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@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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