[Haskell-cafe] ismzero operator possible without equal constraint

[edgar klerks]

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 at 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 <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
> >
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20111203/16bf13cd/attachment.htm>