Yes, actors in BEAM can modify files and there's a small bit of local state which can be set without messages (the process dictionary). However, much like Haske

Yes, actors in BEAM can modify files and there's a small bit of local state which can be set without messages (the process dictionary). However, much like Haskell you'd still need some point where real side effects could occur. It'd be possible to do all file actions via specific actors using messages which could then be modified to support "time-travel".