Fibers are primitives for implementing light weight cooperative concurrency in Ruby. Basically they are a means of creating code blocks that can be paused and resumed, much like threads. The main difference is that they are never preempted and that the scheduling must be done by the programmer and not the VM.
As opposed to other stackless light weight concurrency models, each fiber comes with a small 4KB stack. This enables the fiber to be paused from deeply nested function calls within the fiber block.
When a fiber is created it will not run automatically. Rather it must be be explicitly asked to run using the Fiber#resume method. The code running inside the fiber can give up control by calling Fiber.yield in which case it yields control back to caller (the caller of the Fiber#resume).
Upon yielding or termination the Fiber returns the value of the last executed expression
1 2 FiberError: dead fiber called
The Fiber#resume method accepts an arbitrary number of parameters, if it is the first call to resume then they will be passed as block arguments. Otherwise they will be the return value of the call to Fiber.yield
fiber = Fiber.new do |first| second = Fiber.yield first + 2 end puts fiber.resume 10 puts fiber.resume 14 puts fiber.resume 18
12 14 FiberError: dead fiber called