Re: [Haskell-cafe] Inverting a Monad

6 Feb 2008

      On Feb 6, 2008 12:39 PM, Miguel Mitrofanov  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

Re: [Haskell-cafe] Inverting a Monad

Bas van Dijk