[gmfawcett]

In a language like Mercury where the IO states are explicit, this would be true. You can run IO in reverse in Mercury if you want:

    main(IO_In, IO_Out) :-
      print_line("This will print last", IO_1, IO_Out),
      print_line("This will print first", IO_In, IO_1).

But in Haskell, the world states aren't accessible to the programmer. It's misleading to suggest what could happen in Haskell if you reused a world state, because it's impossible to do so.