[Haskell-cafe] Inverting a Monad

[Miguel Mitrofanov]

> invM :: Maybe a -> Maybe ()
> invM Nothing  = Just ()
> invM (Just _) = Nothing
>
> invL :: [] a -> [] ()
> invL []    = [()]
> invL (_:_) = []
>
>
> How can I define this for an arbitrary Monad m?

Such as Identity?