Why do we need fibers

Question

For Fibers we have got classic example: generating of Fibonacci numbers

fib = Fiber.new do  
  x, y = 0, 1 
  loop do  
    Fiber.yield y 
    x,y = y,x+y 
  end         


        

Answer 1

Unlike closures, which have a defined entry and exit point, fibers can preserve their state and return (yield) many times:

f = Fiber.new do
  puts 'some code'
  param = Fiber.yield 'return' # sent parameter, received parameter
  puts "received param: #{param}"
  Fiber.yield #nothing sent, nothing received 
  puts 'etc'
end

puts f.resume
f.resume 'param'
f.resume

prints this:

some code
return
received param: param
etc

Implementation of this logic with other ruby features will be less readable.

With this feature, good fibers usage is to do manual cooperative scheduling (as Threads replacement). Ilya Grigorik has a good example on how to turn an asynchronous library (eventmachine in this case) into what looks like a synchronous API without losing the advantages of IO-scheduling of the asynchronous execution. Here is the link.