I was convinced it was going to be about pure vs impure (monadic) code, but then he just picked out one particular monad instance (coroutines/asynchronous opera

I was convinced it was going to be about pure vs impure (monadic) code, but then he just picked out one particular monad instance (coroutines/asynchronous operations). Any time you have side effects, you potentially have "red" code, in that it can add arbitrary restrictions on how you must use the functions.