> Mutation is also how the real world works. If you want to record something, you write it down—you've just mutated the world, not encapsulated your operation in the WorldState monad.

But is it, though? Perhaps you just appended something to the world-log in a purely functional way. Time seems to always go in one direction (at least in my experience, YMMV), kind of like a DB primary key that is monotonously increasing. It really depends on how you look at this.