Even if you use a stateful monad (not necessarily the State monad), you can take snapshots of the state of the machine and literally produce a log. You haven't

Even if you use a stateful monad (not necessarily the State monad), you can take snapshots of the state of the machine and literally produce a log. You haven't lost immutability or the time machine, and you can 'deriving Show' the hell out of everything and get human-readable output for free. Fuck, you could even lift functions in such a way that they produce a trace of assertions that each function of (state -> state) must satisfy. A state-debugger-log monad. Not that you'd need a monad for something like this anyway.