I'm a little concerned about it being a hierarchy that doesn't support multiple inheritance. For example, if we want "exceptions with code location info" to be catchable in such a way that we can manipulate that info (even though those exceptions would span across a sensible hierarchy, yet not all exceptions would provide it). It would be possible to model exceptions as a set of classes, e.g. data Exception = Exception [InTypeable] data InTypeable = forall a. (Typeable a{-,perhaps?? Show a-}) => InTypeable a --then sort of similar datas as we're used to from the proposal data IOException = forall a. (IOException a) => IOException a and you can catch exceptions that match some set of classes somehow. Hopefully all exceptions would provide 'show' somehow. Modelling it as a list [InTypeable] seems not ideal, but typeable doesn't provide anything like an Ordering for Set or anything more abstract? I'm not entirely happy with this particular sketch of a proposal, but do people think that my initial issue is something to be concerned about at all? (I'd be glad to be disproved :-) -Isaac