Hi all, The function 'block' and 'unblock' (from Control.Exception) have been deprecated for some time, and are apparantly now being removed (in favour of 'mask'). Generalisations of these functions are (part of) the interface of MonadCatchIO-transformers (the 'MonadCatchIO' class has methods 'block' and 'unblock'). So, the interface would have to change to keep up with base. I'm inclined to deprecate MonadCatchIO-transformers itself, in favour of monad-control. I suspect that most clients do not use 'block' or 'unblock' directly, but use only derived functions, like 'bracket'. (I have partly confirmed this, by inspecting some reverse dependencies on hackage.) This allow an easy transition to monad-control: in many cases, only imports will need to be changed. In the minority of cases where 'block' and 'unblock' are used and/or instances of MonadCatchIO are defined, code will need to be updated. There is a difference in functionality between MonadCatchIO and monad-control. In the former, 'bracket' will not perform the final action if the main action is an ErrorT that throws an error (in contrast with exceptions in the underlying IO monad). In monad-control, 'bracket' will perform the final action in this case. (See this discussion for background: http://www.haskell.org/pipermail/haskell-cafe/2010-October/084890.html.) Probably, in most use cases the behaviour of monad-control is preferred. This seems to be the case also for snap, which uses MonadCatchIO-transformers, but defines its own variant of 'bracket' to get the right behaviour. Would anyone have a problem with a deprecation of MonadCatchIO-transformers, and a failure to update it to work with a base without 'block' and 'unblock'? Regards, Arie