Ok, Foldable is a formal condition for Traversable, but not actually used in the implementation of Traversable Either.  This still leaves room to implement Foldable for Either by

  instance Foldable (Either a) where
    foldMap _ _ = error "Folding Either?  Naah, I don't think this is a good idea."

On 02.03.2017 17:48, David Feuer wrote:

The problem is that we'd then lose the perfectly good Traversable
instance, which would be sad.

On Mar 2, 2017 11:23 AM, "Andreas Abel" <andreas.abel@ifi.lmu.de
<mailto:andreas.abel@ifi.lmu.de>> wrote:

    Today a student came to me wondering why a certain function produced
    a regular result, where he had expected an error.  Turned out he had
    used `concat`, but not on a lists of lists as he had thought, but on
    a lists of `Either a [b]`.

    With the Foldable instance for Either, which considers Either a b to
    be a container of 0-1 elements of b, errors are happily swallowed.

    I think this instance is harmful and should be deprecated (and later
    removed) from base.

    There are similarly pointless Foldable instances as well.

    See a discussion one year ago, which was heated, but had no
    consequences.

    https://mail.haskell.org/pipermail/libraries/2016-February/026678.html
    <https://mail.haskell.org/pipermail/libraries/2016-February/026678.html>


    --
    Andreas Abel  <><      Du bist der geliebte Mensch.

    Department of Computer Science and Engineering
    Chalmers and Gothenburg University, Sweden

    andreas.abel@gu.se <mailto:andreas.abel@gu.se>
    http://www.cse.chalmers.se/~abela/ <http://www.cse.chalmers.se/~abela/>
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--
Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel@gu.se
http://www.cse.chalmers.se/~abela/
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