Improving the instances of Data.Functor.{Product,Sum}

[Zemyla]

Also, keeping it Haskell98 compliant makes it easier for newer users to
grok.

On Fri, Mar 13, 2020, 23:15 Eric Mertens <emertens at gmail.com> wrote:

> I prefer to avoid FlexibleInstances wherever I can. The last thing I'd
> heard about quantified constraints was that they were buggy and I've been
> avoiding relying on them. (I should probably review that assumptions at
> some point.) Having a class like Eq1 is the cleaner option. I'd rather fix
> up these classes than work to get rid of them.
>
> On Fri, Mar 13, 2020 at 8:59 PM chessai . <chessai1996 at gmail.com> wrote:
>
>> Consider the Eq instance for these types. Currently we rely on:
>>
>> (Eq1 f, Eq1 g, Eq a)
>>
>> But some potential improvements include changing to:
>>
>> (Eq (f (g a))) (FlexibleContexts)
>>
>> or
>>
>> (forall x. Eq x => Eq (f x), forall x. Eq x => Eq (g x), Eq a)
>> (QuantifiedConstraints)
>>
>> There was a discussion sometime last year about the same thing
>> regarding Semigroup/Monoid instances for `Compose` [1]. Additionally,
>> the question has been raised again for Data.Functor.{Product,Sum} on
>> Gitlab [2, 3]. There has been no consensus in either case, but that's
>> not too worrying as both discussions have been brief. I'm currently
>> not happy with the {Eq,Ord,Show}{1,2} family of classes, and would
>> hope to work toward their removal, or at least the shrinking of their
>> presence in base. Even though the linked proposals are about a single
>> type, I think it's important that we come up with a decision and stick
>> with it. Having different APIs for different types here would be
>> pretty confusing, and some could even say sloppy.
>>
>> Please let me know your thoughts.
>>
>> [1]: https://mail.haskell.org/pipermail/libraries/2019-July/029771.html
>> [2]: https://gitlab.haskell.org/ghc/ghc/issues/17015
>> [3]: https://gitlab.haskell.org/ghc/ghc/merge_requests/1704
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>
>
> --
> Eric Mertens
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