Quoth Jonathan Cast
An `error' is any condition where the correct response is for the programmer to change the source code :)
That's a broad category, that overlaps with conditions where there are one or more correct responses for the original program as well. If I throw exceptions within the type system, using IO or whatever, and at some later time observe that I have caught one that would have been better handled closer to its source, for example. I've already technically satisfied my requirement, since everything is in an exception monad, but the exception is still a bug. Or if I catch an non-IO error using Control.Exception.catch (if I were using ghc), and take advantage of the opportunity to release locks, post various notices, etc. - I evidently have a bug, but I also may need to have a programmed response for it. Donn