Hi Edward, thanks a lot for your reply.
rnf can be thought of a function which produces a thunk (for unit) which, when forced, fully evaluates the function. With this in hand, it's pretty clear how to use evaluate to enforce ordering:
evaluate (rnf ('a': throw exceptionA))
So if I understand correctly, then if I have evaluate (x_1 `seq` x_2 `seq` x_3 `seq` ... `seq` x_n) throwIO exceptionB it is guaranteed that exceptionB can only happens if none of the xs are "exceptional". I was just going to say that I can give at least one counterexample where this does not hold: evaluate (('a' : undefined) `deepseq` return () :: IO ()) throwIO exceptionB But then I realized that here exceptionA is optimized away altogether. For me this smells like a bug. Is this related to [1]? Cheers, Simon [1] http://hackage.haskell.org/trac/ghc/ticket/2273