We have already right-biased Eithers in Haskell for Functor, Applicative, and Monad. We should be consistent and right-bias it for Foldable, Traversable, and similar typeclasses. Left-biasing can still be accomplished with a newtype wrapper, and perhaps we should consider including this in the standard libraries as well.

Until the change happens, I'd suggest recommending that "errors" users requiring this functionality lean on the "lens" instances.

-- Dan Burton

On Thu, May 30, 2013 at 1:12 PM, Shachaf Ben-Kiki <shachaf@gmail.com> wrote:

On Thu, May 30, 2013 at 12:53 PM, Gabriel Gonzalez <gabriel439@gmail.com> wrote:
> I'm surprised that `Either` does not have a `Foldable` instance of the form:
>
>     import Data.Foldable
>     import Data.Monoid
>
>     instance Foldable (Either e) where
>         foldMap f (Left  _) = mempty
>         foldMap f (Right r) = f r
>
>         foldr _ z (Left  _) = z
>         foldr f z (Right r) = f r z
>
> In other words, the above instance would behave like the `Maybe` `Foldable`
> instance, treating `Left` in the same way as `Nothing`.
>
> This came up in the context of an issue raised on the `errors` package:
>
> https://github.com/Gabriel439/Haskell-Errors-Library/issues/16
>
> I could provide an orphan instance for `Either` in the `errors` package, but
> I wanted to ask if it was possible to incorporate the instance directly into
> `Data.Foldable`.
>

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>.

I wrote a patch for base at one point --
<http://shachaf.net/0001-Add-Foldable-and-Traversable-instances-for-Either-e-.patch>
-- but didn't go through the rest of the process at the time.

Right now `lens` has an orphan instance for this, by the way.

    Shachaf

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