[Haskell-cafe] Is there a generic way to detect "mzero"?

[Antoine Latter]

On Mon, Mar 26, 2012 at 1:33 PM, Ting Lei <tinlyx at hotmail.com> wrote:
> 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.
>

Well, you can sort of do it with only MonadPlus - but it really
depends on your choice of Monad whether or not it does anything like
what you want:

can :: (MonadPlus m) => (a -> m ()) -> a -> m Bool
can f x = (f x >> return True) <|> return false

For 'Maybe' this works great, but for something like 'List' I couldn't
even tell you what it would do without reasoning through it.

So you might be better off with the suggestion from Tobias using Eq

Antoine