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

[Andrew Martin]

You're right. I botched that. The scenario I meant to describe was:

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

The different results are:
* FlexibleContexts approach: `(Eq (Maybe (f a)), Eq [f a])`
* Eq1 typeclass: `(Eq1 f, Eq a)`
* Quantified Constraints: `(forall x. Eq x => Eq (f x), Eq a)`

Only the FlexibleContexts approach mentions Maybe and [] in the constraints.

On Tue, May 5, 2020 at 1:56 PM David Feuer <david.feuer at gmail.com> wrote:

> 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
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>>
>>
>> --
>> -Andrew Thaddeus Martin
>>
>

-- 
-Andrew Thaddeus Martin
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