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
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