Why is there no Foldable instance for Either?

[Erik Hesselink]

+1, I wanted this recently.

Erik

On Thursday, May 30, 2013, Shachaf Ben-Kiki wrote:

> On Thu, May 30, 2013 at 12:53 PM, Gabriel Gonzalez <gabriel439 at gmail.com<javascript:;>>
> 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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