Every asynchronous programming API ever created.

My program spawns and runs in the main thread. It calls the main function. If it want it to run forever, I'll stick a while(1) loop in that function.

Now I want to have a particular thing go off and do some work in parallel with that main loop, and when it's done, I want the next iteration of the main loop to pick up on that fact.

Not once have I encountered a sane API for that use-case as easily expressed as those sentences of prose above.