Re: Why is there no Foldable instance for Either?

30 May 2013


      On 05/30/2013 01:45 PM, Dan Burton wrote:
...
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.
Keep in mind that `errors` can provide the `Left`-biased versions 
because it already has the appropriate newtype for doing so: `EitherR`.
...
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 mailto:shachaf@gmail.com> wrote:
On Thu, May 30, 2013 at 12:53 PM, Gabriel Gonzalez
    mailto: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-...
    -- 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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