
    sum = a
    pow = a
    @fiber = Fiber.new do
      Fiber.yield sum
      pow = pow * a
      sum = sum + pow
    end
  end

  def next
    @fiber.resume
  end

  def each
    loop do
      yield self.next
    end
  end

end

sp = SumPow.new(3)
sp.each do |sum|
  puts sum
end

output:

3
12
tmp/fiber_enum.rb:96:in `resume': dead fiber called (FiberError)
    from tmp/fiber_enum.rb:96:in `next'
    from tmp/fiber_enum.rb:101:in `block in each'
    from tmp/fiber_enum.rb:100:in `loop'
    from tmp/fiber_enum.rb:100:in `each'
    from tmp/fiber_enum.rb:108:in `<main>'

This code on the other hand works completely fine:

fib = Fiber.new do
  x, y = 0, 1
  loop do
    Fiber.yield y
    x, y = y, x + y
  end
end
7.times { puts fib.resume }

output:

1
1
2
3
5
8
13

Answer 1

The code is calling resume regardless of state of the fiber.

Check whether fiber can resume by calling Fiber#alive?

def each
  while @fiber.alive? do
    yield self.next
  end
end

NOTE You need to do require 'fiber' to use alive? method.

---

UPDATE accoridng to the question edit.

The original code yields only once. To iterate indefinitely, you need loop.

def initialize(a)
  sum = a
  pow = a
  @fiber = Fiber.new do
    loop do # <-------------
      Fiber.yield sum
      pow = pow * a
      sum = sum + pow
    end # <-----------------
  end
end