Red/Blue functions allegory of async and sync functions.

The author that if one function is red, all functions using it should be red too.

This is where the “red functions can only be called by red functions” rule comes from. You have to closurify the entire callstack all the way back to main() or the event handler.

One solution could be more generators.
Another is the use of green threads.

[...] Go has eliminated the distinction between synchronous and asynchronous code.