Comment by contravariant

Logging is probably easier, since a tuple with the output and the logging naturally forms a monad. What is really tricky o pull off is allowing interaction with the environment, which basically requires a more fine grained (and customizable) version of the IO monad.

Monads have the problem that they don't commute, which is usually not what you want for handling events (if you've got handlers for two different types of event it shouldn't matter which event handler was registered first) Maybe the algebraic effects solve this problem.