[Haskell-cafe] ismzero operator possible without equal constraint

[Arseniy Alekseyev]

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 <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:
>
> 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
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>