> This twist is very simple to deal with if you have real existential types,
> with the relevant part of the interface looking approximately like
>
> init :: exists a. IO (Inotify a)
> addWatch :: Inotify a -> FilePath -> IO (Watch a)
> rmWatch :: Inotify a -> Watch a -> IO ()

You can still do the ST-like encoding (after all, the ST typing trick
is just an encoding of an existential), with init becoming "like
runST":

> init :: (forall a. Inotify a -> IO b) -> IO b

> addWatch :: Inotify a -> FilePath -> IO (Watch a)
> rmWatch :: Inotify a -> Watch a -> IO ()