Hmmm, my point (perhaps I wasn't clear), is that different effects have different commutability properties. In the case of a file system, you can commute two sequential reads from two different files. This has no effect on the result of the computation, assuming no interference from other programs -- and if there _is_ interference from other programs, then guarantees go out the window, _with or without_ commuting.

Monads are an insufficient structure to capture the fine semantics of effects. Something more powerful is needed. And in general, the way effects combine and commute is quite complicated and needs to be baked into the effect system, rather than being the responsibility of a lowly developer.