Re: [Haskell-cafe] ismzero operator possible without equal constraint

Hi Arseniy, Yes, I see it now. :) . I had some feeling there should be some structural equality: Just _ == Just _ = True Nothing == Nothing = True _ == _ = False But this doesn't work for functions. Thanks for your answer! Greets, Edgar On Sat, Dec 3, 2011 at 6:23 PM, Arseniy Alekseyev < arseniy.alekseyev@gmail.com> wrote:

Of course it is not possible! Take a simple composition of reader and Maybe functors for an example:

miszero :: (b -> Maybe a) -> Bool

I'm pretty sure (b -> Maybe a) for a is a MonadPlus, but you can't implement miszero for it.

Arseniy.

On 3 December 2011 16:55, edgar klerks 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:

miszero :: (Eq (m a), MonadPlus m) => m a -> Bool miszero = ( == mzero )

This works, but not correctly. It adds an Eq constraint that is unneeded. I would prefer to have something like:

miszero :: MonadPlus m => m a -> Bool

Because I am not comparing the contents of the monad. I don't even touch it. Is this possible to write?

with kind regards,

Edgar

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