Re: [Haskell-cafe] Inverting a Monad

Bas van Dijk wrote:

The following obviously doesn't work:

import Control.Monad.Error

inv :: MonadError e m => m a -> m () inv m = (m >> fail "") `catchError` \_ -> (return ())

What about this? inv :: MonadError e m => m a -> m () inv m = join $ (m >> return mzero) `catchError` \_ -> return (return ()) Tillmann