Jeff,
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
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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