Deprecate Foldable for Either

[Matthew Pickering]

Another type can be added to base which does have the Traversable instance.

On Thu, Mar 2, 2017 at 4:48 PM, David Feuer <david.feuer at gmail.com> 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 at 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
>>
>>
>> --
>> Andreas Abel  <><      Du bist der geliebte Mensch.
>>
>> Department of Computer Science and Engineering
>> Chalmers and Gothenburg University, Sweden
>>
>> andreas.abel at gu.se
>> http://www.cse.chalmers.se/~abela/
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>
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