I try to write a function "liftN", but I'm not satisfied with result.

lift :: (MonadTrans t) => Monad m => m a -> t m a

liftN n
š š | n < 1 š š š = error "liftN: n<1"
š š | n == 1 š š = lift
š š | otherwise = lift . (liftN (n-1))

1) I know(?), that it is impossible to write liftN now: typechecker can't
decide which signature it is.
2) Main reason is finite recursive constraint

3) Let's think, it is possible to write
Main candidates: promoted data and data-kinds

For example,
lift . lift . lift
š :: (Monad (t1 (t2 m)), Monad (t2 m), Monad m, MonadTrans t,
š š š MonadTrans t1, MonadTrans t2) =>
š š šm a -> t (t1 (t2 m)) a

My best solution looks like:

data MT = M | T Int MT
liftN :: forall tm. ( forall x n.
š š š š š š š š š šMonad 'M, Monad (x :: MT),
š š š š š š š š š šMonadTrans ('T n),
š š š š š) => Int -> 'M a -> š('T 0) (tm :: MT) a
liftN n
š š š| n < 1 š š = error "liftN: šn < 1"
š š š| n == 1 š š= lift
š š š| otherwise = lift . (liftN (n-1))


Am I miss something?
I think this function could look much prettier.
But I don't know how.



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