On 26 March 2012 21:11, Jeff Shaw <shawjef3@msu.edu> 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