Thank you both for your clarifications - that's very helpful. I think I am just going to have to think hard in each situation whether I want to catch exceptions myself or leave it to the user. I suppose it depends, in some cases I might be able to do something gracefully, but in other cases the user is going to have to be told forcefully that their input isn't sane. I don't suppose there's a hard and fast rule for that, is there?