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

[dag.odenhall at gmail.com]

On 26 March 2012 21:11, Jeff Shaw <shawjef3 at 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
>

can f x = f x /= mzero
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