Deprecate Foldable for Either

[Edward Kmett]

On Thu, Mar 2, 2017 at 11:59 AM, Andreas Abel <andreas.abel at ifi.lmu.de>
wrote:

> 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."
>
>
This would change the semantic of every

forM_ myeither $ \i -> ....

in existing code to silent errors.

Hell no.

-Edward


> 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 at ifi.lmu.de
>> <mailto: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
>>     <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 <mailto:andreas.abel at gu.se>
>>     http://www.cse.chalmers.se/~abela/ <http://www.cse.chalmers.se/~a
>> bela/>
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>>
>
> --
> 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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