Why is there no Foldable instance for Either?

Edward Kmett ekmett at gmail.com
Wed Jun 12 17:01:15 CEST 2013


No reason at all other than an annoying major version bump in transformers.


On Wed, Jun 12, 2013 at 5:10 AM, Shachaf Ben-Kiki <shachaf at gmail.com> wrote:

> On Thu, May 30, 2013 at 6:41 PM, Shachaf Ben-Kiki <shachaf at gmail.com>
> wrote:
> > On Thu, May 30, 2013 at 6:34 PM, Ross Paterson <R.Paterson at city.ac.uk>
> wrote:
> >> On Thu, May 30, 2013 at 01:12:00PM -0700, Shachaf Ben-Kiki wrote:
> >>> This instance should exist. There's been a couple of discussion on
> >>> libraries@ before. One of them is at
> >>> <http://www.haskell.org/pipermail/libraries/2012-July/018246.html>.
> >>
> >> The previous one was in January 2011:
> >>
> >> http://thread.gmane.org/gmane.comp.lang.haskell.libraries/15196
> >>
> >> I've taken the liberty of pushing the previously discussed Foldable and
> >> Traversable instances for Either a and (,) a.
> >>
> >
> > Thank you! While you're at it would it be possible to add (trivial)
> > Foldable and Traversable instances for (Const r), for completeness? I
> > think that was also mentioned in one of the discussions. As in my
> > patch, the instances are:
> >
> > instance Foldable (Const m) where
> >     foldMap _ _ = mempty
> >
> > instance Traversable (Const m) where
> >     traverse f (Const m) = pure (Const m)
> >
> >     Shachaf
>
> Just to make sure this isn't slipping through the cracks -- is there a
> particular reason not to add the instance for (Const r)? It was
> brought up in the last discussion and it has one obviously-correct
> definition. It's a useful instance.
>
> (Right now, as mentioned, lens defines orphans for all three types --
> it would be nice to get rid of all of them in one go.)
>
> Thanks,
>     Shachaf
>
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