[sullyj3]

> you are violating a logical constraint of the system

I'm looking for something even a little more concrete than this. Why would the program be invalid? Can you give an example of what would happen if we just went ahead and accessed a worldstate a second time, logical constraints be damned?

[goto11]

One of the operations using the same worldstate might not get executed, or the operations might happen in unpredictable order.

[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.