[Haskell-cafe] ismzero operator possible without equal constraint

[David Menendez]

On Sat, Dec 3, 2011 at 3:55 PM, Antoine Latter <aslatter at gmail.com> wrote:
> On Sat, Dec 3, 2011 at 10:55 AM, edgar klerks <edgar.klerks at gmail.com> wrote:
>> Hi list,
>>
>> I am using MonadSplit
>> (from http://www.haskell.org/haskellwiki/New_monads/MonadSplit )  for a
>> project and now I want to make a library out of it. This seems to be
>> straightforward, but I got stuck when I tried to move miszero out of the
>> class:
>>
>> miszero :: m a -> Bool
>>
>> It tests if the provided monad instance is empty. My naive attempt was:
>>
>
> You can write:
>
> miszero :: MonadPlus m => m a -> m Bool
> miszero m = (m >> return False) <|> return True
>
> but that will invoke any monadic effects as well as determining the
> nature of the value, which may not be what you want.

It's almost certainly not what you want for the list monad.

-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>