[oconnor663]

Modern fast IO is built on top of facilities like Linux's epoll(), which don't block an entire thread. However, that means that when an IO operation is finished, we no longer have the original callstack around to keep track of what operations were supposed to happen next. Instead we need some sort of state machine that's able to pause and resume every time it does IO. But writing state machines by hand is much less convenient than using the callstack, because you lose all the nice language features that you'd normally use to compose things, like `if` and `for` and `?`. So async/await is all about writing normal-looking code with the usual conveniences that can instead be compiled into a state machine that lives off the callstack.