Deprecate Foldable for Either

[Edward Kmett]

Sadly, doing so in Data.List would lead to massive breakage as Data.List is
specified by the report and is very, very widely imported unqualified just
to get at things like sort.

-Edward

On Thu, Mar 2, 2017 at 2:28 PM, David Feuer <david.feuer at gmail.com> wrote:

> Yes. That is an excellent plan. I'd love to call it Data.List, but others
> will disagree.
>
> On Mar 2, 2017 2:17 PM, "Andreas Abel" <andreas.abel at ifi.lmu.de> wrote:
>
>> We could have a module
>>
>>   Data.List.ReallyJustListsAndNotSomeThingMoreGeneric
>>
>> which implements concat and friends just for lists and could be imported
>> if one wants to have the list operations.  Currently,
>>
>>   import qualified Data.List as List
>>
>> does not give on the list operations as e.g.
>>
>>   List.concat
>>
>> but the generic ones.
>>
>> See also http://ghc.haskell.org/trac/ghc/ticket/13345
>>
>> On 02.03.2017 20:02, Edward Kmett wrote:
>>
>>>
>>> On Thu, Mar 2, 2017 at 11:59 AM, Andreas Abel <andreas.abel at ifi.lmu.de
>>> <mailto: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>
>>>         <mailto: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/0
>>> 26678.html
>>>         <https://mail.haskell.org/pipermail/libraries/2016-February/
>>> 026678.html>
>>>
>>>         <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>
>>>         <mailto:andreas.abel at gu.se <mailto:andreas.abel at gu.se>>
>>>             http://www.cse.chalmers.se/~abela/
>>>         <http://www.cse.chalmers.se/~abela/>
>>>         <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 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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