Why is there no Foldable instance for Either?

Edward Kmett ekmett at gmail.com
Wed Jun 12 21:39:08 CEST 2013

The main concern I would have is that Const actually seems to be in use
everywhere, while Constant is more or less unused by dint of the fact that
Const comes into scope with Control.Applicative, so many more people are
aware of it.


On Wed, Jun 12, 2013 at 1:26 PM, Ross Paterson <R.Paterson at city.ac.uk>wrote:

> On Wed, Jun 12, 2013 at 11:01:15AM -0400, Edward Kmett wrote:
> > On Wed, Jun 12, 2013 at 5:10 AM, Shachaf Ben-Kiki <shachaf at gmail.com>
> wrote:
> >     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.
> >
> > No reason at all other than an annoying major version bump in
> transformers.
> Constant in transformers already has these instances, but Const in base
> doesn't.  Otherwise they're the same (and both of them are my fault).
> One of them should go -- I'm in favour of keeping the full name.
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