The actual writing of course isn't pure. Loading isn't either, but we only support loading on level transition, so we can supply the data already when constructing the game state. Saving is done by gathering all the data that should be saved in a struct, what is pure and happens at a well defined point in the frame, where the game state is known to be consistent (-> I think it's after all systems have been updated), and then this struct is written out to a file.