Deprecate Foldable for Either

[David Feuer]

foldMapDefault and fmapDefault *should* actually perform quite well, by the
way, unless the specialized definitions come with rewrite rules.

On Mar 2, 2017 1:50 PM, "Dan Burton" <danburton.email at gmail.com> wrote:

> Of course errors are swallowed. Look at the type signature.
>
> concat :: Foldable t => t [b] -> [b]
> -- t = Either a
> concat :: Either a [b] -> [b]
>
> The error type "a" does not appear anywhere in the function's output type.
> In the event of an error, the only thing it could possibly produce as
> output is an empty list.
>
> I don't think this means that Either's foldable instance is harmful and
> should be removed. It's a perfectly good instance that behaves in the
> "obvious" Maybe-like way. I do think this is an argument in favor of using
> a custom Prelude (which uses functions specialized to lists) when teaching
> new students.
>
> Any Traversable instance should absolutely be Foldable, with foldMap =
> foldMapDefault, or an optimized version that produces the same result.
> Putting `foldMap = error ...` for any Traversable is out of the question,
> imo.
>
> -- Dan Burton
>
> On Thu, Mar 2, 2017 at 10:12 AM, Kris Nuttycombe <
> kris.nuttycombe at gmail.com> wrote:
>
>> As with all of these discussions, the point of having Either be Foldable
>> is not that you're going to call foldMap on an Either value directly.
>> Instead, it is that you be able to pass whatever sort of foldable thing you
>> choose (which Either certainly is) to a function that only requires
>> foldability of its input. By removing or damaging the Foldable instance for
>> Either, you don't simply prevent people from encountering problems that
>> will be resolved the first time they test their software - you make a whole
>> universe of nicely polymorphic functions unavailable for them to use
>> without additional effort.
>>
>> In particular, the idea that one should make all such functions partial
>> by throwing an error is repugnant.
>>
>> Kris
>>
>> On Thu, Mar 2, 2017 at 10:35 AM, Francesco Ariis <fa-ml at ariis.it> wrote:
>>
>>> On Thu, Mar 02, 2017 at 05:19:26PM +0000, Oliver Charles wrote:
>>> > Personally, I think it would be a shame to lose foldMap on EIther. I
>>> > frequently foldMap over Maybe values (where mempty is suitable in case
>>> of
>>> > "failure"), and I can certainly see myself doing the same thing with
>>> Either.
>>>
>>> I am not trying to be polemic, just to see where the community
>>> stands: regarding Either/Maybe: do you have use cases for length
>>> (sum) too?
>>>
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>>
>>
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>
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