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

[Andrew Martin]

I dislike the FlexibleContexts approach because it gives you constraints
that do not compose. Consider this minimal example:

    foo :: _ => a -> a -> Bool
    foo x y = Compose (Just (Just x)) == Compose (Just (Just y)) && Compose
[Just x] == Compose [Just y]

What do we expect the constraint to be? With the Eq1 machinery or with
QuantifiedConstraints, it's `Eq a` (GHC will infer this). However, with
FlexibleContexts, it's `(Eq (Compose Maybe Maybe a), Eq (Compose [] Maybe
a)`.

On Tue, May 5, 2020 at 12:45 PM David Feuer <david.feuer at gmail.com> wrote:

> I oppose the QuantifiedConstraints version because:
>
> 1. It is more complicated than the FlexibleConstraints one.
>
> 2. It is strictly less general than the FlexibleConstraints one.
>
> 3. There is no apparent benefit to pay for detriments 1 and 2.
>
> On Fri, Mar 13, 2020, 11:59 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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-- 
-Andrew Thaddeus Martin
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