Proposal: Add the missing instances for Traversable (Either b) and Traversable ((,) b)

[Ross Paterson]

On Sat, Jul 28, 2012 at 01:27:47AM +0100, Edward Kmett wrote:
> The following instances are missing and have only one sensible
> definition. I've been bitten by their lack repeatedly and there is
> no place outside of base that they can live without needlessly being
> orphaned. 
> 
> I would like to propose adding the following instances to Data.Foldable
> and Data.Traversable.
> 
> instance Foldable (Either e) where
>    foldMap f (Right m) = f m
>    foldMap _ (Left _) = mempty
> instance Traversable (Either e) where
>    traverse _ (Left e) = pure (Left e)
>    traverse f (Right x) = Right <$> f x
> instance Foldable ((,) e) where
>    foldMap f (_,x) = f x
> instance Traversable ((,) e) where
>    traverse f (e,x) = (,) e <$> f x
> 
> I had thought honestly thought we'd already added them long ago.

You proposed them in January 2011 and everyone agreed (after changing the
pair instance to the strict versions given here) but I don't think you
sent a patch to Ian.

http://thread.gmane.org/gmane.comp.lang.haskell.libraries/15196

It's certainly overdue.