[Haskell-cafe] Inverting a Monad

[Bas van Dijk]

On Feb 6, 2008 12:39 PM, Miguel Mitrofanov <miguelimo38 at yandex.ru> wrote:
> > 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?
>

Well in:

inv :: (Monad m, ...) => m a -> m ()
inv m = ...

I don't mind that there are more constraints on 'm' than just Monad
maybe a MonadPlus constraint or others are needed.


(I was even thinking about a MonadTimes class as in:

class MonadPlus m => MonadTimes m where
    mone :: m ()
    mtimes :: m a -> m a -> m a
)

Thanks,

Bas