Re: Why is there no Foldable instance for Either?

On Thu, May 30, 2013 at 6:41 PM, Shachaf Ben-Kiki wrote:

On Thu, May 30, 2013 at 6:34 PM, Ross Paterson wrote:

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On Thu, May 30, 2013 at 01:12:00PM -0700, Shachaf Ben-Kiki wrote:

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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