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

On 26 March 2012 21:11, Jeff Shaw wrote:

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.

The problem here is that when you match on "f x", your first match is an identifier that matches anything at all, and binds it to mzero. I think what you're looking for is

can f x = case f x of x' | x' == mzero -> False _ -> True

can f x = f x /= mzero