I don't think your code works in general, since it is not guaranteed that x' == mzero is allowed unless (m b) is an instance of Eq. I'm unsure if you are able to test for mzero in general.

Harry

On Mon, Mar 26, 2012 at 3:11 PM, 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

Jeff

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