On 26 March 2012 21:11, Jeff Shaw <[email protected]> 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
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