Your thought that it's the caller makes some sense. Think of the OS level call to read a file, you can either ask for it to block until end of line, or just gi

Your thought that it's the caller makes some sense. Think of the OS level call to read a file, you can either ask for it to block until end of line, or just give you what it has now, based on a flag. But "wait until there's eol, than call this callback" or its cousin the future which is "when complete, read here to find the callback(s) to call" which let's you fill in the callback after the call, that's inherently an async call. Same way sending a letter forces you to wait. Sure you could stand by the mailbox until the reply arrives, but that's just converting the async call to synchronous by waiting. Though I don't really see how futures are that different from a cps, especially if you view them as chaining .then calls, which are exactly continuations. Await just let's you drop some syntax, and unify scopes.