Final thoughts
Before we round off this chapter, I want to point out that it should now be clear to us why coroutines aren’t really pre-emptable. If you remember back in Chapter 2, we said that a stackful coroutine (such as our fibers/green threads example) could be pre-empted and its execution could be paused at any point. That’s because they have a stack, and pausing a task is as simple as storing the current execution state to the stack and jumping to another task.
That’s not possible here. The only places we can stop and resume execution are at the pre-defined suspension points that we manually tagged with wait.
In theory, if you have a tightly integrated system where you control the compiler, the coroutine definition, the scheduler, and the I/O primitives, you could add additional states to the state machine and create additional points where the task could be suspended/resumed. These suspension points could be opaque to the user and treated differently...
