Why is there no Foldable instance for Either?

[Ross Paterson]

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.