Hi, I was writing a code trying to use MonadPlus to detect some error cases (representing missing values etc. in pure code). With the Maybe monad, I can do this: can0 :: (a -> Maybe b) -> a -> Bool can0 f x = case f x of Nothing -> False Just x -> True And I got the expected result: *Main> can0 (\x -> Just x) 1 True But, when I try to generalize this using MonadPlus, as follows: can :: (MonadPlus m) => (a -> m b) -> a -> Bool can f x = case f x of mzero -> False _ -> True I got a warning: __testError.hs:31:11: Warning: Pattern match(es) are overlapped In a case alternative: _ -> ... Ok, modules loaded: Main. And the result is also not as intended (see also can0): *Main> can (\x -> Just x) 1 False Can anyone help to explain why this wouldn't work or if there is a workaround to use Monadplus and mzero (or Monad and fail) to achieve this? Thanks in advance for your help Ting