In base 4.6 addMVarFinalizer is deprecated in favour of mkWeakMVar of type

    mkWeakMVar :: MVar a -> IO () -> IO (Weak (MVar a))

This type makes it inherently non-compositional. For instance, if we have a larger datatype T that contains an MVar somewhere inside then there in no way to define mkWeakT in terms of mkWeakMVar; instead, mkWeakT would have to be defined along the lines of

    mkWeakT :: T a -> IO () -> IO (Weak (T a))
    mkWeakT m@(MkT (MVar m#) _) f = IO $ \s ->
      case mkWeak# m# m f s of (# s1, w #) -> (# s1, Weak w #)

It would be better if the type of mkWeakMVar would change to

    mkWeakMVar :: MVar a -> v -> Maybe (IO ()) -> IO (Weak v)

(i.e., following mkWeak rather than mkWeakPtr).

(The same comment goes for related functions such as mkWeakIORef.)