Re: [Haskell-cafe] Inverting a Monad

On Feb 6, 2008 11:32 AM, Bas van Dijk wrote:

The following obviously doesn't work:

inv m = (m >> mzero) `mplus` return ()

because it will always return ().

How about: inv m = do success <- (m >> return True) `mplus` return False if success then mzero else return () Luke