[eximius]

okay, but that doesnt solve basically the main thing that async paradigms seek to solve: sharing of resources between waiting disjoint processes.

your statemachine blocks the thread. if you had a more complicated state machine, maybe nested machines, theyd block each other because they dont know how to cooperate.

[jude-]

This code is just an example. The state machine can easily run in its own thread, separate from the main thread. As long as the state machine had an IOClient instance that lets it send I/O requests to the IOServer and receive I/O results, you're good. Also, you could imagine an IOClient having an API that takes a StateMachine instance as input, and returns an IOClient::Future that resolved to the machine's main() return value.