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

[David Feuer]

Why is that a problem? `Eq a` is still sufficient.

On Tue, May 5, 2020, 1:51 PM Andrew Martin <andrew.thaddeus at gmail.com>
wrote:

> 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
>>> _______________________________________________
>>> Libraries mailing list
>>> Libraries at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>
>
>
> --
> -Andrew Thaddeus Martin
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20200505/ac954d10/attachment.html>